北邮数据库课程讲义chapter12.ppt

* DataBase System Concepts Chapter 12 Transactions DataBase System Concepts Transaction State Implementation of Atomicity and Durability Concurrent Executions Serializability Recoverability Implementation of Isolation Transaction Definition in SQL Testing for Serializability. The term transaction refers to a collection of operations that form a single logical unit of work. A transaction is a unit of program execution that accesses and possibly updates various data items. DataBase System Concepts Transaction E.g. 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) 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 Atomicity requirement if the transaction fails after step 3 and before step 6, money will be “lost” leading to an inconsistent database state Failure could be due to software or hardware the system should ensure that updates of a partially executed transaction are not reflected in the database 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 even if there are software or hardware failures. DataBase System Concepts Transaction Example DataBase System Concepts Consistency requirement in above example: the sum of A and B is unchanged by the execution of the transaction A transaction must see a consistent database. During transaction execution the database may be temporarily inconsistent. When the transaction completes successfully the database must be consistent Erroneous transaction logic can lead to inconsistency Transaction Example(2) Isolation requirement — if between steps 3 and 6, another transaction T2 is allowed to access the partially updated database, it will see an inconsistent dat