浙江大学数据库系统概念PPT第十五章 对应原版教材第五版
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Commit a transaction Rollback a transaction
Consistency. Execution of a transaction in isolation preserves the consistency of the database.
Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions.
15.3
©Silberschatz, Korth and Sudarshan, Bo Zhou
Example of Fund Transfer
Transaction to transfer $50 from account A to account B:
1. read(A) 2. A := A – 50 3. write(A) 4. read(B) 5. B := B + 50 6. write(B)
A transaction must see a consistent database. During transaction execution the database may be inconsistent. When the transaction is committed, the database must be consistent.
15.2
©Silberschatz, Korth and Sudarshan, Bo Zhou
ACID Properties
To preserve integrity of data, the database system must ensure: Atomicity. Either all operations of the transaction are properly reflected in the database or none are.
Failure could be due to software or hardware
Database System Concepts
15.4
©Silberschatz, Korth and Sudarshan, Bo Zhou
Example of Fund Transfer (Cont.)
Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures. Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).
1. read(A)
4来自百度文库 read(B)
2. A := A – 50
5. B := B + 50
3. write(A)
6. write(B
A transaction is a unit of program execution that accesses and possibly updates various data items.
Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.
Database System Concepts
Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.
Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.
Two main issues to deal with:
Failures of various kinds, such as hardware failures and system crashes
Concurrent execution of multiple transactions
Database System Concepts
Database System Concepts
15.1
©Silberschatz, Korth and Sudarshan, Bo Zhou
Transaction Concept
E.g. Transaction to transfer $50 from account A to account B:
Chapter 15: Transactions
Transaction Concept Concurrent Executions Serializability Testing for Serializability Recoverability Transaction Definition in SQL
Consistency. Execution of a transaction in isolation preserves the consistency of the database.
Isolation. Although multiple transactions may execute concurrently, each transaction must be unaware of other concurrently executing transactions. Intermediate transaction results must be hidden from other concurrently executed transactions.
15.3
©Silberschatz, Korth and Sudarshan, Bo Zhou
Example of Fund Transfer
Transaction to transfer $50 from account A to account B:
1. read(A) 2. A := A – 50 3. write(A) 4. read(B) 5. B := B + 50 6. write(B)
A transaction must see a consistent database. During transaction execution the database may be inconsistent. When the transaction is committed, the database must be consistent.
15.2
©Silberschatz, Korth and Sudarshan, Bo Zhou
ACID Properties
To preserve integrity of data, the database system must ensure: Atomicity. Either all operations of the transaction are properly reflected in the database or none are.
Failure could be due to software or hardware
Database System Concepts
15.4
©Silberschatz, Korth and Sudarshan, Bo Zhou
Example of Fund Transfer (Cont.)
Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures. Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database (the sum A + B will be less than it should be).
1. read(A)
4来自百度文库 read(B)
2. A := A – 50
5. B := B + 50
3. write(A)
6. write(B
A transaction is a unit of program execution that accesses and possibly updates various data items.
Durability. After a transaction completes successfully, the changes it has made to the database persist, even if there are system failures.
Database System Concepts
Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.
Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.
Two main issues to deal with:
Failures of various kinds, such as hardware failures and system crashes
Concurrent execution of multiple transactions
Database System Concepts
Database System Concepts
15.1
©Silberschatz, Korth and Sudarshan, Bo Zhou
Transaction Concept
E.g. Transaction to transfer $50 from account A to account B:
Chapter 15: Transactions
Transaction Concept Concurrent Executions Serializability Testing for Serializability Recoverability Transaction Definition in SQL