Abstract The Web
charlotte's web英文原著
Charlotte's Web is a classic children's novel written by E.B. White, first published in 1952. The story follows the friendship between a pig named Wilbur and a spider named Charlotte, as well as their experiences on a farm. The novel has be a beloved and enduring work of children's literature, and has been adapted into various forms, including stage plays, animated films, and live-action movies.1. Background of the AuthorE.B. White, the author of Charlotte's Web, was an American writer and editor. Born in Mount Vernon, New York in 1899, White had a passion for writing from a young age. He pursued a career in journalism and worked for The New Yorker magazine for many years, where he wrote essays, poems, and editorials. In addition to Charlotte's Web, White also wrote Stuart Little, another popular children's book. He was known for his witty and gentle writing style, and his works often revolved around the themes of friendship, empathy, and the wonders of the natural world.2. Plot SummaryCharlotte's Web is set on a farm in rural M本人ne, where a young pig named Wilbur lives with his friends, including a wiseold sheep named Charlotte. When Wilbur learns that he is destined to be slaughtered, Charlotte promises to help him avoid this fate. Using her web, Charlotte spins messages pr本人sing Wilbur, which attracts attention and admiration from the humans around the farm. As a result, Wilbur bes famous and is ultimately spared from being turned into bacon.3. Themes and MessagesThe novel addresses several important themes, including the values of friendship, loyalty, and the cycle of life and death. Through the friendship between Wilbur and Charlotte, the story emphasizes the significance of empathy, kindness, and standing up for those in need. Charlotte's selfless actions for Wilbur also highlight the importance of sacrifice and the impact of one's choices on others.4. Impact and LegacyCharlotte's Web has left a lasting impact on readers of all ages since its publication. The novel has been celebrated for its heartwarming storytelling, endearing characters, and timeless messages. It has been pr本人sed for its ability to addressplex emotions and moral lessons in a way that is accessible to children, and it continues to be a staple in many classrooms andhomes around the world.5. Adaptations and Cultural SignificanceGiven its popularity, Charlotte's Web has been adapted into various forms of media, including animated and live-action films, as well as stage productions. These adaptations have introduced the story to new generations, further cementing its status as a beloved classic. Charlotte's Web has also been the subject of scholarly analysis, with experts exploring its social and ethical themes, as well as its place within the canon of children's literature.In conclusion, Charlotte's Web is a treasured children's novel that continues to captivate readers with its timeless themes, endearing characters, and poignant storytelling. E.B. White's creation has left a significant mark on the world of literature, influencing generations of readers and serving as a testamentto the enduring power of friendship,passion, and the magic of storytelling.。
Designing Localized Web Sites
Designing Localized Web SitesOlga De Troyer and Sven CasteleynWISEDepartment of Computer Science, Vrije Universiteit Brussel, Pleinlaan 2,1050 Brussel, Belgium{Olga.DeTroyer, Sven.Casteleyn}@vub.ac.behttp://wise.vub.ac.beAbstract.The term World Wide Web (WWW) emphasizes that the Web isglobal and many companies realize that this creates new opportunities. Aconsiderable amount of literature on web site development stresses that, inorder to attract and retain more customers, it is vital to create different versionsof a web site and adapt those versions to the local communities they target. Thisprocess is usually called globalization and the different web site versions arecalled localized web sites. Although content management systems (CMS) forweb sites provide support for multinational web sites, current web site designmethodologies do not consider the issue of globalization. In general, theglobalization effort is done after the web site is designed and implemented. Thismakes globalization much harder. In this paper, we show how to extend anexisting web site design method, WSDM, to support the design of localizedweb sites.1IntroductionWWW pages are accessible from all over the world. This offers opportunities for companies and organizations to attract visitors from across the country borders and to do business with them. Two different approaches are possible to address this issue: develop one single web site to serve everyone or develop ‘localized’ web sites for particular localities. The ‘one size fits all’ approach may be appropriate for particular communities (like researchers) but in general it will be less successful. A considerable amount of literature on web site development stresses that, in order to attract and retain more customers, it is vital to localize a global web site, i.e. to create different web site versions and adapt those versions to the local communities they target. Members of a community do not only share a common language, but also common cultural conventions. Since measurement units, keyboard configurations, default paper sizes, character sets and notational standards for writing time, dates, addresses, numbers, currency, etc differ from one culture to another, it is self-evident that local web sites should address these issues. Some jokes, symbols, icons, graphics or even colors may be completely acceptable in one country, but trigger negative reactions in another country. Sometimes the style or tone of the site’s text might even be considered offensive by a particular cultural entity, as a result of which the text needsto be rewritten rather than merely translated. Next to culturally differences, it may also be necessary to adapt the content to regional differences, like differences in the services and products offered, differences in price, and differences in regulations.The localization issue is not new. Localization of software has been done for years and research has been performed in this context. Nielsen and Del Galdo [5] stress that localization should encompass more than a ‘surface-level’ adaptation, by acknowledging underlying cultural differences. The role of culture in user interface has also been addressed by Evers and Day [11]. Barber and Badre [1] detected the existence of cultural markers, i.e. web design elements that are prevalent in web sites of a particular culture (e.g. color, icons, symbols). Sheppard and Scholtz [21] and Sun [23] conducted pilot studies to determine if the absence or presence of cultural markers affects the user's preference or performance. Cultural differences have also been investigated from an anthropological perspective, looking at the intangible nuances of a culture's social hierarchy, individualism, gender roles, attitude towards uncertainty and time-orientation ([18], [10]). This type of research is commonly approached through Hofstede’s cross-cultural theory [15]. According to Hofstede, cultural differences are based in deeply rooted values that can be categorized along five fundamental dimensions: power distance, collectivism-individualism, masculinity-femininity, uncertainty avoidance, and long and short-term orientation. His research is based on a large-scale survey carried out between 1967 and 1973 and which covered 53 countries representing the major regions of the world. Marcus and Gould [18] attempt to apply those dimensions to global web interface design, providing suggestions and guidelines to produce successfully localized web sites.In the context of localization, three different terms are used: globalization, internationalization and the term localization itself. According to LISA (Localization Industry Standards Association) [17] localization of a thing is adapting it to the needs of a given locale. Globalization is about spreading a thing to several different countries, and making it applicable and useable in those countries. Globalization is never all encompassing; you will never cover all the 600 languages on the planet today. In the context of web sites, globalization usually indicates the process of converting a web site to different languages and communities. Internationalization consists of all preparatory tasks that will facilitate subsequent localization. The purpose of internationalization is to make localization easier, faster, of higher quality and more cost-effective. It may include: creating illustrations in which the text can easily be changed; allowing space for translation into languages that require more space; abstracting content from markup; identification and isolation of culturally specific items. Localization adds cultural context to a previously internationalized web site and includes translation. Translation is only one of the tasks of localization but because it is the most cost expensive, time consuming and most vital task it is often used in the same context as globalization, internationalization and localization.As for classical software, web site globalization is often done once the web site is completely developed and available for a particular community. Content Management Systems (CMS) for web sites usually provide support for multinational web production [4]. However such systems don’t provide a methodology for designing the web site. Nowadays, it is recognized that a proper method should be used to design professional web sites. Although, several methods to design web sites (e.g. OOHDM [22], WebML[3], OO-H [13], UWE [16], WSDM[6]) exist, as far as we are aware of,none of these methods takes globalization issues or one of its aspects (localization, internationalization or translation) into account during the design process. We believe that the globalization process could benefit from taking localization requirements into consideration while designing the web site. If web sites are designed with the need for localization in mind, it may be easier to actually realize globalization because the internationalization activities may already be considered and prepared for during the design process. For this reason, we have extended our own web site design method WSDM in order to support web localization. In this paper, we explain how this has been done. The paper is structured as follows. Section 2 provides a brief overview of WSDM. In section 3, we explain the extensions to WSDM and illustrate them with an example. Section 4 discusses the proposed solution and presents conclusions.2WSDMThe design process of WSDM follows an audience driven design philosophy i.e. the design is based on and driven by the requirements of the different types of users. Figure 1 gives an overview of the different phases of the method.Figure 1 - Overview of WSDMThe method starts with the ‘Mission Statement Specification’. The mission statement should identify the purpose, the subject and the target audience(s) of the web site. Next, the ‘Audience Modeling’ phase is performed. In the sub phase ‘Audience Classification’, the different types of users are identified and classified into so called audience classes. During the sub phase ‘Audience Class Characterization’ the different audience classes are characterized regarding age, experience level, language, etc. The next phase, the ‘Conceptual Design’, is composed of ‘TaskModeling’ and ‘Navigational Design’. During ‘Task Modeling’, the requirements identified during Audience Classification are elaborated and task models and object chunks are created to model the necessary information and functionality needed to fulfill the requirements. The ‘Navigational Design’ is used to design the overall conceptual structure of the web site and the navigational possibilities for each audience class. The fourth phase, the ‘Implementation Design’ contains three sub phases. The ‘Page Design’ translates the conceptual and navigational design to an actual page structure. The look & feel of the website is defined in the ‘Presentation Design’. The ‘Data Design’ is only needed for data-intensive web sites. In case the data will be maintained in a database, the database schema is constructed. It is also possible that the data will not originate from a database but provided by means of another source (e.g. XML). For pure static web pages, the data design step can be omitted; the actual data will be supplied by the designer during the last step of the method, the actual implementation of the website.3Designing Localized Web Sites with WSDMWe will explain how WSDM is extended to allow modeling localized web sites by indicating how each of the different (sub) phases is adapted and illustrate this with an example. For the example, suppose a company wants a web site to offer their products for sale in the US and Belgium. In addition, the product managers of the company should be able to maintain the product information through the web site.First, we introduce a new concept: locality. A locality describes a particular place, situation, or location. Localities are identified by means of a name and a label. Examples of localities are: the US, Japan, and the Flemish community in Belgium. 3.1Mission Statement SpecificationThe mission statement is the starting point of the design. The mission statement should identify the purpose, the subject and the target audience(s) of the web site. If we want to be able to take localization into account during the design process, the mission statement should also mention the different localities for which the web site needs to be developed.For the example web site, we can formulate the following mission statement:“The web site should allow to increase the sales of the company in the company’s country (Belgium) as well as in the US, by allowing people to search for information about the products and to buy the products online. In addition, the web site should act as an easy user interface for the product managers (located in Flanders) to maintain the product information and to keep track of the supply”From this statement, we can identify:•The purpose: increase the sales and provide support for maintaining the product information and the supply•The subject: products of the company•The target audiences: potential buyers, and the company‘s product managers•The target localities: US, Flanders and Wallonia (Flanders and Wallonia are the two main regions in Belgium each with a different language).3.2Audience ModelingThe target audiences identified in the mission statement should be refined into audience classes. Therefore, for each target type of user, the different functional- and informational requirements are identified. Users with the same information and functional requirements become members of the same audience class. Users with additional requirements form audience subclasses. In this way a hierarchy of audience classes can be constructed. The class Visitor is always the top of the audience class hierarchy, grouping the requirements all visitors have in common. During ‘Audience Class Characterization’ the different audience classes are characterized regarding age, experience level, language, etc.In our extension of WSDM, we make a distinction between requirements that are typical for the audience class and requirements that are typical for a locality. Requirements that are typical for a locality will be specified separately from those typical for an audience class. Therefore, an additional sub phase called ‘Locality Specification’has been introduced. The requirements specified in the Audience Classification should be independent of the localities’. In a similar way a sub phase, called ‘Locality Characterization’, is added to allow specifying the characteristics of the localities. The characteristics given for the different audience classes should be independent of the specific characteristics of the different localities. The order in which the sub phases ‘Audience Classification’ - ‘Audience Class Characterization’ and ‘Locality Specification’ - ‘Locality Characterization’ is performed is not important because the information they allow to specify is independent.To express the relationship between the audience classes and the localities another new sub phase is introduced, the ‘Locality Mapping’. This sub phase can only be performed after finishing the four other sub phases. See figure 2 for an overview of the different sub phases of the extended Audience Modeling phase. We now describe the new sub phases into more detail and illustrate them with the example web site.Locality Specification & CharacterizationThe requirements and characteristics that are typical for a locality are related to the language, culture, habits or regulations of the locality. Some examples of locality requirements are: an address should always include the state; for each price it shouldbe indicated if tax is included or not, and if it is not included the percentage of tax that need to be added should be mentioned; all prices should be expressed in EURO. Locality characteristics will typically deal with issues as language use, reading order, use of color, and use of symbols.We illustrate the Audience Modeling phase with the example web site. There are three audience classes: ‘Visitor’, ‘Buyer’ and ‘ProductManager’, and there are three localities: ‘US’, ‘Flanders’ (‘FL’) and ‘Wallonia’ (‘WA’). Due to space limitations we only give the most important requirements and characteristics.•Audience Class ‘Visitor’: Need to be able to browse through the products for sale in the Visitor’s country and obtain detail descriptions of those products.Characteristics: varying age and varying web expertise•Audience Class ‘Buyer’: Need to be able to buy products that are for sale in the Buyer’s country. Characteristics: varying age but older than 18 and varying web expertise•Audience Class ‘ProductManager’: Need to be able to update information and supply information on the products for sale in both countries. Characteristics: familiar with the system and the Web•Locality ‘US’: Each address should include a state; prices must be in US dollars. Characteristics: English speaking•Locality ‘FL & WA’: Contact address of the company must be clearly mentioned; prices must be in EURO with tax included; it must be allowed to pay by means of bank transfer•Locality ‘FL’: Characteristics: Dutch speaking•Locality ‘WA’: Characteristics: French speakingLocality MappingThe localities need to be linked to the different audience classes. An audience class may span different localities, e.g. in the example ‘Visitor’ and ‘Buyer’ are applicable for all localities. Different audience classes may be needed for a locality, e.g. for the locality ‘Flanders’ we need the audience classes ‘Visitor’, ‘Buyer’ and ‘ProductManager’. ‘ProductManager’ is only needed for the locality ‘Flanders’. Therefore, in the ‘Locality Mapping’, for each locality the audience classes that need to be supported are enumerated. For our example, this results in the following sets. Flanders: {Visitor, Buyer, ProductManager}; Wallonia: {Visitor, Buyer}; and US: {Visitor, Buyer}. Graphically, this can be represented by drawing, for each locality, a box in the audience class hierarchy diagram that includes all the audience classes needed and label this box with the locality’ label (see figure 3).Figure 3 – Audience Modeling3.3Conceptual DesignWe now describe how the sub phases of the ‘Conceptual Design’ phase are influenced by the localization requirement.Task ModelingDuring ‘Task Modeling’, a task model is defined for each requirement of each audience class. This is done using an adapted version of CTT diagrams [20] (CTT+). To create such a task model, each task is decomposed into elementary tasks and temporal relationships between tasks indicate the order in which the tasks need to be performed. For each elementary task an object model, called ‘object chunk’, is created modeling the necessary information and functionality needed to fulfill the requirement of the elementary task. An extended form of ORM (Object Role Model) [14] (ORM+) is used as language for the object chunks.In our extended version of WSDM, there are also requirements for the different localities. These requirements also need to be considered during task modeling. When constructing the task models, we need to inspect the locality requirements to check if additional or different steps are needed when decomposing a task. If a task needs to be completely different for a specific locality (which is rarely the case), a different CTT must be created and labeled with this locality. If only some additional steps are needed, then these steps are labeled with the localities for which they are needed. In our example, it was specified that in Belgium it must be possible to pay by bank transfer, in the US only payment by credit card is possible. This is indicated in the subtask ‘payment method’ by means of the labels ‘FL’ and ‘WA’ (see figure 4).When constructing the object chunks, again, we need to inspect the locality requirements to check if additional information is needed. If this is the case, this information is added to the object chunk and labeled with the locality for which it is needed. If the object chunk is created for an elementary task that is labeled, the object chunk itself is labeled in the same way. Figure 5 shows the object chunk ‘shipping details’; the State of an Address is only needed for the locality ‘US’.In the object chunks, we should also indicate which information is dependent on the locality. E.g., the value and the currency of the price may depend on the locality. Also the available set of products may be dependent on the locality. This is indicated by labeling the object types that are locality dependent (see figure 6). The set of ‘Product’ is different for the localities ‘US’ and ‘FL&WA’: other products might be available in the US compared to Belgium (FL&WA). ‘ProductName’ has the three localities US, FL, WA denoting that names of products differ in the three localities.Figure 4 – CCT ‘Payment Method’ Figure 5 – Object Chunk ‘Shipping Details’Figure 6 – Object Chunk with Locality Dependent Object Types In summary, the extended Task Modeling is as follows (the new steps are in italic): For each requirement of each audience class:1.Define a task for the requirement2.Decompose the task into elementary tasks and add temporal relations between thetasks (using CTT+).•If the decomposition of the task or a sub-task depends on the locality (expressed by means of a locality requirement) then either make different CTTs and label the CTT with the appropriated locality label or label the nodes that are specific for a locality with the appropriated locality label(s)3.For each elementary task:Make an object chunk that models the information and/or functionality required by the task (using ORM+)•If the elementary task is labeled, then also label the object chunk with the same labels•If a locality requires additional information or functionality (formulated by means of locality requirements) label the relationships that models these requirements with the label of this locality•If the content of an object type is dependent on the locality, label it with all the localities in which the audience class is involvedNavigational DesignThe Navigational Design defines the conceptual structure of the web site and models how the members of the different audience classes will be able to navigate through the site and perform the tasks. Because of the audience driven approach of WSDM, a navigation track is created for each audience class. A navigation track can be considered as a sub site containing all and only the information and functionality needed by the members of the associated audience class. If an audience track involvesNext, all audience tracks are combined into the Conceptual Structure by means of structural links. The structure defined between the audience tracks should correspond to the hierarchical structure defined between the audience classes in the audience class hierarchy. How this is done is described in detail in [2] and will not be discussed here. It is not influenced by localization.A navigation track is constructed based on the task models developed for the audience class in the Task Modeling. How this is done exactly is outside the scope of this paper and can be found in [8]. Roughly speaking, we can say that for each task model a task navigation model is created and that this task navigational model is based on the structure of the CTT for this task. A task navigational model is composed of components and links. Because we have labeled the tasks and sub-tasks in the task models where necessary, it is easy to indicate in the task navigational models which components and links are locality dependent.3.4Implementation DesignWe now describe the impact on the sub phases of the ‘Implementation Design’. The ‘Page Design’ translates the conceptual and navigational design into an actual page structure. The actual page structure is obtained by grouping (or splitting) the components and links from the navigational design into pages. Usually the page structure will be independent of the locality, i.e. for each locality the page structure will be the same. However, if some task models are very different for different localities, a different page structure may be needed. In that case, alternative page structures must be defined and each page structure must be labelled with the locality to which it applies.The presentation design defines the general look and feel of the web site and for each page in the page design a template is constructed defining the layout of the page. Clearly, in this sub phase we must take the localization characteristics formulated in the ‘Localization Characterization’ into consideration. Each page belongs to exactly one audience class. Therefore, for each page and for each locality needed for thisaudience class, a different template should be created. How to take the localization characteristics into consideration in the presentation design is not treated here. This is described extensively in the literature about localization (see e.g. [12], [17], [19]).When localization is needed, the easiest way to maintain the data is by means of a database (or CMS). In WSDM, the database schema is obtained (during Data Design) by integrating all object chunks into a single schema [9] and map this schema onto a database schema (see e.g. [14]). Here, we also need to take the labeling of the object chunks into consideration when mapping the object chunks into a database schema. We will do this for a mapping to a relational database. Different situations are possible:1.The complete object chunk is labeled. This means that the information modeledin the chunk is only needed for the label’s locality. This can be reflected by using table (and/or attribute) names that include the locality’s label. E.g. TABLE ProductSupply_FL ( ProductId INTEGER, …. )2.A relationship is labeled. This means that the information modeled by therelationship is only needed for the label’s locality. This can be reflected by using an attribute name that includes the locality’s label. Furthermore, the attribute must allow for null values. E.g. TABLE DeliveryAddress (Street STRING NOT NULL, Nr INTEGER NOT NULL, … , State_US STRING)3.An object type is labeled. This means that the content of the object type isdependent on the locality. We distinguish between entity types and label types: •For an entity type, we can add, per locality, a boolean-type attribute to the primary key to indicate if an entity should be considered in the locality or not.E.g. TABLE Product (productId INTEGER, availability_US BOOLEAN,availability_Fl&WA BOOLEAN, …). The value TRUE for the attribute ‘availability_US’ indicates that the product is available in the US.•For a label type, we have to provide an attribute per locality needed. E.g.TABLE Product (productId, …. , productName_US, productName_FL,productName_WA,…). An alternative solution is to put the locality dependent attributes in a separated table. E.g. TABLE Product_locality (productIdINTEGER, locality STRING, productName STRING, price REAL, …)Please note that the mapping described here is only one possible solution. In the proposed mapping, information for different localities is maintained in the same table. If (nearly) all information is locality dependent, it may be better to define different tables for different localities. E.g. Table Product_US (productId, …, productName, …) and Product_FL (productId, …, productName, …). In this way it is also easier to physically separate the information for the different localities in different databases. 4Conclusions & DiscussionWe have shown how to extend an existing web site method, WSDM, in order to support the localization of web sites. First, a new concept ‘locality’ has been introduced. A locality describes a particular place, situation, or location. Then, the different phases of the method were adapted to allow for the specification of different localities. The mission statement should also answer the question: What are thedifferent target localities? The Audience Modeling was extended with some extra sub phases to allow to describe the specific requirements and characteristics of the different localities and to link the localities to the audience classes. During Conceptual Design, the locality requirements are taken into consideration and integrated in rest of the design. In the Implementation Design, for each locality a different Presentation Design may be needed and when using an underlying database, the Data Design should take into consideration that for some tables and attributes more than one version is needed. Although, the extension has been done for WSDM, we believe that the principle of using localities as a starting point is generally applicable and may therefore be used by other web site design methods.We also discuss some of the limitations of the method proposed. First of all, the method is not appropriate for the ‘one size fits all’ approach to globalization. In that case, it is better to use standard WSDM and to mention in the audience class characterization that the members are from different localities. Then, during Presentation Design, these issues can be taking into consideration.Sometimes, the approach needed is a mixture of localization and the ‘one size fits all’ approach. An example of this is our own university web site. ‘Potential Students’ and ‘Researchers’ are two of the target audiences. For researchers, we want to use the ‘one size fits all’ approach, but for potential students we want to localize. Our local students should be addressed in their local language; foreign students should be addressed in English and should only see information that is applicable to them (e.g. only the English programs). We can achieve this dual approach by defining two localities (‘Locals’ and ‘Foreigners’) for the audience class ‘Potential Students’ and no localities for the audience class ‘Researchers’. In the audience characterization of the ‘Researchers’ we can state that the language must be English and that it is an international audience.Next, the approach that we follow assumes that the differences in type of content and structure between the localized web sites are rather small. If the local web sites need to be substantially different, this method may not work well. In that case too many requirements from the audience classes need to be moved to the locality specification and those requirements may not all express requirements that are “specific” for the locality. Then, it may be better to define different audience classes, e.g. ‘US-Tourists’ and ‘Europe-Tourists’ instead of one audience class ‘Tourists’ and two localities ‘US’ and ‘Europe’.5References1.Barber, W. and Badre, A.: Culturability: The Merging of Culture and Usability. In:Proceedings of the 4th Conference on Human Factors and the Web, at /conf/hfweb/proceedings/barber/ (1998)2.Casteleyn, S. and De Troyer, O.: Structuring Web Sites Using Audience Class Hierarchies.In: Proceedings of DASWIS 2001 (ER) workshop. Yokohama, Japan (2001)3.Ceri S., Fraternali P., and Bongio A: Web Modeling Language (WebML): a modelinglanguage for designing Web sites. In: WWW9 Conference, First ICSE Workshop on Web Engineering, International Conference on Software Engineering (2000)4.CMSWatch, at (accessed date: May 2004)。
清华大学硕士论文博士论文格式及范文详解
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清华大学硕士论文博士论文格 式及范文详解
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Chemical Abstracts(CA)Web Edition的使用
Chemical Abstracts(CA)Web Edition的使用一、首先进入天津理工大学图书馆主页。
如图1所示：图1 天津理工大学图书馆主页二、点击“外文数据库”，出现图2画面。
图2 三、点击“（化学文摘）电子版”，出现图3画面。
图3四、输入用户名（Username）和密码（Password），进入图4界面。
图4五、选择索引组，例如点击图4画圈处，则出现一组标签，见图5。
图5六、从出现的这组标签中选择欲查索引，例如分别选择作者索引（Authors），CA 文摘号索引（CA Abstract Numbers），文献类型（Document Types），期刊名（Journal Titles），语言（Langugers），单位机构名称（Organization Names），专利号索引（Patent Numbers），发布时间（Publication Y ears）等，则分别出现图6~13的界面。
图6图7图8图9图10图11图12图13七、在图4界面中点击其它标签（索引），则分别出现图14~25。
图14图15图16图17图18图19图20图21图22图23图24图25八、根据“Browse for”框下出现的的提示（模板），在框中输入相应的检索词，点击“GO!按钮，即可进行搜索。
以分子式，CA登记号检索为例（参见图23~25），然后在搜索结果中选择你所需要内容左方的选择框，点击“Show Title”（显示文献题目）按钮，则出现图26中的搜寻结果（右侧栏）。
从中选择感兴趣的文献，点击题目，则可出现图27所示的较详细的信息。
图26九、点击图27中相应的“▼”，可获得文摘等较为详细的信息，见图28。
十、选择图28中红色圆圈中包括的按钮，可进行打印，保存等操作。
图27图28。
ABSTRACT Text Joins in an RDBMS for Web Data Integration
Text Joins in an RDBMS for Web Data IntegrationLuis Gravano Panagiotis G.Ipeirotis Nick Koudas Divesh Srivastava Columbia University AT&T Labs–Research {gravano,pirot}@{koudas,divesh}@ABSTRACTThe integration of data produced and collected across autonomous, heterogeneous web services is an increasingly important and chal-lenging problem.Due to the lack of global identifiers,the same entity(e.g.,a product)might have different textual representations across databases.Textual data is also often noisy because of tran-scription errors,incomplete information,and lack of standard for-mats.A fundamental task during data integration is matching of strings that refer to the same entity.In this paper,we adopt the widely used and established cosine similarity metric from the information retrievalfield in order to identify potential string matches across web sources.We then use this similarity metric to characterize this key aspect of data inte-gration as a join between relations on textual attributes,where the similarity of matches exceeds a specified puting an exact answer to the text join can be expensive.For query process-ing efficiency,we propose a sampling-based join approximation strategy for execution in a standard,unmodified relational database management system(RDBMS),since more and more web sites are powered by RDBMSs with a web-based front end.We implement the join inside an RDBMS,using SQL queries,for scalability and robustness reasons.Finally,we present a detailed performance evaluation of an im-plementation of our algorithm within a commercial RDBMS,us-ing real-life data sets.Our experimental results demonstrate the efficiency and accuracy of our techniques.Categories and Subject DescriptorsH.2.5[Database Management]:Heterogeneous Databases;H.2.4 [Database Management]:Systems—Relational databases,Tex-tual databases;H.2.8[Database Management]:Database Appli-cations—Data miningGeneral TermsAlgorithms,Measurement,Performance,ExperimentationKeywordstext indexing,data cleaning,approximate text matching1.INTRODUCTIONThe integration of information from heterogeneous web sources is of central interest for applications such as catalog data integra-tion and warehousing of web data(e.g.,job advertisements and an-nouncements).Such data is typically textual and can be obtained from disparate web sources in a variety of ways,including web Copyright is held by the author/owner(s).WWW2003,May20–24,2003,Budapest,Hungary.ACM1-58113-680-3/03/0005.site crawling and direct access to remote databases via web proto-cols.The integration of such web data exhibits many semantics-and performance-related challenges.Consider a price-comparison web site,backed by a database,that combines product information from different vendor web sites and presents the results under a uniform interface to the user.In such a situation,one cannot assume the existence of global identifiers (i.e.,unique keys)for products across the autonomous vendor web sites.This raises a fundamental problem:different vendors may use different names to describe the same product.For example,a ven-dor might list a hard disk as“Western Digital120Gb7200rpm,”while another might refer to the same disk as“Western Digi r al HDD120Gb”(due to a spelling mistake)or even as“WD120Gb 7200rpm”(using an abbreviation).A simple equality comparison on product names will not properly identify these descriptions as referring to the same entity.This could result in the same product entity from different vendors being treated as separate products, defeating the purpose of the price-comparison web site.To effec-tively address the integration problem,one needs to match multiple textual descriptions,accounting for:•erroneous information(e.g.,typing mistakes)•abbreviated,incomplete or missing information•differences in information“formatting”due to the lack of standard conventions(e.g.,for addresses)or combinations thereof.Any attempt to address the integration problem has to specify a measure that effectively quantifies“closeness”or“similarity”be-tween string attributes.Such a similarity metric can help establish that“Microsoft Windows XP Professional”and“Windows XP Pro”correspond to the same product across the web sites/databases,and that these are different from the“Windows NT”product.Many ap-proaches to data integration use a text matching step,where sim-ilar textual entries are matched together as potential duplicates. Although text matching is an important component of such sys-tems[1,21,23],little emphasis has been paid on the efficiency of this operation.Once a text similarity metric is specified,there is a clear require-ment for algorithms that process the data from the multiple sources to identify all pairs of strings(or sets of strings)that are sufficiently similar to each other.We refer to this operation as a text join.To perform such a text join on data originating at different web sites, we can utilize“web services”to fully download and materialize the data at a local relational database management system(RDBMS). Once this materialization has been performed,problems and incon-sistencies can be handled locally via text join operations.It is de-sirable for scalability and effectiveness to fully utilize the RDBMS capabilities to execute such operations.In this paper,we present techniques for performing text joins ef-ficiently and robustly in an unmodified RDBMS.Our text joins rely on the cosine similarity metric[20],which has been successfully used in the past in the WHIRL system[4]for a similar data inte-gration task.Our contributions include:•A purely-SQL sampling-based strategy to compute approxi-mate text joins;our technique,which is based on the approxi-mate matrix multiplication algorithm in[2],can be fully exe-cuted within standard RDBMSs,with no modification of the underlying query processing engine or index infrastructure.•A thorough experimental evaluation of our algorithms,in-cluding a study of the accuracy and performance of our ap-proach against other applicable strategies.Our experiments use large,real-life data sets.•A discussion of the merits of alternative string similarity met-rics for the definition of text joins.The remainder of this paper is organized as follows.Section2 presents background and notation necessary for the rest of the dis-cussion,and introduces a formal statement of our problem.Sec-tion3presents SQL statements to preprocess relational tables so that we can apply the sampling-based text join algorithm of Sec-tion4.Then,Section5presents the implementation of the text join algorithm in SQL.A preliminary version of Sections3and5ap-pears in[12].Section6reports a detailed experimental evaluation of our techniques in terms of both accuracy and performance,and in comparison with other applicable approaches.Section7discusses the relative merits of alternative string similarity metrics.Section8 reviews related work.Finally,Section9concludes the paper and discusses possible extensions of our work.2.BACKGROUND AND PROBLEMIn this section,wefirst provide notation and background for text joins,which we follow with a formal definition of the problem on which we focus in this paper.We denote withΣ∗the set of all strings over an alphabetΣ.Each string inΣ∗can be decomposed into a collection of atomic“enti-ties”that we generally refer to as tokens.What constitutes a token can be defined in a variety of ways.For example,the tokens of a string could simply be defined as the“words”delimited by special characters that are treated as“separators”(e.g.,‘’).Alternatively, the tokens of a string could correspond to all of its q-grams,which are overlapping substrings of exactly q consecutive characters,for a given q.Our forthcoming discussion treats the term token as generic,as the particular choice of token is orthogonal to the design of our ter,in Section6we experiment with different token definitions,while in Section7we discuss the effect of token choice on the characteristics of the resulting similarity function. Let R1and R2be two relations with the same or different at-tributes and schemas.To simplify our discussion and notation we assume,without loss of generality,that we assess similarity be-tween the entire sets of attributes of R1and R2.Our discussion extends to the case of arbitrary subsets of attributes in a straight-forward way.Given tuples t1∈R1and t2∈R2,we assume that the values of their attributes are drawn fromΣ∗.We adopt the widely used vector-space retrieval model[20]from the information retrievalfield to define the textual similarity between t1and t2. Let D be the(arbitrarily ordered)set of all unique tokens present in all values of attributes of both R1and R2.According to the vector-space retrieval model,we conceptually map each tuple t∈R i to a vector v t∈ |D|.The value of the j-th component v t(j) of v t is a real number that corresponds to the weight of the j-th token of D in v t.Drawing an analogy with information retrieval terminology,D is the set of all terms and v t is a document weight vector.Rather than developing new ways to define the weight vector v t for a tuple t∈R i,we exploit an instance of the well-established tf.idf weighting scheme from the information retrievalfield.(tf.idf stands for“term frequency,inverse document frequency.”)Our choice is further supported by the fact that a variant of this gen-eral weighting scheme has been successfully used for our task by Cohen’s WHIRL system[4].Given a collection of documents C,a simple version of the tf.idf weight for a term w and a document d is defined as tf w log(id f w),where tf w is the number of times that w appears in document d and id f w is|C|w,where n w is the num-ber of documents in the collection C that contain term w.The tf.idf weight for a term w in a document is high if w appears a large num-ber of times in the document and w is a sufficiently“rare”term in the collection(i.e.,if w’s discriminatory power in the collection is potentially high).For example,for a collection of company names, relatively infrequent terms such as“AT&T”or“IBM”will have higher idf weights than more frequent terms such as“Inc.”For our problem,the relation tuples are our“documents,”and the tokens in the textual attribute of the tuples are our“terms.”Consider the j-th token w in D and a tuple t from relation R i. Then tf w is the number of times that w appears in t.Also,id f w is|R i|w,where n w is the total number of tuples in relation R i that contain token w.The tf.idf weight for token w in tuple t∈R i is v t(j)=tf w log(id f w).To simplify the computation of vector similarities,we normalize vector v t to unit length in the Euclidean space after we define it.The resulting weights correspond to the impact of the terms,as defined in[24].Note that the weight vec-tors will tend to be extremely sparse for certain choices of tokens; we shall seek to utilize this sparseness in our proposed techniques.D EFINITION 1.Given tuples t1∈R1and t2∈R2,let v t1and v t2be their corresponding normalized weight vectors and let D be the set of all tokens in R1and R2.The cosine similarity(or just similarity,for brevity)of v t1and v t2is defined as sim(v t1,v t2)=|D|j=1v t1(j)v t2(j).Since vectors are normalized,this measure corresponds to the cosine of the angle between vectors v t1and v t2,and has values be-tween0and1.The intuition behind this scheme is that the magni-tude of a component of a vector expresses the relative“importance”of the corresponding token in the tuple represented by the vector. Intuitively,two vectors are similar if they share many important to-kens.For example,the string“ACME”will be highly similar to “ACME Inc,”since the two strings differ only on the token“Inc,”which appears in many different tuples,and hence has low weight. On the other hand,the strings“IBM Research”and“AT&T Re-search”will have lower similarity as they share only one relatively common term.The following join between relations R1and R2brings together the tuples from these relations that are“sufficiently close”to each other,according to a user-specified similarity thresholdφ:D EFINITION 2.Given two relations R1and R2,together with a similarity threshold0<φ≤1,the text join R1 IφR2returns all pairs of tuples(t1,t2)such that t1∈R1and t2∈R2,and sim(v t1,v t2)≥φ.The text join“correlates”two relations for a given similarity thresh-oldφ.It can be easily modified to correlate arbitrary subsets of attributes of the relations.In this paper,we address the problem of computing the text join of two relations efficiently and within an unmodified RDBMS:P ROBLEM 1.Given two relations R1and R2,together with a similarity threshold0<φ≤1,we want to efficiently compute(an approximation of)the text join R1 IφR2using“vanilla”SQL in an unmodified RDBMS.In the sequel,wefirst describe our methodology for deriving, in a preprocessing step,the vectors corresponding to each tuple of relations R1and R2using relational operations and represen-tations.We then present a sampling-based solution for efficiently computing the text join of the two relations using standard SQL in an RDBMS.3.TUPLE WEIGHT VECTORSIn this section,we describe how we define auxiliary relations to represent tuple weight vectors,which we later use in our purely-SQL text join approximation strategy.As in Section2,assume that we want to compute the text join R1 IφR2of two relations R1and R2.D is the ordered set of all the tokens that appear in R1and R2.We use SQL expressions to create the weight vector associated with each tuple in the two rela-tions.Since–for some choice of tokens–each tuple is expected to contain only a few of the tokens in D,the associated weight vec-tor is sparse.We exploit this sparseness and represent the weight vectors by storing only the tokens with non-zero weight.Specifi-cally,for a choice of tokens(e.g.,words or q-grams),we create the following relations for a relation R i:•RiTokens(tid,token):Each tuple(tid,w)is associated with an occurrence of token w in the R i tuple with id tid.This relation is populated by inserting exactly one tuple(tid,w) for each occurrence of token w in a tuple of R i with tuple id tid.This relation can be implemented in pure SQL and the implementation varies with the choice of tokens.(See[10] for an example on how to create this relation when q-grams are used as tokens.)•RiIDF(token,idf):A tuple(w,id f w)indicates that token w has inverse document frequency id f w(Section2)in relation R i.The SQL statement to populate relation RiIDF is shown in Figure1(a).This statement relies on a“dummy”relation RiSize(size)(Figure1(f))that has just one tuple indicating the number of tuples in R i.•RiTF(tid,token,tf):A tuple(tid,w,tf w)indicates that token w has term frequency tf w(Section2)for R i tuple with tuple id tid.The SQL statement to populate relation RiTF is shown in Figure1(b).•RiLength(tid,len):A tuple(tid,l)indicates that the weight vector associated with R i tuple with tuple id tid has a Eu-clidean norm of l.(This relation is used for normalizing weight vectors.)The SQL statement to populate relation RiLength is shown in Figure1(c).•RiWeights(tid,token,weight):A tuple(tid,w,n)indicates that token w has normalized weight n in R i tuple with tuple id tid.The SQL statement to populate relation RiWeights is shown in Figure1(d).This relation materializes a compact representation of thefinal weight vector for the tuples in R i.•RiSum(token,total):A tuple(w,t)indicates that token w hasa total added weight t in relation R i,as indicated in relationRiWeights.These numbers are used during sampling(see Section4).The SQL statement to populate relation RiSum is shown in Figure1(e).Given two relations R1and R2,we can use the SQL statements in Figure1to generate relations R1Weights and R2Weights with a compact representation of the weight vector for the R1and R2 tuples.Only the non-zero tf.idf weights are stored in these tables. Interestingly,RiWeights and RiSum are the only tables that need to be preserved for the computation of R1 IφR2that we describe in the remainder of the paper:all other tables are just necessary to construct RiWeights and RiSum.The space overhead introduced by these tables is moderate.Since the size of RiSum is bounded by the size of RiWeights,we just analyze the space requirements for RiWeights.Consider the case where q-grams are the tokens of choice.(As we will see,a good value is q=3.)Then each tuple R i.t j of relation R i can contribute up to approximately|R i.t j|q-grams to relation RiWeights,where|R i.t j|is the number of characters in R i.t j.Furthermore,each tuple in RiWeights consists of a tuple id tid,the actual token(i.e.,q-gram in this case),and its associated weight.Then,if C bytes are needed to represent tid and weight, the total size of relation RiWeights will not exceed|R i|j=1(C+q)·|R i.t j|=(C+q)·|R i|j=1|R i.t j|,which is a(small)constant times the size of the original table R i.If words are used as the token of choice,then we have at most|R i.t j|tokens per tuple in R i.Also,to store the token attribute of RiWeights we need no more than one byte for each character in the R i.t j tuples.Therefore,we can bound the size of RiWeights by1+C times the size of R i. Again,in this case the space overhead is linear in the size of the original relation R i.Given the relations R1Weights and R2Weights,a baseline ap-proach[13,18]to compute R1 IφR2is shown in Figure2.This SQL statement performs the text join by computing the similar-ity of each pair of tuples andfiltering out any pair with similar-ity less than the similarity thresholdφ.This approach produces an exact answer to R1 IφR2forφ>0.Unfortunately,as we will see in Section6,finding an exact answer with this approach is prohibitively expensive,which motivates the sampling-based tech-nique that we describe next.4.SAMPLING-BASED TEXT JOINSThe result of R1 IφR2only contains pairs of tuples from R1and R2with similarityφor ually we are interested in high values for thresholdφ,which should result in only a few tuples from R2typically matching each tuple from R1.The baseline ap-proach in Figure2,however,calculates the similarity of all pairs of tuples from R1and R2that share at least one token.As a result, this baseline approach is inefficient:most of the candidate tuple pairs that it considers do not make it to thefinal result of the text join.In this section,we describe a sampling-based technique[2] to execute text joins efficiently,drastically reducing the number of candidate tuple pairs that are considered during query processing. The sampling-based technique relies on the following intuition: R1 IφR2could be computed efficiently if,for each tuple t q of R1, we managed to extract a sample from R2containing mostly tuples suspected to be highly similar to t q.By ignoring the remaining (useless)tuples in R2,we could approximate R1 IφR2efficiently. The key challenge then is how to define a sampling strategy that leads to efficient text join executions while producing an accurate approximation of the exact query results.The discussion of the technique is organized as follows:•Section4.1shows how to sample the tuple vectors of R2to estimate the tuple-pair similarity values.•Section4.2describes an efficient algorithm for computing an approximation of the text join.The sampling algorithm described in this section is an instance of the approximate matrix multiplication algorithm presented in[2], which computes an approximation of the product A=A1·...·A n, where each A i is a numeric matrix.(In our problem,n=2.)The actual matrix multiplication A =A2·...·A n happens during a preprocessing,off-line step.Then,the on-line part of the algorithm works by processing the matrix A1row by row.4.1Token-Weighted SamplingConsider tuple t q∈R1with its associated token weight vector v tq,and each tuple t i∈R2with its associated token weight vector v ti.When t q is clear from the context,to simplify the notation we useσi as shorthand for sim(v tq,v ti).We extract a sample of R2 tuples of size S for t q as follows:•Identify each token j in t q that has non-zero weight v tq(j), 1≤j≤|D|.INSERT INTO RiIDF(token,idf)SELECT T.token,LOG(S.size)-LOG(COUNT(UNIQUE(*)))FROM RiTokens T,RiSize S GROUP BY T.token,S.size INSERT INTO RiTF(tid,token,tf)SELECT T.tid,T.token,COUNT(*)FROM RiTokens TGROUP BY T.tid,T.token (a)Relation with token idf counts(b)Relation with token tf countsINSERT INTO RiLength(tid,len)SELECT T.tid,SQRT(SUM(I.idf*I.idf*T.tf*T.tf))FROM RiIDF I,RiTF T WHERE I.token =T.token GROUP BY T.tidINSERT INTO RiWeights(tid,token,weight)SELECT T.tid,T.token,I.idf*T.tf/L.len FROM RiIDF I,RiTF T,RiLength L WHERE I.token =T.token AND T.tid =L.tid (c)Relation with weight-vector lengths (d)Final relation with normalized tuple weight vectors INSERT INTO RiSum(token,total)SELECT R.token,SUM(R.weight)FROM RiWeights R GROUP BY R.tokenINSERT INTO RiSize(size)SELECT COUNT(*)FROM Ri (e)Relation with total token weights(f)Dummy relation used to create RiIDFFigure 1:Preprocessing SQL statements to create auxiliary relations for relation R i .SELECTr1w.tid AS tid1,r2w.tid AS tid2FROM R1Weights r1w,R2Weights r2w WHERE r1w.token =r2w.token GROUP BY r1w.tid,r2w.tidHAVING SUM(r1w.weight*r2w.weight)≥φFigure 2:Baseline approach for computing the exact value of R 1 IφR 2.•For each such token j ,perform S Bernoulli trials over each t i ∈{t 1,...,t |R 2|},where the probability of picking t i in a trial depends on the weight of token j in tuple t q ∈R 1and in tuple t i ∈R 2.Specifically,this probability is p ij =v t q (j )·v t i (j )T V (t q ),where T V (t q )= |R 2|i =1σi is the sum of the similarity of tuple t q with each tuple t i ∈R 2.In Section 5we show how we can implement the sampling step even if we do not know the value of T V (t q ).Let C i be the number of times that t i appears in the sample of size S .It follows that:T HEOREM 1.The expected value ofC i S·T V (t q )is σi .PThe proof of this theorem follows from an argument similar to that in [2]and from the observation that the mean of the process that generates C i is |D |j =1v t q (j )v t i (j )T V (t q )=σi T V (t q ).Theorem 1establishes that,given a tuple t q ∈R 1,we can obtain a sample of size S of tuples t i such that the frequency C i of tuple t i can be used to approximate σi .We can then report t q ,t i aspart of the answer of R 1 IφR 2for each tuple t i ∈R 2such that its estimated similarity with t q (i.e.,its estimated σi )is φ or larger,where φ =(1− )φis a threshold slightly lower 1than φ.Given R 1,R 2,and a threshold φ,our discussion suggests thefollowing strategy for the evaluation of the R 1 IφR 2text join,in which we process one tuple t q ∈R 1at a time:•Obtain an individual sample of size S from R 2for t q ,using vector v t q to sample tuples of R 2for each token with non-zero weight in v t q .•If C i is the number of times that tuple t i appears in the sam-ple for t q ,then use CiS T V (t q)as an estimate of σi .•Include tuple pair t q ,t i in the result only if C iS T V (t q)≥φ (or equivalently C i ≥S T V (t q )φ ),and filter out the re-maining R 2tuples.1For all practical purposes, is treated as a positive constant less than 1.This strategy guarantees that we can identify all pairs of tuples withsimilarity of at least φ,with a desired probability,as long as we choose an appropriate sample size S .So far,the discussion has focused on obtaining an R 2sample of size S individually for each tuple t q ∈R 1.A naive implementation of this sampling strat-egy would then require a scan of relation R 2for each tuple in R 1,which is clearly unacceptable in terms of performance.In the next section we describe how the sampling can be performed with only one sequential scan of relation R 2.4.2Practical Realization of SamplingAs discussed so far,the sampling strategy requires extracting a separate sample from R 2for each tuple in R 1.This extraction of a potentially large set of independent samples from R 2(i.e.,one per R 1tuple)is of course inefficient,since it would require a large number of scans of the R 2table.In this section,we describe how to adapt the original sampling strategy so that it requires one single sample of R 2,following the “presampling”implementation in [2].We then show how to use this sample to create an approximateanswer for the text join R 1 IφR 2.As we have seen in the previous section,for each tuple t q ∈R 1we should sample a tuple t i from R 2in a way that depends on the v t q (j )·v t i (j )values.Since these values are different for each tuple of R 1,a straightforward implementation of this sampling strategy requires multiple samples of relation R 2.Here we describe an alter-native sampling strategy that requires just one sample of R 2:First,we sample R 2using only the v t i (j )weights from the tuples t i of R 2,to generate a single sample of R 2.Then,we use the single sample differently for each tuple t q of R 1.Intuitively,we “weight”the tuples in the sample according to the weights v t q (j )of the t q tuples of R 1.In particular,for a desired sample size S and a targetsimilarity φ,we realize the sampling-based text join R 1 IφR 2in three steps:1.Sampling:We sample the tuple ids i and the correspond-ing tokens from the vectors v t i for each tuple t i ∈R2.We sample each token j from a vector v t i with probabil-ity v t i (j ).(We define Sum (j )as the total weight of the j -th token in relation R 2,Sum (j )= |R 2|i =1v t i (j ).These weights are kept in relation R2Sum .)We perform S trials,yielding approximately S samples for each token j .We in-sert into R2Sample tuples of the form i,j as many times as there were successful trials for the pair.Alternatively,we can create tuples of the form i,j,c ,where c is the number of successful trials.This results in a compact representation of R2Sample ,which is preferable in practice.2.Weighting:The Sampling step uses only the token weights from R 2for the sampling,ignoring the weights of the tokensSELECT rw.tid,rw.token,rw.weight/rs.total AS PFROM RiWeights rw,RiSum rsWHERE rw.token=rs.tokenFigure3:Creating an auxiliary relation that we sample to cre-ate RiSample(tid,token).in the other relation,R1.The cosine similarity,however,uses the products of the weights from both relations.During the Weighting step we use the token weights in the non-sampled relation to get estimates of the cosine similarity,as follows.For each R2Sample tuple i,j ,with c occurrences in thetable,we compute the value v tq (j)·Sum(j)·c,which isan approximation of v tq (j)·v ti(j)·S.We add this value toa running counter that keeps the estimated similarity of thetwo tuples t q and t i.The Weighting step thus departs from the strategy in[2],for efficiency reasons,in that we do not use sampling during the join processing.3.Thresholding:After the Weighting step,we include the tu-ple pair t q,t i in thefinal result only if its estimated similar-ity is no lower than the user-specified threshold(Section4.1). Such a sampling scheme identifies tuples with similarity of at leastφfrom R2for each tuple in R1.By sampling R2only once, the sample will be correlated.As we verify experimentally in Sec-tion6,this sample correlation has a negligible effect on the quality of the join approximation.As presented,the join-approximation strategy is asymmetric in the sense that it uses tuples from one relation(R1)to weight sam-ples obtained from the other(R2).The text join problem,as de-fined,is symmetric and does not distinguish or impose an ordering on the operands(relations).Hence,the execution of the text join R1 IφR2naturally faces the problem of choosing which relation to sample.For a specific instance of the problem,we can break this asymmetry by executing the approximate join twice.Thus,we first sample from vectors of R2and use R1to weight the samples. Then,we sample from vectors of R1and use R2to weight the sam-ples.Then,we take the union of these as ourfinal result.We refer to this as a symmetric text join.We will evaluate this technique experimentally in Section6.In this section we have described how to approximate the text join R1 IφR2by using weighted sampling.In the next section,we show how this approximate join can be completely implemented ina standard,unmodified RDBMS.5.SAMPLING AND JOINING TUPLE VEC-TORS IN SQLWe now describe our SQL implementation of the sampling-based join algorithm of Section4.2.Section5.1addresses the Sampling step,while Section5.2focuses on the Weighting and Thresholding steps for the asymmetric versions of the join.Finally,Section5.3 discusses the implementation of a symmetric version of the approx-imate join.5.1Implementing the Sampling Step in SQL Given the RiWeights relations,we now show how to implement the Sampling step of the text join approximation strategy(Sec-tion4.2)in SQL.For a desired sample size S and similarity thresh-oldφ,we create the auxiliary relation shown in Figure3.As the SQL statement in thefigure shows,we join the relations RiWeights and RiSum on the token attribute.The P attribute for a tuple inthe result is the probability RiWeights.weightRiSum.total with which we shouldpick this tuple(Section4.2).Conceptually,for each tuple in the output of the query of Figure3we need to perform S trials,pick-ing each time the tuple with probability P.For each successfulINSERT INTO RiSample(tid,token,c)SELECT rw.tid,rw.token,ROUND(S*rw.weight/rs.total,0)AS c FROM RiWeights rw,RiSum rsWHERE rw.token=rs.token ANDROUND(S*rw.weight/rs.total,0)>0Figure4:A deterministic version of the Sampling step,which results in a compact representation of RiSample.SELECT r1w.tid AS tid1,r2s.tid AS tid2FROM R1Weights r1w,R2Sample r2s,R2Sum r2sum,R1V r1vWHERE r1w.token=r2s.token ANDr1w.token=r2sum.token ANDr1w.tid=r1v.tidGROUP BY r1w.tid,r2s.tid,HAVING SUM(r1w.weight*r2sum.total/)≥S∗φ / Figure5:Implementing the Weighting and Thresholding steps in SQL.This query corresponds to the asymmetric execution of the sampling-based text join,where we sample R2and weight the sample using R1.trial,we insert the corresponding tuple tid,token in a relation RiSample(tid,token),preserving duplicates.The S trials can be implemented in various ways.One(expensive)way to do this is as follows:We add“AND P≥RAND()”in the WHERE clause of the Figure3query,so that the execution of this query corresponds to one“trial.”Then,executing this query S times and taking the union of the all results provides the desired answer.A more efficient al-ternative,which is what we implemented,is to open a cursor on the result of the query in Figure3,read one tuple at a time,perform S trials on each tuple,and then write back the result.Finally,a pure-SQL“simulation”of the Sampling step deterministically de-fines that each tuple will result in Round(S·RiWeights.weightRiSum.total)“suc-cesses”after S trials,on average.This deterministic version of the query is shown in Figure42.We have implemented and run exper-iments using the deterministic version,and obtained virtually the same performance as with the cursor-based implementation of sam-pling over the Figure3query.In the rest of the paper–to keep the discussion close to the probabilistic framework–we use the cursor-based approach for the Sampling step.5.2Implementing the Weighting and Thresh-olding Steps in SQLSection4.2described the Weighting and Thresholding steps as two separate steps.In practice,we can combine them into one SQL statement,shown in Figure5.The Weighting step is implemented by the SUM aggregate in the HA VING clause.We weight each tuple from the sample according to R1W eights.weight·R2Sum.totalR1V.T V, which corresponds to v t q(j)·Sum(j)V q(see Section4.2).Then,we can count the number of times that each particular tuple pair ap-pears in the results(see GROUP BY clause).For each group,the result of the SUM is the number of times C i that a specific tuple pair appears in the candidate set.To implement the Thresholding step,we apply the countfilter as a simple comparison in the HA V-ING clause:we check whether the frequency of a tuple pair at least matches the count threshold(i.e.,C i≥ST V(t q)φ ).Thefinal out-put of this SQL operation is a set of tuple id pairs with expected similarity of at leastφ.The SQL statement in Figure5can be fur-ther simplified by completely eliminating the join with the R1V 2Note that this version of RiSample uses the compact representation in which each tid-token pair has just one associated row.。
火星生命探测中一种潜在的生物标志物磷酸盐
0254-6124/2021/41(1)-129-04 Chin. J. Space Sci.空间科学学报ZHAO Yufen. LIU Yan, HUANG Biling, GAO Xinchang. A potential biomarker phosphate for life exploration on Mars (in Chinese). Chin. J. Space Sci, 2021, 41(1): 129-132. D01:10.11728/cjss2021.01.129火星生命探测中一种潜在的生物标志物磷酸盐赵玉芬1>2’3刘艳2黄碧玲1高新昌31(宁波大学新药技术研究院天体化学与空间生命#学森空间科学协同研究中心宁波315211)门大学化学化工学院ffl门361005)3(清华大学化学系北京100084)f商要地外生命探索是国际上广泛关注的深空探测重要目标之一.中国第一个火星探测器天问-号成功发射，开启了对火星表面形貌、生命迹象等进行科学探索的旅程.作为太阳系中与地球最为相似的星球.火星带给人类无 穷的遐想.火星上是否存在生命，未来人类是否可以移民火星，磷作为重要的生命元素，在生命的整个进化过程具t不可替代的作用.磷酸盐可以作为-种潜在的生命标志物，为火星生命探测提供新的思路和线索.关键词火星，磷酸盐，生命探索中图分类号V476A P oten tial Biom arker P h osp h ate for Life E xplorationon M ars Biom arker for LifeZHAO Yufen1’2,3LIU Yan2HUANG Biling1GAO Xinchang3\[Instit,ute of Drug Discovery Technology, Qian Xuesen Collaborative Research Center of Astrochemistryand Space Life Science, Ningbo University, Ningbo315211)2(College of Chemistry and Chemical Engineering, Xiamen University, Xiamen361005)^(Department of Chemistry, Tsinghua University, Beijing100084)Abstract T h e e x p l o r a t i o n f o r e x t r a t e r r e s t r i a l life is o n e o f t h e m o s t i m p o r t a n t t a r g e t s o f d e e ps p a c e e x p l o r a t i o n.T h e s u c c e s s f u l l a u n c h o f C h i n a’s first M a r s p r o b e“T i a n w e n-1”h a s s t a r t e d t h e s c i e n t i f i c e x p l o r a t i o n a b o u t t h e s u r f a c e m o r p h o l o g y a n d life s i g n s o f M a r s.A s t h e m o s t s i m i l a r p l a n e t t o t h e e a r t h i n t h e s o l a r s y s t e m,M a r s b r i n g s h u m a n b e i n g e n d l e s s r e v e r i e.I s t h e r e life o n M a r s?C a n h u m a n b e i n g s i m m i g r a t e t o M a r s i n t h e f u t u r e?P h o s p h o r u s,o n e o f t h e i m p o r t a n t e l e m e n t s f o rlife, p l a y s a n i r r e p l a c e a b l e r o l e i n t h e w h o l e e v o l u t i o n o f life. P h o s p h a t e c a n b e u s e d a s a p o t e n t i a lb i o m a r k e r t o p r o v i d e n e w i d e a s a n dc l u e s f o r life e x p l o r a t i o n o n M a r s.Key words M a r s,P h o s p h a t e,L i f e e x p l o r a t i o n2020-11-06收到原稿E-mail: *****************.cn;**************.cn;*******************.cnDNARNABaseOHBaseOH OH〇引言2020年7月23日12时41分，在中国文昌航天 发射场，长征五号遥四运载火箭将中国第一个火星探 测器天问一号成功发射.随着天问一号直刺苍穹，中 国开启了对火星表面形貌、物质成分、生命迹象等进 行科学探索的旅程.1火星上是否存在生命2019年度诺贝尔物理学奖授予了吉姆■皮布尔 斯（Jam es P eeb les )、米歇尔.麦耶（M ichel M a y o r)和 迪迪埃.奎洛兹(D idier Q u e lo z )三位科学家，以表彰 他们在宇宙物理学和太阳系外行星领域做出的贡献. 迄今在银河系中已发现了 4000多颗系外行星，这些 系外行星的发现改变了地球在宇宙中的地位.这些 行星中有类似地球的岩石行星，地球在宇宙中是否独 一无二，其他行星中是否也存在生命尚未可知.火星上是否有生命存在对于人类未来在火星建 立基地，进行火星移民有着非常重要的意义.据中国 古籍中记载，火星被古人称为“荧惑”，意为不详之星， 而西方则认为火星是罗马战神，通常被称为红色星 球.火星是离太阳第四近的行星，是太阳系四颗类地 行星之一.科学家们发现在太阳系中火星是与地球 最为相似的星球.首先，火星所处轨道接近太阳系的 宜居地带，尽管现在的火星是一个干燥、寒冷、贫瘠 的星球，却可能在以前或者现在正孕育着生命;其次， 火星的地表环境砾石遍布，北部是平原而南部是布 满陨石坑的高地，其地表环境特征与地球极为相似; 第三，目前发现火星上存在少量水，并在火星盖尔陨 石坑内发现富含矿物盐的沉积物；第四，火星大气层130中存在甲烷,构成甲烷的碳、氢元素是地球生命出现 的基础.因此，火星上具备了与生命存在相关的基本 条件.但是，科学家们尚未发现火星上生命存在的标 记物，无法确认火星上是否有生命存在.2磷是遗传物质不可替代的基本元素生命由核酸、蛋白质等物质组成.遗传信息的传 递遵循遗传学中心法则,即遗传物质贮存于脱氧核糖 核酸（D NA ), D NA进行自我复制.将遗传信息传递给子代D N A ,子代D NA 经转录过程将遗传信息传递给核糖核酸（R N A ), R NA在核糖体中经翻译过程形成生命活动的主要承担者——蛋白质，从而将遗传 信息传递给了蛋白质.磷酸二酯键是构成DN A 和R N A的基本骨架（见图1)，磷在核酸中的含量为9%. D N A作为遗传物质，要求其有足够的稳定性，遗传信息才能长期保存并代代传递.而磷在保证D N A稳定性方面起到了关键性作用.1987年W e s t h e i m e r W研究提出：DN A的半衰期为1〇5年；当由硫替代磷酸二酯键中的磷时，其 半衰期为2.7h ;当由硅或砷替代磷时其半衰期则仅 为1〜2 m i n ;而由矾替代磷时极其不稳定，几乎立即 水解.另外,磷酸二酯键及其负电荷性质保护了D N A不被水解,并且使其保留于细胞内而不会穿过磷脂细胞膜.由此可见，遗传物质DN A中的磷是不可替代的.因此,磷元素的存在也应是行星生命探索的必要 条件之一.地球上最古老的蛋白是37亿年前产生的AT P结合蛋白其次是磷转移蛋白.二者在地球上出现氧气之前便已存在.在地球早期海洋中，金属F e , Mn,Chin . J . Space Sci .空间科学学报 2021,41(1)〇|o LV O—=p -—o -io .i-〇l p —〇二〇〇图1D N A 与R N A 结构片段F ig. 1D N A a n d R N A s tru c tu r a l fra g m e ntsC o, Ni含量较高.这些金属可以催化偏三磷酸钠与腺苷反应，将腺苷转化为A T P l '而A T P可以诱导A T P 结合蛋白的产生.生物体的生命活动都必须 有AT P的参与.人体每天可以产生并消耗的AT P量与自身体重相当ATP结合盒转运体（A T P-b i n d i n gc a s s e t t e t r a n s p o r t e r , A B C转运体)是一类A T P结合蛋白，其利用AT P水解释放的能量执行转运细胞内外物质的功能.保证细胞营养物质的充 足与代谢产物的清除.A B C转运体若发生功能障碍,将V 某些疾病的发生密切相关，如阿尔兹海默症、谷固醇血症等【5】.由此可见，磷元素是生命体必不可少的基本元 素.是不可替代的生命元素.在原始生命的形成过程 中具有重要作用.3地球上的磷矿生物沉集中国主要磷矿形成于距今6.5亿至5.5亿年的震 旦纪晚期及寒武纪成矿年代，当时中国长江流域的四 川、陕西、贵州、云南、湖北等省还是一片汪洋大海. 古称扬子古海.气候温和、阳光充足.繁殖有大量的 藻类.扬子古海的磷矿源于海洋中的生物遗骸.经 过1亿年，蓝绿藻在海洋中沉积了超过105亿吨的 胶磷矿（平均每年沉积约100吨)，占中国磷资源总量 的75%.因此.磷酸盐可能是生物的遗骸，应该是很 好的生命标志物.4火星上_的探测基丁-磷元素对生命的重要意义.1957年度的诺 贝尔化学奖获得者L o r d (A l e x a n d e r R ) To d d教授于1981年在日本大阪召开的一次国际会议上做了 一个主旨为：“哪里有生命，哪里就有憐”的精彩报 告，并提出••我猜想，如果宇宙的其他地方有生命存 在的话，那么极有可能是发生在具有充足磷元素的星球上”161.因此.磷酸盐是生命探索的一种潜在标志 物.对地外生命的探索.也需探究地外星球上是否有 憐的存在.2〇13 年 A^u r e G e o s c i e n c e发表了 A d c o c k 等丨的研究成果.他们认为.通过勇气号火星探测车的a 粒子X 射线光谱法测到火星表面有磷.火星上的磷赵玉芬等：火星生命探测中一种潜在的生物标志物磷醆盐不同于地球.地球上的磷矿石主要存在形式是氟磷 灰石C a 5(P 04)3F ，而氣离子对生物体是有毒性的.因 此，炼磷矿过程中需将氟离子去除，不含氟离子的磷 肥才可用尹促进农作物的生长.而火星磷矿石的主 要存在形式是氯磷灰石C a 5(P 04)3C I ,氯离子是与生 命体共存的.是生物体可以利用的基本元素.通过对 矿石的溶解速率与溶解度分别进行比较分析,发现氯 磷灰石释放磷的速率远快于氟磷灰石.火星上水-岩 相互作用时.磷酸盐的释放速率是地球上的45倍.早 期火星上潮湿环境中的磷酸盐浓度可能是早期地球 上的2倍这些都表明火星上的磷酸盐可利用性 高于地球.5火星上的磷是否来自生物氧存在三种稳定同位素：160. 170, 180.三者在 地球圈的含量分别为99.759%, 0.037%和0.204%.磷 酸根P〇i 广中的氧原子16〇对非生物过程的同位素交换有很强的抵抗力，即在没有生物或酶参与下.p〇r与其他物质发生氧同位素交换很慢.实验证据表明，羟基磷灰石来自骨骼和牙齿的磷酸根，其180 含量高，说明同位素可以快速交换.而非生物的磷矿 石和花岗石18〇含量低[8,91.生物几乎参与了磷循环的每个过程.酶是细胞活 性的指示物.酶催化的磷酸盐与水分子之间的氧同位 素交换表明生命的存在.L i a n g 等11U 1和B l a k e等I 11)提出，P 18〇疒的<518〇p 值可能提供了存在地外生命的重要生物特征.如果将火星上的磷矿石带回地球, 那么磷酸根P 18〇〗_的<518O p 值可以作为火星生命 探索的标志物.<518〇p 的定义I 11]为Q x 1_.(1)\ "VSMOWf18〇式中，"R sample 为样品中的比值，尺VSMOW 为维也纳标准平均海水（V i e n n a S t a n d a r d Me a nO c e a n18〇W a t e r, VS M O W)中元石的比值.因此，若能知道宇宙样品中磷酸盐中氧同位素f 8〇P 原始值，将样品的测量值W 该原始值进行 比较，即可获知该样品中的磷是否曾被生物利用，便 可探测生命是否存在.1316火星上的磷可否被生物利用火星上的磷水溶性很好，但是火星上的盐为高氯酸盐，地球上的生物不能直接利用，必须将高氯酸盐 还原分解为氯离子才能实现生物利用.例如，可以利 用耐辐射的特殊微生物将其还原分解为氯离子，并进 一步利用多种功能微生物将火星上的磷、氮及钾解 离出来.氮及钾对地球上的植物仍是必须的.可以设 想将多种极端生物，如地衣、水熊虫、嗜冷菌、嗜压 菌等有机结合成一个小型生态圈，实现对火星土壤及 磷资源的有效利用.虽然上面的描述仅仅是设想，但 是理论上是可行的，值得潜心尝试研究，以期为今后 的“火星移民”提供重要的理论支撑及实验验证.7展望由于火星与地球相似并具备一些适宜的条件，其可能是一个潜在的可居住环境.人类若考虑对外星 球进行移民，火星将是人类的首选星球.但是，火星 对人类来说充满了未知，火星过去是否有生命存在. 磷是否来自火星生物遗骸，地球上的生命能否利用火 星上的磷及其他原位资源来生存，这些重要课题亟待 人类去研究探索，并激发人类为之不懈努力.天问一 号的成功发射，必将为中国的火星探测乃至深空探测 的研究带来前所未有的惊喜和收获.参考文献[1] WESTHEIMER F H. Why nature chose phosphates [J].Science, 1987, 235:1173-1178132[2] KIM K M, QIN T, JIANG Y Y, et al. Protein domainstructure uncovers the origin of aerobic metabolism and the rise of planetary oxygen [J ].2012，20:67-76[3] CHENG C, FAN C, WAN R, et al. Phosphorylation of adenosine with trimetaphosphate under simulated pre- biotic conditions [J]. Orig. Life Evol. Biosph., 2002, 32:219-224[4] TORNROTH-HORSEFIELD S, NEUTZE R. Opening and closing the metabolite gate [J]. Proc. Natl. Acad. Sci., 2008, 105:19565-19566[5] TARLING E J, DE AGUIAR VALLIM T Q, EDWARDS P A. Role of ABC transporters in lipid transport and human disease [J]. Trends Endocrinol. Metab.^ 2013, 24:342-350[6] TODD L. Where There's Life, There's Phosphorus [M ]. Tokyo: Japan Sciencs Society Press, 1981:275-279[7] ADCOCK C T, HAUSRATH E M, FORSTER P M. Readily available phosphate from minerals in early aqueous environments on Mars[J]. Nat. Geosci., 2013. 6:824- 827[8] SCHWARTZ A W. Phosphorus in prebiotic chemistry [J].Philos. Trans. R. Soc. Lond. B. Biol. Sci., 2006, 361: 1743-1749[9] LECUYER C, GRANDJEAN P, SHEPPARD S M F. Oxygen isotope exchange between dissolved phosphate and water at temperatures ( 135°C: inorganic versus biological fractionations [J]. Geochim. Cosmochim. Ac., 1999, 63:855-862[10] LIANG Y, BLAKE R E. Oxygen isotope fractionation between apatite and aqueous-phase phosphate: 20~45°C [J]. Chem. Geol, 2007, 238: 121-133[11] BLAKE R E, ALT J C, MARTINI A M. Oxygen isotoperatios of PO 4: an inorganic indicator of enzymatic activity and P metabolism and a new biomarker in the search for life[J]. Proc. Natl. Acad. Sci., 2001. 98:2148-2153Chin . J . Space Sci .空间科学学报 2021, 41(1)。
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WebsiteFrom Wikipedia, the free encyclopediaA website is a collection of related web pages, images, videos or other digital assets that are addressed relative to a common Uniform Resource Locator (URL), often consisting of only the domain name, or the IP address, and the root path in an Internet Protocol-based network. A web site is hosted on at least one web server, accessible via a network such as the Internet or a private local area network.A web page is a document, typically written in plain text interspersed with formatting instructions of Hypertext Markup Language (HTML, XHTML).A web page may incorporate elements from other websites with suitable markup anchors.Web pages are accessed and transported with the Hypertext Transfer Protocol (HTTP), which may optionally employ encryption (HTTP Secure, HTTPS) to provide security and privacy for the user of the web page content. The user's application, often a web browser, renders the page content according to its HTML markup instructions onto a display terminal.All publicly accessible websites collectively constitute the World Wide Web.the pages of a website can usually be accessed from a simple Uniform Resource Locator called the homepage. The URLs of the pages organize them into a hierarchy, although hyperlinking between them conveys the reader's perceived site structure and guides the reader's navigation of the site.Some websites require a subscription to access some or all of their content. Examples of subscription sites include many business sites, parts of many news sites, academic journal sites, gaming sites, message boards, web-based e-mail, social networking websites, and sites providing real-time stock market data.1.HistoryThe World Wide Web (WWW) was created in 1989 by CERN physicist Tim Berners-Lee.On 30 April 1993, CERN announced that the World Wide Web would be free to use for anyone.Before the introduction of HTML and HTTP, other protocols such as filetransfer protocol and the gopher protocol were used to retrieve individual files from a server. These protocols offer a simple directory structure which the user navigates and chooses files to download. Documents were most often presented as plain text files without formatting or were encoded in word processor formats.2.OverviewOrganized by function, a website may be（1）a personal website（2）a commercial website（3）a government website（4）a non-profit organization websiteIt could be the work of an individual, a business or other organization, and is typically dedicated to some particular topic or purpose. Any website can contain a hyperlink to any other website, so the distinction between individual sites, as perceived by the user, may sometimes be blurred.Websites are written in, or dynamically converted to HTML (Hyper Text Markup Language) and are accessed using a software interface classified as a user agent. Web pages can be viewed or otherwise accessed from a range of computer-based and Internet-enabled devices of various sizes, including desktop computers, laptops, PDAs and cell phones.A website is hosted on a computer system known as a web server, also called an HTTP server, and these terms can also refer to the software that runs on these systems and that retrieves and delivers the web pages in response to requests from the website users. Apache is the most commonly used web server software (according to Netcraft statistics) and Microsoft's Internet Information Server (IIS) is also commonly used.3. Static websiteA static website is one that has web pages stored on the server in the format that is sent to a client web browser. It is primarily coded in Hypertext Markup Language (HTML).Simple forms or marketing examples of websites, such as classic website, a five-page website or a brochure website are often static websites, because they present pre-defined, static information to the user. This may includeinformation about a company and its products and services via text, photos, animations, audio/video and interactive menus and navigation.This type of website usually displays the same information to all visitors. Similar to handing out a printed brochure to customers or clients, a static website will generally provide consistent, standard information for an extended period of time. Although the website owner may make updates periodically, it is a manual process to edit the text, photos and other content and may require basic website design skills and software.In summary, visitors are not able to control what information they receive via a static website, and must instead settle for whatever content the website owner has decided to offer at that time.They are edited using four broad categories of software:(1) Text editors, such as Notepad or TextEdit, where content and HTML markup are manipulated directly within the editor program(2) Wysiwyg offline editors, such as Microsoft FrontPage and Adobe Dreamweaver (previously Macromedia Dreamweaver), with which the site is edited using a GUI interface and the final HTML markup is generated automatically by the editor software(3) Wysiwyg online editors which create media rich online presentation like web pages, widgets, intro, blogs, and other documents.(4) Template-based editors, such as Rapidweaver and iWeb, which allow users to quickly create and upload web pages to a web server without detailed HTML knowledge, as they pick a suitable template from a palette and add pictures and text to it in a desktop publishing fashion without direct manipulation of HTML code.4. Dynamic websiteA dynamic website is one that changes or customizes itself frequently and automatically, based on certain criteria.Dynamic websites can have two types of dynamic activity: Code and Content. Dynamic code is invisible or behind the scenes and dynamic content is visible or fully displayed.(1) Dynamic codeThe first type is a web page with dynamic code. The code is constructed dynamically on the fly using active programming language instead of plain,static HTML.A website with dynamic code refers to its construction or how it is built, and more specifically refers to the code used to create a single web page. A dynamic web page is generated on the fly by piecing together certain blocks of code, procedures or routines. A dynamically-generated web page would call various bits of information from a database and put them together in a pre-defined format to present the reader with a coherent page. It interacts with users in a variety of ways including by reading cookies recognizing users' previous history, session variables, server side variables etc., or by using direct interaction (form elements, mouse overs, etc.). A site can display the current state of a dialogue between users, monitor a changing situation, or provide information in some way personalized to the requirements of the individual user.(2) Dynamic contentThe second type is a website with dynamic content displayed in plain view. Variable content is displayed dynamically on the fly based on certain criteria, usually by retrieving content stored in a database.A website with dynamic content refers to how its messages, text, images and other information are displayed on the web page and more specifically how its content changes at any given moment. The web page content varies based on certain criteria, either pre-defined rules or variable user input. For example, a website with a database of news articles can use a pre-defined rule which tells it to display all news articles for today's date. This type of dynamic website will automatically show the most current news articles on any given date. Another example of dynamic content is when a retail website with a database of media products allows a user to input a search request for the keyword Beatles. In response, the content of the web page will spontaneously change the way it looked before, and will then display a list of Beatles products like CD's, DVD's and books.5. Software systemsThere is a wide range of software systems, such as Java Server Pages (JSP), the PHP and Perl programming languages, Active Server Pages (ASP), Yuma and ColdFusion (CFML) that are available to generate dynamic web systems and dynamic sites. Sites may also include content that is retrievedfrom one or more databases or by using XML-based technologies such as RSS.Static content may also be dynamically generated either periodically, or if certain conditions for regeneration occur (cached) in order to avoid the performance loss of initiating the dynamic engine on a per-user or per-connection basis.Plugins are available to expand the features and abilities of web browsers, which use them to show active content, such as Microsoft Silverlight, Adobe Flash, Adobe Shockwave or applets written in Java. Dynamic HTML also provides for user interactivity and real time element updating within web pages (ie pages don't have to be loaded or reloaded to effect any changes), mainly using the Document Object Model (DOM) and JavaScript, support which is built-in to most modern web browsers.Turning a website into an income source is a common practice for web developers and website owners. There are several methods for creating a website business which fall into two broad categories, as defined below.(1) Content-based siteSome websites derive revenue by selling advertising space on the site (see Contextual advertising).(2) Product- or service-based sitesSome websites derive revenue by offering products or services for sale. In the case of e-commerce websites, the products or services may be purchased at the website itself, by entering credit card or other payment information into a payment form on the site. While most business websites serve as a shop window for existing brick and mortar businesses, it is increasingly the case that some websites are businesses in their own right; that is, the products they offer are only available for purchase on the web.Websites occasionally derive income from a combination of these two practices. For example, a website such as an online auctions website may charge the users of its auction service to list an auction, but also display third-party advertisements on the site, from which it derives further income.网站文章来自维基百科，自由的百科全书网站是一组相关的网页，图片，视频或其他数字资产的集合，是针对相对的一个共同的统一资源定位符（URL），这个定位器往往由域名，或组成的IP地址在以网络为基础的互联网协议上的根路径构成。
抽象类作文英语模版
抽象类作文英语模版英文回答：Introduction.In the realm of object-oriented programming, the concept of abstraction occupies a pivotal position. Abstraction allows us to encapsulate the essential characteristics and behaviors of real-world entities into classes, providing a structured and reusable blueprint for creating objects. Among the various class types, abstract classes hold a unique distinction, serving as a foundation for defining common interfaces and preventing the instantiation of incomplete or invalid objects.Definition of an Abstract Class.An abstract class is a class that cannot be instantiated directly but must be subclassed to create concrete objects. It serves as a template that defines theinterface and shared functionality of a group of related classes. By declaring at least one of its methods as abstract, an abstract class ensures that subclasses must implement the method with specific behavior before they can be used.Characteristics of Abstract Classes.1. Cannot be instantiated: Abstract classes cannot be used to create objects directly. They exist solely to provide a blueprint for subclasses to inherit and expand upon.2. Contain abstract methods: Abstract classes declare one or more abstract methods, which are method signatures without implementations. Subclasses must override these abstract methods with concrete implementations to provide specific functionality.3. May contain concrete methods: In addition to abstract methods, abstract classes can also include concrete methods, which provide default implementationsthat subclasses can inherit or override.4. Act as a contract: Abstract classes define a contract that subclasses must adhere to. By implementing the abstract methods, subclasses guarantee that they possess the required functionality and follow the established interface.Uses of Abstract Classes.Abstract classes offer various advantages, including:1. Enforce method signatures: They ensure that subclasses implement the required methods with the correct signatures, preventing inconsistencies and enforcing a common interface.2. Promote code reusability: By providing a shared interface, abstract classes facilitate code reusability across subclasses, reducing duplication and promoting maintainability.3. Prevent incomplete objects: By prohibiting direct instantiation, abstract classes prevent the creation of invalid or incomplete objects, ensuring that only objects with complete and consistent functionality are created.4. Promote polymorphism: Abstract classes support polymorphism, allowing objects of different subclasses to be treated as objects of the abstract class, fostering flexibility and extensibility in code.Example of an Abstract Class.Consider the following abstract class representing the concept of a shape:java.public abstract class Shape {。
New Characterizations of Input to State Stability
New Characterizations of Input to State StabilityEduardo D.SontagYuan WangAbstract—We present new characterizations of the Input to State Stability property.As a consequence of these re-sults,we show the equivalence between the ISS property and several(apparent)variations proposed in the literature.I.IntroductionThis paper studies stability questions for systems of the general formΣ:˙x=f(x,u),(1) with states x(t)evolving in Euclidean space R n and con-trols u(·)taking values u(t)∈U⊆R m,for some positive integers n and m(in all the main results,U=R m).The questions to be addressed all concern the study of the size of each solution x(t)—its asymptotic behavior as well as maximum value—as a function of the initial condition x(0)and the magnitude of the control u(·).One of the most important issues in the study of control systems is that of understanding the dependence of state trajectories on the magnitude of inputs.This is especially relevant when the inputs in question represent disturbances acting on a system.For linear systems,this leads to the consideration of gains and the operator-theoretic approach, including the formulation of H∞control.For not necessar-ily linear systems,there is no complete agreement as yet regarding what are the most useful formulations of system stability with respect to input perturbations.One candi-date for such a formulation is the property called“input to state stability”(ISS),introduced in[12].Various authors, (see e.g.[4],[5],[6],[10],[17]have subsequently employed this property in studies ranging from robust control and highly nonlinear small-gain theorems to the design of ob-servers and the study of parameterization issues;for expo-sitions see[14]and most especially the textbooks[7],[8]. The ISS property is defined in terms of a decay estimate of solutions,and is known(cf.[15])to be equivalent to the validity of a dissipation inequalitydV(x(t))dt≤σ(|u(t)|)−α(|x(t)|)holding along all possible trajectories(this is reviewed be-low),for an appropriate“energy storage”function V and comparison functionsσ,α.(A dual notion of“output-to-state stability”(OSS)can also be introduced,and leads to the study of nonlinear detectability;see[16].)E.Sontag is with SYCON-Rutgers Center for Systems and Control,Department of Mathematics,Rutgers University,New Brunswick,NJ08903.e-mail:sontag@.This re-search was supported in part by US Air Force Grant F49620-95-1-0101.Y.Wang is with Department of Mathematics,Florida Atlantic Uni-versity,Boca Raton,FL33431.e-mail:ywang@. This research was supported in part by NSF Grants DMS-9457826 and DMS-9403924In some cases,notably in[2],[6],[18],authors have suggested apparent variations of the ISS property,which are more natural when solving particular control problems. The main objective of this paper is to point out that such variations are in fact theoretically equivalent to the orig-inal ISS definition.(This does not in any way diminish the interest of these other authors’contributions;on the contrary,the alternative characterizations are of great in-terest,especially since the actual estimates obtained may be more useful in one form than another.For instance, the“small-gain theorems”given in[6],[2]depend,in their applicability,on having the ISS property expressed in a particular form.This paper merely states that from a the-oretical point of view,the properties are equivalent.For an analogy,the notion of“convergence”in R n is independent of the particular norm used—e.g.all L p norms are equiv-alent—but many problems are more naturally expressed in one norm than another.)One of the main conclusions of this paper is that the ISS property is equivalent to the conjunction of the following two properties:(i)asymptotic stability of the equilibrium x=0of the unforced system(that is,of the system defined by Equation(1)with u≡0)and(ii)every trajectory of(1) asymptotically approaches a ball around the origin whose radius is a function of the supremum norm of the control being applied.We prove this characterization along with many others.Since it is not harder to do so,the results are proved in slightly more generality,for notions relative to an arbitrary compact attractor rather than the equilibrium x=0.A.Basic Definitions and NotationsEuclidean norm in R n or R m is denoted simply as|·|. More generally,we will study notions relative to nonempty subsets A of R n;for such a set A,|ξ|A=d(ξ,A)= inf{d(η,ξ),η∈A}denotes the point-to-set distance from ξ∈R n to A.(So for the special case A={0},|ξ|{0}=|ξ|.) We also let,for eachε>0and each set A:B(A,ε):={ξ||ξ|A<ε},B(A,ε):={ξ||ξ|A≤ε}. Most of the results to be given are new even for A={0}, so the reader may wish to assume this,and interpret|ξ|A simply as the norm ofξ.(We prefer to deal with arbi-trary A because of potential applications to systems with parameters as well as the“practical stability”results given in Section VI.)The map f:R n×R m→R n in(1)is assumed to be locally Lipschitz continuous.By a control or input we mean a measurable and locally essentially bounded function u: I→R m,where I is a subinterval of R which contains theorigin,so that u (t )∈U for almost all t .Given a system with control-value set U ,we often consider the same system but with controls restricted to take values in some subset O ⊆U ;we use M O for the set of all such controls.Given any control u defined on an interval I and any ξ∈R n ,there is a unique maximal solution of the initial value problem ˙x =f (x,u ),x (0)=ξ.This solution is defined on some maximal open subinterval of I ,and it is denoted by x (·,ξ,u ).(For convenience,we allow negative times t in the expression x (t,ξ,u ),even though the interest is in behavior for t ≥0.)A forward complete system is one such that,for each u defined on I =R ≥0,and each ξ,the solution x (t,ξ,u )is defined on the entire interval R ≥0.The L m ∞-norm (possibly infinite)of a control u is denoted by u ∞.That is, u ∞is the smallest number c such that |u (t )|≤c for almost all t ∈I .Whenever the domain I of a control u is not specified,it will be understood that I =R ≥0.A function F :S →R defined on a subset S of R n containing 0is positive definite if F (x )>0for all x ∈S ,x =0,and F (0)=0.It is proper if the preimage F −1(−D,D )is bounded,for each D >0.A function γ:R ≥0→R ≥0is of class N (or an “N function”)if it is continuous and nondecreasing;it is of class N 0(or an “N 0function”)if in addition it satisfies γ(0)=0.A function γ:R ≥0→R ≥0is of class K (or a “K function”)if it is continuous,positive definite,and strictly increasing,and is of class K ∞if it is also unbounded (equivalently,it is proper,or γ(s )→+∞as s →+∞).Finally,recall that β:R ≥0×R ≥0→R ≥0is said to be a function of class KL if for each fixed t ≥0,β(·,t )is of class K and for each fixed s ≥0,β(s,t )decreases to zero as t →∞.(The notations K ,K ∞,and KL are fairly standard;the notations N and N 0are introduced here for convenience.)B.A Catalog of PropertiesWe catalog several properties of control systems which will be compared in this paper.Much of the terminology —except for “ISS”and the names for properties of unforced systems —is not standard,and should be considered ten-tative.A zero-invariant set A for a system Σas in Equation (1)is a subset A ⊆R n with the property that x (t,ξ,0)∈A for all t ≥0and all ξ∈A ,where 0denotes the control which is identically equal to zero on R ≥0.From now on,all definitions are with respect to a given forward-complete system Σas in Equation (1),and a given compact zero-invariant set A for this system.The main definitions follow.We first recall the definition of the (ISS)property:∃γ∈K ,β∈KL st :∀ξ∈R n ∀u (·)∀t ≥0|x (t,ξ,u )|A ≤β(|ξ|A ,t )+γ( u ∞).(ISS)This was the form of the original definition of (ISS)given in [12].It is known that a system is (ISS)if and only if it satisfies a dissipation inequality,that is to say,there exists a smooth V :R n →R ≥0and there are functions αi ∈K ∞,i =1,2,3and σ∈K so thatα1(|ξ|A )≤V (ξ)≤α2(|ξ|A )(2)and∇V (ξ)f (ξ,v )≤σ(|v |)−α3(|ξ|A )(3)for each ξ∈R n and v ∈R m .See [15],[14]for proofs and an exposition,respectively.A very useful modification of this characterization due to [11]is the fact that the (ISS)property is also equivalent to the existence of a smooth V satisfying (2)and Equation (3)replaced by an estimate of the type ∇V (ξ)f (ξ,v )≤−V (ξ)−α3(|ξ|A ).(This can be understood as:“for some positive definite and proper functions y =V (x )and v =W (u )of states and outputs respectively,along all trajectories of the system we have ˙y =−y +v ”.)The main purpose of this paper is to establish further equivalences for the (ISS)property.It will be technically convenient to first introduce a local version of the property (ISS),by requiring only that the estimate hold if the initial state and the controls are small,as follows:∃ρ>0,γ∈K ,β∈KL st :∀|ξ|A ≤ρ,∀ u ∞≤ρ|x (t,ξ,u )|A ≤β(|ξ|A ,t )+γ( u ∞)∀t ≥0.(LISS)Several standard properties of the “unforced”system ob-tained when u ≡0will appear as technical conditions.We review these now.The 0-global attraction property with re-spect to A (0-GATT)holds if every trajectory x (·)of the zero-input system(Σ0):˙x =f (x,0)(4)satisfies lim t →∞|x (t,ξ,0)|A →0;if this is merely requiredof trajectories with initial conditions satisfying |x (0)|A <ρ,for some ρ>0,we have the 0-local attraction property with respect to A (0-LATT).The 0-local stability property with respect to A (0-LS)means that for each ε>0there is a δ>0so that |ξ|A <δimplies that |x (t,ξ,0)|A <εfor all t ≥0.Finally,the 0-asymptotic stability property with respect to A (0-AS)is the conjunction of (0-LATT)and (0-LS),and the 0-global asymptotic stability property with respect to A (0-GAS)is the conjunction of (0-GATT)and (0-LS).Note that (0-GAS)is equivalent to the conjunction of (0-AS)and (0-GATT).It is useful (see e.g.[3],[12],[7])to express these properties in terms of comparison functions:∃β∈KL st :∀ξ∈R n ∀t ≥0|x (t,ξ,0)|A ≤β(|ξ|A ,t ).(0-GAS)and∃ρ>0,β∈KL st :∀|ξ|A <ρ∀t ≥0|x (t,ξ,0)|A ≤β(|ξ|A ,t )(0-AS)respectively.Next we introduce several new concepts.The limit prop-erty with respect to A holds if every trajectory must at some time get to within a distance of A which is a function of the magnitude of the input:∃γ∈N0st:∀ξ∈R n∀u(·)inft≥0|x(t,ξ,u)|A≤γ( u ∞).(LIM) Observe that,if this property holds,then it also holds with someγ∈K∞.However,the caseγ≡0will be of interest, since it corresponds to a notion of attraction for systems in which controls u are viewed as disturbances.The asymptotic gain property with respect to A holds if every trajectory must ultimately stay not far from A, depending on the magnitude of the input:∃γ∈N0st:∀ξ∈R n∀u(·)limt→+∞|x(t,ξ,u)|A≤γ( u ∞).(AG) Again,if the property holds,then it also holds with some γ∈K∞,but the caseγ≡0will be of interest later.The uniform asymptotic gain property with respect to A holds if the limsup in(AG)is attained uniformly with respect to initial states in compacts and all u:∃γ∈N0∀ε>0∀κ>0∃T=T(ε,κ)≥0st:∀|ξ|A≤κsupt≥T|x(t,ξ,u)|A≤γ( u ∞)+ε∀u(·).(UAG)The boundedness property with respect to A holds if bounded initial states and controls produce uniformly bounded trajectories:∃σ1,σ2∈N st:∀ξ∈R n∀u(·)supt≥0|x(t,ξ,u)|A≤max{σ1(|ξ|A),σ2( u ∞)}.(BND)(This is sometimes called the“UBIBS”or“uniform bounded-input bounded-state”property.)The global sta-bility property with respect to A holds if in addition small initial states and controls produce uniformly small trajec-tories:∃σ1,σ2∈N0st:∀ξ∈R n∀u(·)supt≥0|x(t,ξ,u)|A≤max{σ1(|ξ|A),σ2( u ∞)}.(GS)Observe that,if this property holds,then it also holds with bothσi∈K∞.The local stability property with respect to A holds if we merely require a local estimate of this type:∃δ>0,α1,α2∈N0st:∀|ξ|A≤δ∀ u ∞≤δsupt≥0|x(t,ξ,u)|A≤max{α1(|ξ|A),α2( u ∞)}.(LS)If this property holds,then it also holds with bothαi∈K∞,i=1,2Theorem1:Assume given any forward-complete system Σas in Equation(1),with U=R m,and a compact zero-invariant set A for this system.The following properties are equivalent:A.(ISS)B.(LIM)&(0-AS)C.(UAG)D.(LIM)&(0-GAS)E.(AG)&(0-GAS)F.(AG)&(LISS)G.(AG)&(LS)H.(LIM)&(LS)I.(LIM)&(GS)J.(AG)&(GS)This theorem will follow from a several technical facts which are stated in the next section and proved later in the paper.These technical results are of interest in themselves.C.List of Main Technical StepsWe assume given a forward-complete systemΣas in Equation(1),with U=R m,and a compact zero-invariant set A for this system.For ease of reference,wefirst list several obvious implications:(UAG)=⇒(AG).(5)(AG)=⇒(LIM).(6)(ISS)=⇒(0-GAS).(7)(LISS)=⇒(0-AS).(8)(LISS)=⇒(LS).(9) Because(LIM)implies(0-GATT)and(0-GAS)is the same as(0-AS)plus(0-GATT),we have:(LIM)&(0-GAS)⇐⇒(LIM)&(0-AS).(10)It was shown in[15]that(ISS)⇐⇒(UAG)&(LS).(11)It turns out that(LS)is redundant,so(UAG)is in fact equivalent to(ISS):Proposition I.1:(UAG)⇒(LS).This observation generalizes a result which is well-known for systems with no controls(for which see e.g.[1,Theo-rem1.5.28]or[3,Theorem38.1]).It should be noted that the standing hypothesis that A is compact is essential for this implication;in the general case of noncompact sets A, the local stability property with respect to A is not redun-dant.From Proposition I.1and Equation(7),we know then that:(UAG)=⇒(0-GAS).(12) We also prove these results:Lemma I.2:(0-GAS)=⇒(LISS).Lemma I.3:(BND)&(LS)⇐⇒(GS).Lemma I.4:(LIM)&(GS)⇐⇒(AG)&(GS). Lemma I.5:(LIM)⇒(BND).The converse of Lemma I.5is of course false,as illustrated by the autonomous system˙x=0(with n=m=1),which even satisfies(GS)but does not satisfy(LIM).From Lem-mas I.3and I.5,we have that:(LIM)&(LS)⇐⇒(LIM)&(GS).(13)The most interesting technical result will be this: Proposition I.6:(LIM)&(LS)⇒(UAG).We now indicate how the proof of Theorem1follows from all these technical facts.•(A⇐⇒C):by Proposition I.1and Equation(11).•(C⇒E):by(5)and(12).•(E⇒F):by Lemma I.2.•(F⇒G):by Equation(9).•(G⇒H):by Equation(6).•(H⇒I):by Equation(13).•(I⇒J):by Lemma I.4.•(J⇒G):obvious.•(H⇒C):this is Proposition I.6.•(E⇒D):by Equation(6).•(B⇐⇒D):by Equation(10).•(D⇒H):by Lemma I.2and Equation(9).A very particular consequence of the main Theorem is worth focusing upon:A⇐⇒J,i.e.(ISS)is equivalent to having both the global stability property with respect to A and the asymptotic gain property with respect to A. Consider this property:∃γ∈N0st:∀ξ∈R n∀u(·)lim t→+∞|x(t,ξ,u)|A≤γlimt→+∞|u(t)|(14)(the limsup being understood in the“essential”sense,of holding up to a set of measure zero;note also that sinceγis continuous and nondecreasing,the right-hand term equalslim t→+∞γ(|u(t)|)).It is easy to show(see Lemma(II.1))thatthis is equivalent to(AG).The conjunction of(14)and(GS)is the“asymptotic L∞stability property”proposedby Teel and discussed in the survey paper[2](in that paper, A={0});it thus follows that asymptotic L∞stability is precisely the same as(ISS).In[18],Tsinias considered the following property(in thatpaper,A={0}):∃γ∈K st:∀ξ∈R n∀u(·)[|x(t,ξ,u)|A≥γ(|u(t)|)∀t≥0]⇒limt→∞|x(t,ξ,u)|A=0(15)which obviously implies(LIM).The author considered the conjunction of(15)and(LS)(more precisely,the author also assumed a local stability property that implies(LS), namely f(x,u)=Ax+Bu+o(x,u),with A Hurwitz); because of the equivalence A⇐⇒H,this conjunction is also equivalent to(ISS).The outline of the rest of the paper is as follows.In Section II wefirst prove Proposition I.1,Lemmas I.2, I.3,and I.4,and the equivalence between Property(14) and(AG),all of which are elementary.Section III con-tains the proof of the basic technical step needed to prove the main result,as well as a proof of Lemma I.5.After this,Section IV establishes a result showing that uniform global asymptotic stability of systems with disturbances(or equivalently,of an associated differential inclusion)follows from the non-uniform variant of the concept;this would appear to be a rather interesting result in itself,and in any case it is used in Section V to provide the proof of Propo-sition I.6.Finally,in Section VI we make some remarks characterizing so-called“practical”ISS stability in terms of ISS stability with respect to compact sets.II.Some Simple ImplicationsWe start with the proof of Proposition I.1.Proof:We will show the following property,which is equivalent to(LS):∀ε>0∃δ>0st:∀|ξ|A≤δ∀ u ∞≤δsupt≥0|x(t,ξ,u)|A≤ε.(16)Indeed,assume givenε>0.Let T=T(ε/2,1).Pick anyδ1>0so thatγ(δ1)<ε/2.Then:for all|ξ|A≤1and u ∞≤δ1supt≥T|x(t,ξ,u)|A≤ε/2+γ( u ∞)<ε.(17)By continuity(at u≡0and states in A)of solutions with respect to controls and initial conditions,and compactness and zero-invariance of A,there is also someδ2=δ2(ε,T)> 0so that|η|A≤δ2and u ∞≤δ2⇒supt∈[0,T]|x(t,η,u)|A≤ε.Together with(17),this gives the desired property with δ:=min{1,δ1,δ2}.We now prove Lemma I.2.Proof:Wefirst note that the0-global asymptotic sta-bility property with respect to A implies the existence of a smooth function V such thatα1(|ξ|A)≤V(ξ)≤α2(|ξ|A)∀ξ∈R n,for someα1,α2∈K∞,and∇V(ξ)f(ξ,0)≤−α3(|ξ|A)∀ξ∈R n,for someα3∈K∞(this is well-known;see for instance,[9] for one such a converse Lyapunov theorem).Following ex-actly the same steps as in the proof of Lemma3.2in[13], one can show that there exists some functionχ∈K∞such that for allχ(|v|)≤|ξ|A≤1,∇V(ξ)f(ξ,v)≤−α3(|ξ|A)/2.(18) (Here we note that in the proof of Lemma3.2in[13],the function g(s)=1for s∈[0,1].)Using exactly the same arguments used on page441 of[12],one can show that there exist a KL-functionβand a K∞-functionγso that if|x(t,ξ,u)|A≤1for all t∈[0,T) for some T>0,then it holds that|x(t,ξ,u)|A≤max{β(|ξ|A,t),γ( u ∞)}(19) for all t∈[0,T).Letρ=min{κ−1(1/2),γ−1(1/2)},where κ(r)=β(r,0)for r≥0.Note here thatρ≤κ−1(1/2)≤1/2.We now show that the(LISS)property holds with theseβ,γ,andρ.Fix anyξand u with|ξ|A≤ρandu ∞≤ρ.First note that|x(t,ξ,u)|<1for t small enough.Claim:|x(t,ξ,u)|A≤1for all t≥0.Assume the claim is false.Then witht1=inf{t:|x(t,ξ,u)|A≥1},it holds that0<t1<∞.Note then that|x(t,ξ,u)|A<1 for all t∈[0,t1).This then implies that|x(t,ξ,u)|A≤max{β(ρ,0),γ(ρ)}≤1/2∀t∈[0,t1). By continuity,|x(t1,ξ,u)|A<1,contradicting to the defi-nition of t1.This shows that t1=∞,i.e.,|x(t,ξ,u)|A≤1 for all t≥0.Thus the estimate in(19)holds for all t,as desired.Next we prove Lemma I.3:boundedness property with respect to A and local stability property with respect to A implies global stability property with respect to A(the converse is obvious).Proof:Assume that Equations(BND)and(LS)hold, for a given choice ofδ,σ1,σ2,α1,α2.Pick a constant c≥0and two class-K functionsβ1andβ2so that,for each i=1,2,σi(s)≤βi(s)+c for all s≥0.Pick two class-K functionsγ1andγ2so that,for each i=1,2,it holds that:γi(s)≥αi(s)∀0≤s≤δ,γi(s)≥2βi(s)∀s≥0,γi(s)≥2[βi(s)+2c]∀s≥δ.Consider anyξand u.Then Equation(GS)holds.In-deed,if both|ξ|A≤δand u ∞≤δthen this follows from Equation(LS).Assume now that|ξ|A>δ.Thus Equation(BND)implies that,for all t≥0,|x(t,ξ,u)|A≤σ1(|ξ|A)+σ2( u ∞)≤β1(|ξ|A)+c+β2( u ∞)+c≤β1(|ξ|A)+2c+(1/2)γ2( u ∞)≤(1/2)[γ1(|ξ|A)+γ2( u ∞)]≤max{γ1(|ξ|A),γ2( u ∞)}.The case u ∞>δis similar.Lemma I.4says that the limit property with respect to A plus the global stability property with respect to A imply the asymptotic gain property with respect to A;it is shown as follows.Proof:Letσ1,σ2,γ∈Nbe as in(LIM)and(GS). We claim that(AG)holds with:γ(s):=max{(σ1◦γ)(s),σ2(s)}.Pick anyξ,u,and anyε>0.By(LIM),there is some T≥0so that|x(T,ξ,u)|A≤γ( u ∞)+ε.Applying(GS) to the initial state x(T,ξ,u)and the control v(t):=u(t+T) we conclude thatlim t→+∞|x(t,ξ,u)|A≤supt≥T|x(t,ξ,u)|A≤max{σ1(γ( u ∞)+ε),σ2( u ∞)}and takingε→0provides the conclusion.Finally,we show:Lemma II.1:Property(14)is equivalent to(AG).Proof:Sinceγ(limt→+∞|u(t)|)≤γ( u ∞),Property(14)implies(AG),with the sameγ.Conversely,assume that(AG)holds;we next show that Property(14)holds withthe sameγ.Pick anyξ∈R n,control u,andε>0.Letr:=limt→+∞|u(t)|.Let h>0be such thatγ(r+h)−γ(r)<ε.Pick T>0so that|u(t)|≤r+h for almost all t≥T,andconsider the functions z(t):=x(t+T)and v(t):=u(t+T)defined on R≥0.Note that v is a control with v ∞≤r+hand that z(t)=x(t,ζ,v),whereζ=x(T,ξ,u).By thedefinition of the asymptotic gain property with respect toA,applied with initial stateζand control v,limt→+∞|x(t,ξ,u)|A=limt→+∞|z(t,ζ,v)|A≤γ( v ∞)≤γ(r+h)<γ(r)+ε.Lettingε→0gives Property(14).III.Uniform Reachability TimeLet(1)be a forward-complete system.For each subsetO of the input-value space U,each T≥0,and each subsetC⊆R n,we denoteR T O(C):={x(t,ξ,u)|0≤t≤T,u∈M O,ξ∈C}andR O(C):={x(t,ξ,u)|t≥0,u∈M O,ξ∈C}=T≥0R T O(C).In[9,Proposition5.1],it is shown that:Fact III.1:Let(1)be a forward-complete system.Foreach bounded subset O of the input-value space U,eachT≥0,and each bounded subset C⊆R n,R T O(C)isbounded.PGiven afixed system(1)which is forward-complete,apointξ∈R n,a subset S⊆R n,and a control u,one mayconsider the“first crossing time”τ(ξ,S,u):=inf{t≥0|x(t,ξ,u)∈S}with the convention thatτ(ξ,S,u)=+∞if x(t,ξ,u)∈Sfor all t≥0.The following result and its corollary are central.Theystate in essence that,for bounded controls,ifτ(ξ,S,u)isfinite for all u then this quantity is uniformly bounded overu,up to small perturbations ofξand S,and(the Corollary)uniformly on compact sets of initial states as well.(Observethat we are not making the assumption that f is convex oncontrol values and that the set of such values is compactand convex,which would make the result far simpler,bymeans of a routine weak- compactness argument.)Theresult will be mainly applied in the following special case:O is a closed ball in R m,W=R n,and for a given compactset A,C(in the Corollary)is a closed ball of the typeB (A ,2s ),p ∈C ,Ω=B (A ,2s ),and K =B (A ,(3/2)s ).But the general case is not harder to prove,and it is of independent interest.Lemma III.2:Let (1)be a forward-complete system.As-sume given:•an open subset Ωof the state-space R n,•a compact subset K ⊂Ω,•a bounded subset O of the input-value space U ,•a point p ∈R n,and •a neighborhood W of p ,so thatsup u ∈M Oτ(p,Ω,u )=+∞.(20)Then there is some point q ∈W and some v ∈M O suchthatτ(q,K,v )=+∞.(21)Proof:Let p 0=p be as in the hypotheses.Thus for each integer k ≥1we may pick some d k ∈M O so that x (t,p 0,d k )∈Ωfor all 0≤t ≤k .For each j ≥1,we let θj (t )=x (t,p 0,d j ),t ≥0.Consider first {θj (t )}j ≥1as a sequence of functions de-fined on [0,1].From Fact III.1we know that there exists some compact subset S 1of R n such that x (t,p 0,d j )∈S 1for all 0≤t ≤1,for all j ≥1.Let M =sup {|f (ξ,λ)|:ξ∈S 1,λ∈O}.Then d θj (t ) ≤M for all j and almostall 0≤t ≤1.Thus the sequence {θj (t )}j ≥1is uniformly bounded and equicontinuous on [0,1],so by the Arzela-Ascoli Theorem,we may pick a subsequence {σ1(j )}j ≥1of {j }j ≥1with the property that {θσ1(j )(t )}j ≥1converges to a continuous function κ1(t ),uniformly on [0,1].Now we consider {θσ1(j )(t )}j ≥1as a sequence of functions defined on [1,2].Using the same argument as above,one proves that there exists a subsequence {σ2(j )}j ≥1of {σ1(j )}j ≥1such that {θσ2(j )(t )}j ≥1converges uniformly to a func-tion κ2(t )for t ∈[1,2].Since {σ2(j )}is a subsequence of {σ1(j )},it follows that κ2(1)=κ1(1).Repeating the above procedure,one obtains inductively on k ≥1a subse-quence {σk +1(j )}j ≥1of {σk (j )}j ≥1such that the sequence {θσk +1(j )(t )}j ≥1converges uniformly to a continuous func-tion κk +1on [k,k +1].Clearly,κk (k )=κk +1(k )for all k ≥1.Let κbe the continuous function defined by κ(t )=κk (t )for t ∈[k −1,k )for each k ≥1.Then on each interval [k −1,k ],κ(t )is the uniform limit of {θσk (j )(t )}.Since the complement of Ωis closed and the θj ’s have images there,it is clear that κremains outside Ω,and hence outside K .If κwould be a trajectory of the system corresponding to some control v ,the result would be proved (with q =p 0).The difficulty lies,of course,in the fact that there is no reason for κto be a trajectory.However,κcan be well approximated by trajectories,and the rest of the proof consists of carrying out such an approximation.Some more notations are needed.For each control d with values in O ,we will denote by ∆d the control given by ∆d (t )=d (t +1)for each t in the domain of d (so,for instance,the domain of ∆d is [−1,+∞)if the domain of d was R ≥0).We will also consider iterates of the ∆operator,∆k d ,corresponding to a shift by k .Since K is compact and Ωis open,we may pick an r >0such thatB (K,r )⊆Ω.We pick an r 0smaller than r/2and so that the closed ball of radius r 0around p 0is included in the neighborhood W in which q must be found.Finally,let p k =κ(k )for each k ≥1.Observe that both p 0and p 1are in S 1by construction.Next,for each j ≥1,we wish to study the trajectory x (−t,p 1,∆d σ1(j ))for t ∈[0,1].This may be a priori un-defined for all such t .However,since S 1is compact,wemay pick another compact set S1containing B (S 1,r )in its interior,and we may also pick a function f:R n ×R m →R n which is equal to f for all (x,u )∈ S1×O and has compact support;now the system ˙x = f(x,u )is complete,meaning that solutions exist for all t ∈(−∞,∞).We use x (t,ξ,u )to denote solutions of this new system.Observe that foreach trajectory x (t,ξ,u )which remains in S1,x (t,ξ,u )is also defined and coincides with x (t,ξ,u ).In particu-lar, x (−t,θσ1(j )(1),∆d σ1(j ))=x (−t,θσ1(j )(1),∆d σ1(j )),for each j ,since these both equal x (1−t,p 0,d σ1(j )),for each t ∈[0,1].The set of states reached from S 1,using the modified system,in negative times t ∈[−1,0],is in-cluded in some compact set (because the modified system is complete,and again appealing to Fact III.1).Thus,by Gronwall’s estimate,there is some L ≥0so that,for all j ≥1and all t ∈[0,1],x (−t,p 1,∆d σ1(j ))−x (−t,θσ1(j )(1),∆d σ1(j ))≤L p 1−θσ1(j )(1) ,(no “∼”needed in the second solution,since it is also a solution of the original system).Since θσ1(j )(1)→p 1,it follows that there exists some j 1such that for all j ≥j 1,x (−t,p 1,∆d σ1(j ))−x (−t,θσ1(j )(1),∆d σ1(j )) <r 02(22)for all t ∈[0,1].Note that this means in particular thatx (−t,p 1,∆d σ1(j ))∈B (S 1,r/4)⊆ S1for all such t ,for all j ≥j 1,so “∼”can be dropped in Equation (22)for all j ≥j 1.Now let 0<r 1<r 0be such thatx (−t,p,∆d σ1(j 1))−x (−t,p 1,∆d σ1(j 1)) <r 02(23)for all t ∈[0,1],for all p ∈B (p 1,r 1).As this impliesin particular that x (−t,p,∆d σ1(j 1))∈B (S 1,r/2)⊆ S1,again tildes can be bining (22)and (23),it follows that for each p ∈B (p 1,r 1),x (−t,p,∆d σ1(j 1))is defined for all t ∈[0,1]andx (−t,p,∆d σ1(j 1))−x (−t,θσ1(j 1)(1),∆d σ1(j 1)) <r 0(24)for all t ∈[0,1].Let w 1(t )=d σ1(j 1)(t ).Then (24)implies that for each p ∈B (p 1,r 1)it holds that x (−1,p 1,∆w 1)∈B (p 0,r 0),and,since x (−t,θσ1(j 1),∆d σ1(j 1))∈Ωfor all t ∈[0,1],x (−t,p,∆w 1)∈B (K,r/2)∀t ∈[0,1].In what follows we will prove,by induction,that for each i ≥1,there exist 0<r i <r i −1and w i of the form。
(完整版)家教信息平台的设计与实现毕业设计
家教信息平台的设计与实现Online Tutoring Platform Design Of AndImplementation目录内容摘要．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．I．．．．．．．．．Abstract．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．II ．．．．．．．．．．．．．．．．1 导言．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．1．．．．．．．2 系统分析．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2．．．．．．．2.1现状分析．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2．．．．．．2.2可行性分析．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．3．．．．3 系统需求分 4析．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．3.1系统的综合需求．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．4 3.2数据流程分析．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．5 3.3数据字典．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．6 4 软件设计．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．8 4.1总体设计．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．8 4.2详细设计．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．1 05 系统实2施．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．0 5.1 系统编程．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2 05.2 软硬件实现．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2 25.3系统测试．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2 36 总结．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2 6参考文献．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2 7致谢．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．．2 8内容摘要教育信息化是我国信息化的重要组成部分，家教网建设是我国教育信息化的基础。
基于WEB的公交查询系统设计
┊┊┊┊┊┊┊┊┊┊┊┊┊装┊┊┊┊┊订┊┊┊┊┊线┊┊┊┊┊┊┊┊┊┊┊┊┊基于WEB的公交查询系统设计毕业论文摘要为推进地名信息服务工作，开发的“南京市公交查询系统”软件（以下简称“本软件”），用于提供快速的、简便的城市公交站点、线路查询服务，也可以用于制作地铁、公路等交通信息查询系统。
本软件的核心是对选择好的车次进行路线的查询，或者输入所要查询的车站名，点击“查询”按钮，查询所有含有该站的车次及相应的停靠站。
此处既可以“精确查询”也可以是“模糊查询”，“模糊查询”主要方便那些对站名不是很清楚，但知道其中的一部分的乘客，系统可以帮助他们快速的查出。
本软件用JSP作为开发语言并结合SQL数据库，只要将公交站点等信息输入数据库，就可以很快构建成一个简单、实用的本地公交查询系统。
关键词： JSP、 SQL数据库、车次管理，公交查询系统。
┊┊┊┊┊┊┊┊┊┊┊┊┊装┊┊┊┊┊订┊┊┊┊┊线┊┊┊┊┊┊┊┊┊┊┊┊┊English AbstractTo promote the geographical names information service work, I developed the "Nanjing Public Transport Query System" (hereinafter referred to as the "Software"), used to provide fast, convenient city bus stops, in line inquiry service can also be used to make subway , highway and other transportation information inquiry system. The core of this software is a good choice of trips to routes of inquiry, or enter the station name to the query, click the "Search" button, check all the trips with the station and the corresponding stops. Here both "Exact" can be "fuzzy query," "Fuzzy query," the main benefit of those on the station were not clear, but that one part of the passenger, the system can help them quickly find out. This software is developed using JSP as a language and SQL database integration, as long as the bus stops and other information entered into the database, you can quickly build into a simple, practical system of local public transport inquiry。
Java技术与Web设计外文文献翻译
文献信息文献标题：A Comparative Study of Top Web Design Models that are using Java Technologies（使用Java技术的顶级Web设计模型的比较研究）文献作者及出处：Sathyaseelan B, Cordova R S. A Comparative Study of Top Web Design Models that are using Java Technologies[J].International Journal of Innovations and Advancement in Computer Science. 2016,5(5):41-44字数统计：英文1614单词，8806字符；中文2779汉字外文文献A Comparative Study of Top Web Design Models that areusing Java TechnologiesAbstract In today's professional workplace, enterprise applications are complex, adaptable, distributed, part-based, and mission-critical. They might be conveyed as an assortment of platforms across corporate systems, intranets, or the Internet. They are information driven, easy to use, and should meet stringent prerequisites for security, organization, and support. To put it plainly, they are highly complex systems. Various enterprise applications have been in the market today and there are numerous methods available for developing enterprise applications.This paper discusses the major differences between two competing design models for developing Web applications using Java technologies which are Struts and Java Server Faces (JSF). Struts is an open source java framework and is an action-based framework.On the other hand, JSF is a new technology that supports ready-to-use components for rapid Web application development. These two technologies will be analyzed depending on the factors such as the controller flexibility or event handling, navigation, page development, integration and extensibility.Keywords: web design models, Struts, Java Server Faces and Java technologiesI.IntroductionA STRUT is a web application system that has been popular over Java Servlets. It envelops the ordinarily utilized MVC design pattern. A STRUT automates normal tasks, liberating up to explore more of the Action- to-result pages mapping utilizing SML-based configurations. The system makes the progress level subtle elements of the usage of Web-based applications and gives a bit of structural software. JavaServer Faces (JSF) abridges the development of web application user interfaces, usually by defining a user interface component model tied to a well-defined request processing lifecycle. It characterizes a set of UI components— basically, a balanced mapping of the html structure component set along with some extras—that could a be utilized as an application programming interface for extending and modifying standard components or developing new components.II.Key DifferencesThe figures below illustrate the key differences between a STRUTS and JSF request-response scenario. This is necessary in order to fully understand the architecture of each web design models.Figure 1. Struts Request-Response ScenarioIn figure 1, the client sends request to server through a web-browser for aparticular resource. This request is forwarded to the server through a Controller. A servlet goes about as a controller, accepting all requests from the customer. The servlet hands off the request to a different business layer for handling. When processing is finished, the servlet advances or diverts the request to a JSP, which is exclusively in charge of creating the following view for the client. There is no business logic within the JSP.Figure 2. JSF Request-Response ScenarioIn figure 2, the server handles requests from the client, starting from an initial request to a postback request. When a client makes an underlying request for a page, it is asking for the page for the first time. When a client executes a postback, it presents the structure contained on a page that was beforehand stacked into the program as a result of executing an underlying request. When the life cycle handles an underlying request, it just executes the restore view and renders response phases in light of the fact that there is no client information or activities to prepare. Then again, when the life cycle handles a postback, it executes the majority of the phases.III.MethodologyIn this study we will compare JSF and Struts. We will create two modelsemploying the JSF and Struts request-response scenario. JSF is a “component” framework while Struts is an “action” framework.This paper compares and evaluates the ease of application development and the performance of two design models (Struts and JSF) by presenting the two design models and critically analyzes each technology.IV.ChallengesWeb Development ChallengesThe challenges in Web development are much more than the application development. The main challenge is in hosting the application over a webserver and then handling the network issues while accessing those via network. The performances of the web-based application are based on the network availability and its speed. These are not challenges for desktop applications as they run on the local host and the resources are based only on that system.Challenges in STRUTSDespite the fact that Struts accompanies a rundown of remarkable elements however it is important not to neglect the few negative applications about Struts and would require loads of improvements.Greater expectation to learn and adapt - To utilize MVC with STRUTS, you must be alright with the standard JSP, Servlet APIs and a substantial and elaborate system.Poor documentation - Compared to the standard servlet and JSP APIs, STRUTS has less online resources, and some first-time clients locate the online Apache documentation confusing and inadequately sorted out.Less straightforward -With STRUTS applications, there is significantly more going ahead in the background than with ordinary Java-based Web applications which makes it hard to comprehend the system.Challenges in JSFJSF is a very powerful innovation for creating Java-based web applications. It is intended to improve the development of client interfaces for Java Enterprise Edition (Java EE) applications via programmed handling of low level HTTP requests andclient information processing. JSF utilizes a part based model for web advancement. Utilizing the visual JSF web application instrument offered by NetBeans () integrated development environment (IDE), segments can be actually "painted" on a virtual JSF page by relocating them from a palette of JSF segment library.Event handlers can then be characterized for every part the same route concerning creating standalone Java graphical user interface (GUI) application. Route standards are determined for every page from a central XML arrangement document (faces-config.xml). Client activities on a web interface will trigger an event which in return figures out which page is to be shown along the rules specified on that page.This methodology encourages measured and adaptable design, making web application development much more straightforward and faster.The route decides determined for that page. This methodology encourages measured and adaptable configuration, making web application advancement much less difficult and speedier.V.AnalysisIn this study, STRUTS and JSF were analyzed in terms of their controller flexibility or event handling, navigation, page development, integration and extensibility. This is important to compare each web design model and present the advantages and disadvantages of each technology.Controller Flexibility/Event HandlingController is considered as the heart of the Struts. It uses the Front Controller Pattern and Command Pattern. A single servlet takes a request, and then translates HTTP parameters into a Java ActionForm, and passes the ActionForm into a Struts Action class. The URI designates which Action class need to be executed. The Struts framework has one single event handler for the HTTP request. When the request is met by the event handler, the Action returns the result back to the front controller. This in turn will be used to choose the navigation destination.Controller Pattern is been used by the JSF. Each face request goes through asingle servlet, whose responsibility is to get a faces page with components. It will then trigger the events for each component and render the components using a render toolkit. It is also possible to bound the components to the data from its model. JSF have several event handlers on a page while Struts is geared to one event per request. In addition, with Struts, the ActionForms have to extend Struts classes, creating another layer of complicated coding while JSF gives the ability to peg into the model without breaking layering.NavigationNavigation is a key feature of both Struts and JSF. There are 2 types of navigation: static navigation - when the output of a particular page is known so that it is very easy to predict its expected output; and dynamic navigation – when the output is unpredictable and some business logic determines the output of that particular page. Both frameworks have a declarative navigation model and define navigation using rules inside their XML configuration file. Both JSF and Struts support both types of navigation.Page DevelopmentJSF was built with component models which support RAD development, while struts don’t have that support. The Struts framework provides custom libraries to peg into Action Forms and offers some utilities. JSF provides the ability to build components from a variety of view technologies and does it in such a way to be component based.IntegrationStruts are model neutral, so there is no special hook into a model layer. The page data has to be moved from the Action Form to another Model input format which requires heavy complicated coding. The ActionForm class, provides an extra layer of tedious coding and state transition.JSF, on the other hand, encapsulates and hides the details of any data inside the component tree. Data grids and similar rich components can be bound to any Java class. This allows the combination of JSF and SDO which are powerful RAD development.ExtensibilityBoth Struts and JSF offers the extension of framework to meet expanding requirements. Struts have RequestProcessor class that has various callback methods and is the major grip for struts throughout the life-cycle of a request. JSF also have similar functionality to outspread special life-cycle interfaces. Apart from this, JSF totally decouples the render phase from the controller. This considered as on of the main important feature of JSF that Struts does not incorporate.VI.ConclusionJSF is a much more flexible framework while struts is a sturdy framework and works well. JSF is a strong framework because of its flexible controller and navigation. Furthermore, JSF is built with integration and extensibility. From a strategic direction and programming model, JSF can be a target of new applications.中文译文使用Java技术的顶级Web设计模型的比较研究摘要在当今的专业工作环境中，企业应用程序是复杂的、可适应的、分布式的、基于部件的和任务关键的。
Text 2 Electronic Threats of a Computer
Text 2 Electronic Threats of a ComputerAbstract:英文:Any computer which connected to the Internet has raised a dauting range of electronic threats.this paper attempts to point out some main electronic threats like “viruses and worms”,zombies and trojans” ,and so on. On the basis of electric threats of a computer,we divided it into several parts to clarify.From these threats,we can find that the biggest single threat to any computer is the humble software bug;seeming harmless errors can also be exploited to force entry into a computer and also provide the weak spots that allow computer worms and virus to multiply;and the attacker can convert the bug into an “exploit”that can be used to cause major damage.While the Internet has transformed global communication beyond recognition,we should be ready to the tasks of preventing these threats.中文：任何一台计算机连接到互联网，都会引发了一系列的电子威胁。
(完整word版)如何写英文Abstract
How to Write an Abstract一、什么是摘要Abstract?an abstract comprises one paragraph which describes the main content of a paper and appears at the very beginning of the paper.摘要是叙述文章主要内容的一个段落，并且位于文章的开头部分.摘要是以梗概形式呈现的一篇文章要点的总结,它强调了一篇文章所包含的重要的信息.它也可以帮助读者快速的了解到是否这篇文章是他们感兴趣的，是否他们需要来阅读整篇文章。
而且，国家或国际出版社的编辑通常通过浏览投稿文章的摘要来决定是否投稿人的文章是可以被录用的。
因此，对于学者和研究人员来说，写一份好的摘要至关重要。
二、写作Abstract的目的对于科技论文的摘要，Abstract的目的有以下几点：1.Introduce journal articles.rm readers about article`s content.3.Help readers decide whether or not to read article.4.Overview conference programs,abstract collections，and book chapters.三、学习写作Abstract的必要性1。
Helps you present complex information in a clear,concise manner。
2.Helps you read abstracts more effectively.3。
Helps you conduct research.4.Helps you write abstracts for future publications。
5.Helps you condense report information into a short format for database searches。
英文abstract范文
英文abstract范文In the realm of academic writing, an abstract is a concise summary of a research paper or study, presenting the key points and findings without delving into the details. It serves as a window into the content, enticing readers to explore further.The abstract should begin with a brief introduction of the research topic, highlighting its significance and relevance in the field. It is crucial to establish the context and purpose of the study to pique the interest of the reader.Next, the methodology section of the abstract should succinctly describe the approach taken in the research. Whether it's a qualitative study, quantitative analysis, or a mix of both, the abstract should outline the techniques used to gather and analyze data.Following the methodology, the abstract must present the main findings of the research. These should be presented in a clear and concise manner, avoiding any jargon that could confuse the reader. The goal is to convey the essence of the results without overwhelming the reader with data.The conclusion of the abstract should succinctly summarize the implications of the findings. It should highlight how the research contributes to the existing bodyof knowledge and what the potential impact might be on the field.Lastly, the abstract may include a few keywords that encapsulate the core themes of the research. These keywords help in the indexing of the paper and facilitate easy retrieval by interested readers.Remember, the abstract is often the first and sometimes the only part of a paper that readers will see. It is, therefore, the key to capturing their attention and convincing them of the value of your research.。
网络谣言研究比较分析——以图书情报学科与新闻传播学科为例
第1期(总279期)No. 1(Serial No. 279)2021 年 1 月Jan. 2021情报探索Information Research 网络谣言研究比较分析**收稿日期:2020-07-20*本文系国家社会科学基金重点项目“面向智慧服务的多源多维公共文化数据治理及政策保障研究”（项目编号：19ATQ001） 成果之一。
作者简介:齐美妮（1996—）,女,2019级硕士研究生，研究方向为信息传播;钱鹏（1972—）,男，博士，研究馆员，硕士生导师，研 究方向为信息资源管理。
——以图书情报学科与新闻传播学科为例 齐美妮1钱鹏2(1.东南大学经济管理学院江苏南京211189)(2.东南大学图书馆江苏南京211189)摘要：[目的/意义]探究图书情报学科与新闻传播学科在网络谣言研究中的发展现状与研究热点，比较分析两门学科在 网络谣言研究中的共性与差异性。
[方法/过程]采用文献计量与内容分析方法,对发表于图书情报学科与新闻传播学科的 CSSCI 与北大中文核心期刊中关于网络谣言研究的文献，从发文机构、核心作者以及关键词这三个方面进行了比较分析。
[结果/结论]两门学科关于网络谣言的研究有着网络谣言研究方法的差异性、谣言传播研究的不同方向、网络谣言研究对象的相 似性，同时也发现两门学科在论文发表方面存在着融合研究。
建议加强本学科的相互合作、加强两门学科间的融合研究、两门 学科间互相吸纳研究方法。
关键词:谣言;网络谣言;比较分析;文献计量中图分类号：G203 文献标识码:A doi ：10.3969/j .issn.1005-8095.2021.01.013Comparative Analysis of Internet Rumor Research : Case Study of Library & Information Science and Journalism & CommunicationQi Meini 1 Qian Peng 2( 1. School of Economics and Management ， Southeast University ， Nanjing Jiangsu 211189)( 2. Southeast University Library ， Nanjing Jiangsu 211189)Abstract : [ Purpose/significance ]The paper is to explore the development status and research hotspots of library and informationscience as well as journalism and communication science in the studies of internet rumor ， and compares the similarities and differencesbetween the two disciplines in the studies of internet rumor. [ Method/process ] By using the methods of bibliometrics and content analy- sis ， the paper makes a comparative analysis of the literatures on internet rumor published in journals of library and information science as well as journalism and communication included in CSSCI and Chinese Core Journals of Peking University from the three aspects of pub lishing organization , core authors and keywords. [ Result,/conclusion ] The researches of the two disciplines on internet rumor have differences in research methods ， different directions of rumor propagation research ， and similarities in the research objects. At the sametime ， it is also found that there is a fusion research on paper publishing between the two disciplines. It is recommended to strengthen the mutual cooperation of the disciplines ， strengthen the fusion research between the two disciplines ， and absorb the research methods be tween the two disciplines.Keywords : rumor ； internet rumor ； comparative analysis ； bibliometrics0引言“没有事实根据的消息”。
英语学术论文中Abstract写作总结！
英语学术论文中Abstract写作总结！作者简介：晨星，男，湖北武汉人，副高职称，博士，高级程序员，IAMG（国际数学地质协会）会员。
目前研究方向为地理信息数据挖掘与知识发现、地学信息三维可视化。
当年我在攒第一篇 SCI 论文的时候，是先写的 Abstract，然后再写的正文。
在我将自以为很用心撰写的论文交给副导师审阅后，就被他立即请到办公室去畅谈了一番心声：「不是 Abstract 的位置放在正文前面，就将Abstract 放在论文前面写的！你没看到你的Abstract 有问题吗？和全文对得起来吗？」那么，好的 Abstract 应该是怎么样呢？在 SCI 论文写作中，Abstract 是很重要的一部分。
所谓 Abstract，就是对所写论文主要内容的精炼概括。
Abstract 是美国人的说法，英国的科技期刊喜欢称之为Summary。
在英文中，有资料是这么对其定义的：Abstract is a sketchy summary of the main points of an argument or theory in a paper.一篇英文科技论文特别是SCI 论文，是否能被编辑发出审稿乃至最后被录用，在很大程度上取决于论文的 Abstract 质量。
事实上，摘要由于其独特的功能，通常被认为是科技论文最重要的组成部分之一（有的文献认为是最重要的，没有之一）。
据我了解，英文科技论文的 Abstract 主要功能有以下三点：1. 提纲携领，方便检索Abstract 作为对英文科技论文的最主要内容进行不加注释和评论的简短陈述那一部分。
与整篇论文的其他部分相比，它起着至关重要的概括要点的作用。
从本质上讲，Abstract 是把论文的内容浓缩成一篇简短、简洁、概括性强的一个段落的文本。
Abstract 被看成是论文的缩略版或缩写，概括了主体的内容，例如背景、主题/问题、方法、结论和意义。
精心撰写的Abstract是检索科技文献的有用工具（反之，如果 Abstract 写得不佳，对检索该篇论文是个误导），它能使读者能够更快、更方便地识别论文的基本内容，确定论文与其个人兴趣相关性，并最终决定是否需要继续阅读整篇论文。
基于web停车场管理系统的设计与实现
基于Web停车场管理系统的设计与实现摘要：采用MyEclipse2014平台和Java语言开发了一款基于B/S结构的Web停车场管理系统。
系统前台采用JSP技术动态生成Web网页，后台采用MVC开发模式降低代码的耦合度、提高代码的重用性。
主要包含了系统信息管理、车位信息管理、IC卡信息管理、临时车辆停车管理和系统功能操作等模块。
系统信息管理模块主要是对角色以及用户进行管理，车位信息管理模块主要是新增车位信息和车位信息的管理，IC卡信息管理模块主要是新增IC卡类型和IC卡基本信息的管理，临时车辆停车管理模块主要是新增临时停放车辆、车辆的出场信息和收取停车费用的管理，系统功能操作模块主要是对用户密码进行管理以及设置退出系统功能。
该系统不仅具有良好的性能和扩展性，而且还具有较高的响应速度和效率，为实际生活中的停车场供了一个快速、简单、规范的管理平台。
关键字：J2EE；停车场管理；MyEclipse2014；MySQL；MVCDesign and Implementation of Parking Lot ManagementSystem Based on WebAbstract: A B/S-based Web parking management system has been developed using the MyEclipse 2014 platform and the Java language. The system foreground uses JSP technology to dynamically generate Web pages, and the background uses MVC development mode to reduce the coupling of code and improve the reusability of code. It mainly includes the modules of system information management, parking information management, IC card information management, temporary vehicle parking management and system function operation. System information management module is mainly to the role and user management, parking information management module is mainly the new parking information and parking information management, IC card information management module is mainly the new IC card type and IC card basic information management, temporary vehicle parking management module is mainly new temporaryparking vehicles, vehicle appearance information and parking fee management, system function operation module is mainly to manage the user password and set up exit system function. The system not only has good performance and extensibility, but also has high responsespeed and efficiency, which provides a fast, simple and standardized management platform for the parking lot in real life.Keywords: J2EE; parking management; MyEclipse 2014; MySQL; MVC引言随着社会的进步发展以及人们生活水平的提高，许多家庭都有了购置车辆的想法，伴随着车辆数量的迅猛增长，城市交通逐渐陷入瓶颈状态。
抽象类英语作文模板
抽象类英语作文模板英文回答：Abstract Classes。
An abstract class in Java is a class that is declared abstract—it may or may not include abstract methods. Abstract classes cannot be instantiated, but they can be subclassed. When an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, then the subclass must also be declared abstract.Syntax。
The syntax for an abstract class in Java is:```。
public abstract class ClassName {。
// Abstract methods and/or concrete methods。
}。
```。
Abstract Methods。
An abstract method is a method that is declared without an implementation. The syntax for an abstract method is:```。
public abstract void methodName();```。
Concrete Methods。
A concrete method is a method that is implemented in the abstract class. The syntax for a concrete method is:```。
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AbstractThe Web services platform offers a distributed computing environment where autonomous applications interact using standard Internet technology. In this environment, diverse applications and systems become the components of intra- and inter-enterprise integration. Yet, transactional reliability, an often critical requirement on such integration, is presently missing from the Web services platform. In this paper, we address this shortcoming and propose the WSTx framework as an approach to Web service reliability. WSTx introduces transactional attitudes to explicitly describe the otherwise implicit transactional semantics, capabilities, and requirements of individual applications. We show how explicit transactional attitude descriptions can be used by a middleware system to automate the reliable composition of applications into larger Web transactions, while maintaining autonomy of the individual applications.1.IntroductionThe development of software systems frequently entails the need to integrate diverse applications within an enterprise and across enterprises. Different kinds of application integration technologies, such as object-ori-ented middleware, message-oriented middleware, and, more recently, the Web services platform, have been proposed for this purpose.Transactions are a commonly employed approach to address system reliability and fault-tolerance. Object-oriented middleware and message-oriented middleware support transaction processing; examples of middleware transaction models are CORBA OTS/JTS distributed object transactions, EJB declarative transactions, and messaging transactions. The Web services platform, however, is lacking reliability features; no Web transac-tion model and corresponding Web transaction infra-structure support has been established so far.In this paper, we address this shortcoming of the Web services platform and propose a solution for intro-ducing transactional reliability to Web services.Web transactions are a difficult matter, as unlike con-ventional middleware transactions, no common transac-tion semantic, transaction context representation, and coordination protocol can be assumed to exist for trans-action participants (the individual applications) in a Web services environment. It is more likely that partici-pants are autonomous (with respect to their implementa-tion and execution environment), that different, seemingly incompatible transaction models and middle-ware technologies may be involved in the same Web transaction, and that context representation and service coordination and management must be achieved in a decoupled, decentralized manner.In this paper, we describe first results in the develop-ment of the WSTx (Web Services Transactions) frame-work addressing these challenges. We introduce the concept of “transactional attitudes”, where Web ser-vice providers declare their individual transactional capabilities and semantics, and Web service clients declare their transactional requirements (of providers). We further propose a Web service middleware, based on intermediaries, which supports different Web service transactional capabilities, and provides global context management for execution monitoring, transaction com-pletion, failure-detection, and recovery.2.Web ServicesThe Web services platform comprises different kinds of technologies and standards that are organized into the five layers of network, transport, packaging, descrip-tion, and discovery, as illustrated in Figure 1.The Web services standard of primary interest in this paper is WSDL [1]. WSDL is the Web Services Descrip-tion Language; it is used to describe a Web service in terms of ports (addresses implementing the service), port types (the abstract definition of operations and ordered exchanges of messages), and bindings (the con-crete definition of which packaging protocols, such as SOAP [2], are used).Figure 1. Web Services Technology StackTransactional Attitudes:Reliable Composition of Autonomous Web ServicesThomas Mikalsen, Stefan Tai, Isabelle RouvellouIBM T.J. Watson Research Center, Hawthorne, New York, USA{tommi | stai | rouvellou@}WSDL is an interface standard that abstracts from any platform and programming language-specific details of how application code is actually invoked. As such, Web services are independent of any particular implementation technology used; they can be deployed anywhere that common Internet technology is sup-ported.Therefore, individual (implementations of) Web ser-vices may use different kinds of (middleware) transac-tion technology. A Web services transaction model that aims to support transactions across Web services hence must tolerate such autonomy; a Web transaction is a global transaction that spans across diverse transac-tional (and non-transactional) service implementations.3.WSTxIn this section, we introduce the Web services trans-action (WSTx) framework comprising provider transac-tional attitudes (Section 3.2) and client transactional attitudes (Section 3.3.). We then show how this frame-work is supported as middleware (Section 3.4). We begin by describing a travel booking scenario (Section 3.1), which is used throughout the paper to motivate and illustrate the WSTx approach.3.1.Travel Booking ScenarioIn the Travel Booking scenario illustrated in Figure 2, a client application wishes to arrange a trip using three independent travel services: a flights Web service, a rooms Web service, and a taxis Web services. These services are defined below.The Flights Web Service. The Flights Web ser-vice provides the three operations reserveFlight, confirmReservation, and cancelReservation. The reserveFlight operation returns a reservation number to the client, which the client needs to either confirm a reservation, or to cancel a reservation. This service is defined by the following WSDL specifica-tion:<?xml version="1.0" encoding="UTF-8" ?><!-- Flights reservation Web service --><definitions name="Flights"xmlns="/wsdl/" ...<!-- abstract messages -->...<message name="ReserveFlightOut"><part name="resvNo" type="xsd:string"/></message><message name="ConfirmReservationIn"><part name="resvNo" type="xsd:string"/></message><message name="CancelReservationIn"><part name="resvNo" type="xsd:string"/></message>...<!-- port type --><portType name="FlightsPortType"><operation name="reserveFlight"><input message="ReserveFlightIn"/><output message="ReserveFlightOut"/></operation><operation name="confirmReservation"><input message="ConfirmReservationIn"/><output message="ConfirmReservationOut"/></operation><operation name="cancelReservation"><input message="CancelReservationIn"/><output message="CancelReservationOut"/></operation></portType></definitions>The Taxis Web Service. The Taxis Web service offers operations to group multiple taxi leg requests into a single (atomic) reservation. The newBooking operation is used to start a reservation; it returns a booking number which the client uses for all subse-quent operations. The addLeg operation is used to add taxi legs, the submitBooking operation is used to sub-mit the request, and the confirmBooking and the cancelBooking operations are used to complete or cancel a reservation. The Taxis Web service is described as follows:<?xml version="1.0" encoding="UTF-8" ?><!-- Taxis reservation Web service --><definitions name="Taxis"xmlns="/wsdl/" ...<!-- abstract messages -->...<message name="NewBookingOut"><part name="bookingNo" type="xsd:string"/></message><message name="ConfirmBookingOut"><part name="confNo" type="xsd:string"/></message>...<!-- port type --><portType name="TaxisPortType"><operation name="newBooking"><input message="NewBookingIn"/><output message="NewBookingOut"/></operation>Figure 2. Travel Booking Services<operation name="addLeg"> ...</operation><operation name="submitBooking"> ...</operation><operation name="confirmBooking"><input message="ConfirmBookingIn"/><output message="ConfirmBookingOut"/></operation><operation name="cancelBooking"> ...</operation></portType></definitions>The Rooms Web Service. The Rooms Web service provides the two operations reserveRoom and can-celReservation. The reserveRoom operation returns a confirmation number to the client, which is needed for the client to cancel (undo) a reservation later on. The following WSDL fragment shows the interface of the service:<?xml version="1.0" encoding="UTF-8" ?><!-- Hotel room reservation Web service --><definitions name="Rooms"xmlns="/wsdl/"...><!-- types --><types ...><schema><complexType name=”ConfInfo”><sequence><element name=”confNo” type=”xsd:string”/>...</sequence></schema></complexType></types><!-- abstract messages -->...<message name="ReserveRoomOut"><part name="conf" type="types:ConfInfo"/></message><message name="CancelReservationIn"><part name="confNo" type="xsd:string"/></message>...<!-- port type --><portType name="RoomsPortType"><operation name="reserveRoom"><input message="ReserveRoomIn"/><output message="ReserveRoomOut"/></operation><operation name="cancelReservation"><input message="CancelReservationIn"/><outputmessage="CancelReservationOut"/></operation></portType></definitions>Each Web service implements a different transac-tional behavior, which, using standard WSDL, is only implicitly described. For example, in the Flights Web service, the cancelReservation operation aborts a tentative reservation, whereas in the Rooms Web ser-vice, the cancelReservation operation compensates for a completed reservation.If the client application wishes to establish a transac-tional dependency between the individual services, so that only well defined combinations of reservations/ bookings are possible, ad hoc code will be required to weave these services into a meaningful transaction.3.2.Provider Transactional AttitudesIn order to avoid ad hoc programming of composi-tions of Web services that have different transactional semantics, we propose the concept of Provider Trans-actional Attitudes.Provider transactional attitudes (PTAs) are a mecha-nism for Web service providers to explicitly describe their specific transactional behavior. By making trans-actional semantics explicit, PTAs can be used in auto-mating the composition of individual transactional Web services into larger transactional patterns, while main-taining the autonomy of the individual services.A PTA includes the name of an abstract transac-tional pattern, plus any additional port-specific infor-mation needed to make the pattern concrete. The abstract pattern implies a well-defined state machine describing the valid transactional states, state transi-tions, and transition-triggering events. The name and semantic of each state is implied by the name of the transactional pattern. State transitions are triggered by externally observable events, such as operation invoca-tions on ports and time-outs. To complete a pattern, a provider must•identify transactional port operations (i.e., the port operations that can trigger transactional-statetransitions),•describe how invocations on these operations are associated with corresponding state machinesinstances (e.g., operation correlation encoding),and•describe how transactional-operation outcomes are encoded.We propose using the standard WSDL extensibility mechanisms (so called extensibility elements) to anno-tate port bindings with transactional semantics (i.e., to define an XML vocabulary for transactions).The WSTx includes the following extension ele-ments (applied to wsdl:binding):• a binding extension element for declaring the transactional attitude (i.e., transactional pattern)of a port,•operation extension elements for declaring trans-actional-event triggering operations,•input/output extension elements for describing operation correlation encoding, and•output/fault extension elements for describing operation-outcome encoding.For example, a WSTx port binding element is used to declare a port’s transactional attitude: annotating a port binding with<wstx:bindingattitude=’pending-commit’/>declares that the port has a pending-commit attitude towards transactions. The name pending-commit refers to a well-defined WSTx transactional pattern.The WSTx vocabulary presently defines three pro-vider transactional attitudes:•“pending-commit” (PC),•“group-pending-commit”(GPC), and•“commit-compensate” (CC).We have found these PTAs to describe transactional behavior that is common to many Web services; in par-ticular, these PTAs can be used to describe the transac-tional behavior of the three Web services in our Travel Booking scenario. We expect the number attitudes sup-ported by the WSTx to grow as additional transactional Web service patterns emerge.Following, we describe each of these transactional attitudes in detail, and describe the associated WSTx extension elements used to represent them.3.2.1.Pending-commit PTAThe pending-commit (PC) attitude describes a trans-actional port of a single Web service where the effect of a single forward operation invocation can be held in a pending state; the operation-effect remains pending until the subsequent occurrence of an event (e.g., the invocation of a commit or abort operation) triggers either acceptance or rejection (of the operation-effect).Forward operations are annotated with a <wstx:forwardOperation> element. The effect of a forward operation is brought to a pending state if the operation is invoked successfully; otherwise, the effect is rejected.The effect of a forward operation can be explicitly realized by invoking a commit operation. Commit oper-ations are annotated using a <wstx:commitOpera-tion> operation extension element.The effect of a forward operation can be explicitly dismissed by invoking an abort operation. Abort opera-tions are annotated using a <wstx:abortOperation> operation extension element.In order to associate a forward operation with a com-mit (or abort) operation, a PC attitude must include a correlation encoding. WSTx provides the <wstx:sim-pleCorrelation> input/output extension element for describing simple correlation encodings, where a corre-lation identifier is embedded in a message part. This element allows the provider to specify the part, and optionally the sub-part (e.g., using an XPath [3] expres-sion), of a message that holds the correlation identifier.Returning to the Travel Booking scenario from above, the Flights Web service implicitly describes the transactional semantics of a pending-commit PTA: the process of reserving a flight includes a pending state, before the reservation is either committed (using the confirmReservation operation) or aborted (using the cancelReservation operation). This transac-tional behavior can be made explicit in a WSDL bind-ing using WSTx extension elements as follows1:<binding name="FlightsPortBinding"type="tns:FlightsPortType"><wstx:binding attitude="pending-commit"/><operation name="reserveFlight"><wstx:forwardOperation/><output><wstx:simpleCorrelation partName="resvNo"/> </output></operation><operation name="confirmReservation"><wstx:commitOperation/><input><wstx:simpleCorrelation partName="resvNo"/> </input></operation><operation name="cancelReservation"><wstx:abortOperation/><input><wstx:simpleCorrelation partName="resvNo"/> </input></operation></binding>3.2.2.Group-pending-commit PTAThe group-pending-commit (GPC) attitude describes a transactional port of a single Web service where the effects of a group of forward operation invocations can be held in a pending state; the group-effect remains pending until the subsequent occurrence of an event (e.g., the invocation of a commit or abort operation) triggers either acceptance or rejection (of the group-effect).A new operation group is created either explicitly with a begin operation, or implicitly as part of a for-ward operation.Explicit begin operations are annotated with a <wstx:beginOperation> element. Forward opera-tions are annotated with a <wstx:forwardOpera-tion> element.The group-effect is brought to a pending state by invoking a single prepare operation. Prepare operations are annotated with a <wstx:prepareOperation> element. The group-effect is brought to a pending state if the prepare operation is invoked successfully; other-wise, the group-effect is rejected.The group-effect of all forward operations can be explicitly accepted by invoking a single commit opera-tion. A commit operation is annotated with a <wstx:commitOperation> element. The group-effect is accepted if the commit operation is invoked successfully; otherwise, the group-effect is rejected.The group-effect of all forward operations can be explicitly rejected by invoking a single abort operation. Abort operations are annotated using the <wstx:abortOperation> element.In order to associate individual operations (forward operations, prepare operations, etc.) with a group, a GPC attitude must include a correlation encoding. As with the pending-commit PTA (see ”Pending-commit PTA”above), simple correlation encodings can be1.The bindings illustrated here have been abbreviated to savespace; a complete binding would likely include additional communication protocol elements, such as SOAP extension elements.described using the <wstx:simpleCorrelation> element.The “Taxis” Web service (from the scenario above) implicitly describes a group-pending-commit PTA: the Web service provides control operations to begin, pre-pare, commit or abort a group of actions, and it pro-vides a forward operation that represents the group action. This transactional behavior can be described using WSTx as follows:<binding name="BookingBinding"type="tns:BookingPortType"><wstx:bindingattitude="group-pending-commit"/><operation name="newBooking"><wstx:beginOperation/><output><wstx:simpleCorrelationpartName="bookingNo"/></output></operation><operation name="addLeg"><wstx:forwardOperation/><input><wstx:simpleCorrelationpartName="bookingNo"/></input></operation><operation name="submitBooking"><wstx:prepareOperation/><input><wstx:simpleCorrelationpartName="bookingNo"/></input></operation><operation name="confirmBooking"><wstx:commitOperation/><input><wstx:simpleCorrelationpartName="bookingNo"/></input></operation><operation name="cancelBooking"><wstx:abortOperation/><input><wstx:simpleCorrelationpartName="bookingNo"/></input></operation></binding>mit-compensate PTAThe commit-compensate (CC) attitude describes a transactional port of a single Web service where the effect of a single forward operation invocation is immediately accepted, yet can later be semantically reversed by invoking an associated compensation oper-ation on the port.Forward operations are annotated with a <wstx:forwardOperation> element. The effect of a forward operation is immediately accepted if the opera-tion is invoked successfully; otherwise, the effect is rejected.The effect of a forward operation can be semanti-cally reversed by invoking a compensation operation. Compensation operations are annotated using a <wstx:compensationOperation> element.The transactional behavior of the Rooms Web ser-vice (from above) is consistent with the commit-com-pensate PTA: an incoming request that can be executed is immediately committed. A compensating operation (the cancelReservation operation) is defined for a client to undo a previously committed reservation. This behavior can be made explicit using the following WSTx port binding:<binding name="ReservationBinding"type="tns:ReservationPortType"><wstx:binding attitude="commit-compensate"/><operation name="reserveRoom"><wstx:forwardOperation/><input><output><wstx:simpleCorellationpartName="conf" select="confNo"/></output></operation><operation name="cancelReservation"><wstx:compensationOperation/><input><wstx:simpleCorrelation partName="confNo"/> </input></operation></binding>In the examples above, a default operation-outcome encoding is assumed. Each operation, in all three Web services, has an associated single fault message (which has been elided to save space). If an operation generates a fault, then the operation-outcome is failure; otherwise, the operation-outcome is suc-cess.2 The meaning of success and failure for a given operation is defined by the PTA; for example, failure of a prepareOperation in the group-pending-commit PTA results in the immediate rejection of the group-effect (as if an abortOperation was invoked).Figure 3 illustrates in summary the provider transac-tional attitudes of the three autonomous Web services of the Travel Booking scenario. Using WSTx, the oth-erwise implicit transactional behavior of the Web ser-vices are now explicit as part of the WSDL interface.3.3.Client Transactional AttitudesReconsider the client of the travel booking example. Let us assume that the client wants to reserve two flights, one hotel room, and multiple taxis, using the three independent Web services. The client wishes to establish a transactional dependency between the ser-vices, so that only the following defined outcomes of the combined use of the Web services are possible:2.Other, more flexible, operation-outcome encodings are beingconsidered.•the two flight reservations, the hotel reservation, and the taxis reservation are all committed •the two flight reservations and the hotel reserva-tion are committed, but the taxis reservation isaborted•the two flight reservations, the hotel reservation, and the taxis reservation are all aborted.Therefore, the flights reservations and the hotel res-ervation in combination are vital to the success of the client transaction, whereas the taxis reservation is not critical to the success of the transaction (but is never-theless part of the transaction).3.3.1.Implicit client attitudesWithout the WSTx framework, the client needs to manually program the use of the Web services to ensure that exactly one of the desired outcomes is reached. This is a complex exercise; a possible naive solution (neglecting any management of system failures) is illustrated below.// pseudo-code for client without WSTxcR = Rooms.reserveRoom // commit roompF1 = Flights.reserveFlight //prepare flight1 pF2 = Flights.reserveFlight //prepare flight2 Taxis.newBooking {Taxis.addLeg *pT = Taxis.submit // prepare taxis}if(cR && pF1 && pF2) { // commit condition cF1 = Flights.confirmFlight // commit flight1 cF2 = Flights.confirmFlight // commit flight2 if(cF1 && cF2) {if(pT){cT = Taxis.confirmBooking // commit }} else {Rooms.cancelResv; // compensate roomif(pT){Taxis.cancelBooking // abort}}} else { // rollbackif(pF1) {Flights.cancelResv // abort flight1 }if(pF2) {Flights.cancelResv // abort flight2 }if(pT) {Taxis.cancelBooking // abort taxis }if(cR) {Rooms.cancelResv // compensate room }}Here, the client implicitly describes the transactional expectations: which of the providers’commit opera-tions are critical (form an atomic group), and which providers’commit operations are non-critical, but asso-ciated to the atomic group.3.3.2.Explicit client attitudesThe WSTx framework proposes to describe such cli-ent expectations explicitly using Client Transactional Attitudes (CTAs).A client’s transactional attitude is established by its use of a particular WSDL port type to manage (create, complete, etc.) Web transactions, where the port type represents some pre-defined transactional pattern. Within the scope of a Web transaction, the client exe-cutes one or more named actions, where each action represents a provider transaction (associated with some PTA) executing within the context of the larger Web transaction.The client initiates an action by binding to an action port, which serves as a proxy to a participating pro-vider’s transactional port. Each action port represents a unique provider transaction executing within the con-text of the client’s Web transaction. When using an action port, the client may invoke only the forward operations of the provider; that is to say, the client can-not invoke completion operations (commit, abort, and compensation operations). (Actions and action ports are described in more detail below in Section 3.4.) Flexible Atom CTA. The flexible atom (FA) attitude describes a client transaction where a set of client actions (i.e., provider transactions) are grouped into an atomic group that can have one out of a set of defined group outcomes; that is to say, some actions are declared to be critical to the success of the transaction, whereas other actions are part of the transaction though not pivotal to its success. The client specifies the acceptable outcomes as an outcome condition, described in terms of the success or failure of the indi-vidual actions, and when ready (i.e., after executing the forward operations of these actions), requests the com-pletion of the flexible atom according to that condition. The WSTx middleware (described below in Section 3.4) then attempts to satisfy the condition by invoking the appropriate completion operations on the providers represented by the associated actions.The code below illustrates the client for the Travel Booking example using the WSTx:// Create a new Flexible AtomwstxId = flexAtom.beginAtom();Figure 3. Travel Booking Services with PTAs。