GEODATABASE和ArcSDE

Building Geodatabases I
Enterprise geodatabases and ArcSDE
4-1
This lesson will discuss the additional functionality that an enterprise geodatabase offers over a personal geodatabase. It introduces ArcSDE, the ESRI software that enables spatial data to be stored in an RDBMS and interact with ArcGIS applications. Topics include: the client/server communication model; connection methods to an enterprise geodatabase; and the ArcSDE client command line interface for administering ArcSDE.
Building Geodatabases I
Enterprise geodatabases and ArcSDE
4-4
Managing changes The versioning software and procedures enable users to manage changes to the enterprise geodatabase. Multiple, self-consistent versions of a table may exist simultaneously. ArcSDE tracks groups of changes and presents them to its users through named versions. Each user working with a multi-versioned table appears to have his or her own copy of that table. In actuality, only one copy of the table exists in the enterprise geodatabase along with two tables that record changes to it. By selecting logically-consistent subsets of rows from these three tables, ArcSDE can represent multiple, alternate versions of the original table while avoiding the overhead of storing redundant data. In addition to saving storage space, this technique allows ArcSDE to reconcile differences between alternate versions of the table. Reconciling involves determining which changes are unique to each of two versions and applying the changes from one of the versions to the other. In some cases, differences can be reconciled in one step, preserving all changes from both versions. Other times, each version of the table may contain an alternate representation of the same row. Alternate representations of a row may exist in different versions. But a row may only be represented by one of those alternatives in any given version. Therefore, reconciling introduces the potential of creating conflicts between the versions. When a conflict arises, a user must choose which copy of the row will be preserved in the current version, at the expense of its alternate representation. The graphic example above illustrates such a case.
g Geodatabases I Enterprise geodatabases and ArcSDE 4-3
One advantage that an enterprise geodatabase has over a personal geodatabase is the ability to create versions. Multiple users can access the same version or a single user may access multiple versions. Versions are snapshots of the entire geodatabase When you spin off a new version of the geodatabase, all editable feature classes and ArcSDE tables participate, even those that you do not have access to. To make feature classes editable, you have to “register” them as “versioned” in ArcCatalog. It creates an additions and a deletions table for each of them. Making a version is separate from making a feature class editable. You must register a feature class as versioned before you can edit the feature class. A version is created for the entire enterprise geodatabase, not simply those feature classes and tables to which you have edit privileges. All have the same set of feature classes and SDE tables All versions in the enterprise geodatabase have the same schema (e.g., the same set of feature classes and ArcSDE tables). You may not add a feature class to only one version; it is added to all versions. You may not add a column to a table in one version without adding the column to all the other versions of that table. Whenever you add a feature class or add or drop a column to a table, it is included in each version. Creating a new version is not a copy of the database. The version identifies the changes that have occurred, presenting the user with a different picture or view of the database. Only stores edits The “multi-versioned” enterprise geodatabase is efficient because it stores unchanged features (rows) only once. If a row is changed in a version, then the geodatabase stores the updated row and the original row (which is still referenced by other versions). Therefore, you could see a picture of the data in one version and a different picture of the data in another version.
Building Geodatabases I
Enterprise geodatabases and ArcSDE
4-2
ArcSDE offers the same benefits for your spatial data that an RDBMS does for your non-spatial data. GIS users employ ArcSDE to provide efficient access to large geodatabases by multiple concurrent users. These systems are designed to be reliable, secure, and scalable. ArcSDE relies on the RDBMS for its operation, so you can take advantage of database functionality for managing spatial data. Enterprise geodatabases RDBMSs provide a layer of abstraction between the physical data stored on disk and the end user. This allows the user to interact with the database at a logical level while the database administrator manages the physical files of the database. Users can access a table containing one million rows as if it were a single entity, although it may be stored across five different disk drives. ArcSDE stores spatial data in a feature class. Feature classes are logical entities that are physically implemented as a collection of RDBMS tables. As the feature class grows, the ArcSDE administrator can manage its underlying tables to provide efficient access to the data without needing to spatially divide the feature class into tiles. Tiling spatial data is a method for managing large file-based geographic databases. There is no data management software between the end user and the physical files, therefore, GIS users and applications are exposed to the individual tiles and must compensate for changes in the file storage. Concurrent use ArcSDE and the RDBMS allow multiple users to read and write to the spatial database concurrently. Users are not required to maintain individual copies of the data to enforce consistency and integrity, nor must they check out data for editing. Access to the data is dynamic and is managed by the ArcSDE and RDBMS servers. Integrated spatial and tabular data Since all ArcSDE data is stored in the database, you do not need to maintain a link between file-based spatial data and attributes stored in the RDBMS tables. This allows for more efficient access and better data integrity.