《数据库系统原理Database System Concepts》双语课件-ch10(transaction)

Transacfrom account A to account B in account (account_number, balance) .
Update account Set balance = balance - 50 Where account_number = ‘A’
process private area
xi=X
read write
buffer block
buffer
B
input output
disk
B
physical block
Simple Model of Database (cont.)
read_item(X) command includes the following steps: Find the address of the disk block that contains item X. Copy that disk block into a buffer in main memory (if that disk block is not already in some main memory buffer).- Input (X) Copy item X from the buffer to the program variable named xi.
The basic unit of data movement between the disk and main memory is a block.
Database Buffer Memory Stable Database Disk
Simple Model of Database (cont.)
Data access: two basic operations
read_item(X): Reads a database item X into a program variable xi.
write_item(X): Writes the value of program variable xi into the database item X.
Simple Model of Database (cont.)
write_item(X) command includes the following steps:
Find the address of the disk block that contains item X. Copy that disk block into a buffer in main memory (if
that disk block is not already in some main memory buffer). Copy item X from the program variable named xi into its correct location in the buffer. Store the updated block from the buffer back to disk (either immediately or at some later point in time) Output (X).
Chapter Outline
Simple Model of Database Transaction Concept Transaction State Concurrent Executions Serializability Recoverability
Review of Database System Architecture
Database Users Interface & Tools
Query processor Storage & Transaction
Processor
Data Storage
Simple Model of Database
Data storage: database permanently resides on disk, but some portion of it is temporarily residing in main memory. Main memory for database buffer caching currently used data. Disk for stable database storing main database.
Transaction boundary
T1: begin read(A); A:= A–50; write (A); read (B); B := B+50; write(B); commit;
Transaction Concepts
A transaction is a logical unit of work that includes one or more database access operations (read -retrieval, write - insert or update, delete). a sequence of operations including database operations that is atomic with respect to concurrency and recovery. atomic execution unit that, when applied to a consistent database, generates a consistent but possibly different database. a sequence of operations with the database which represents one meaningful activity in the user's environment.
Solution: Define this two operations within a transaction.
Transaction Example (cont.)
Define a funds-transfer transaction using read/ Write operations.
Update account Set balance = balance + 50 Where account_number = ‘B’
Requirement: Either all two operations are completed or none are.
What is your solution?