Transaction Concept A transaction is a unit of program execution that accesses and possibly updates various data items. A transaction must see a consistent database. During transaction execution the database may be temporarily inconsistent. When the transaction completes successfully(is committed),the database must be consistent. After a transaction commits,the changes it has made to the database persist,even if there are system failures. Multiple transactions can execute in parallel. 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-5th Edition,Sep 10,2005. 15.3 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 5 15.3 ©Silberschatz, Korth and Sudarshan th Edition, Sep 10, 2005. Transaction Concept A transaction is a unit of program execution that accesses and possibly updates various data items. A transaction must see a consistent database. During transaction execution the database may be temporarily inconsistent. When the transaction completes successfully (is committed), the database must be consistent. After a transaction commits, the changes it has made to the database persist, even if there are system failures. Multiple transactions can execute in parallel. Two main issues to deal with: Failures of various kinds, such as hardware failures and system crashes Concurrent execution of multiple transactions