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 inconsistent When the transaction is committed the database must be consistent Two main issues to deal with x Failures of various kinds, such as hardware failures and system crashes x Concurrent execution of multiple transactions 标 Database System Concepts 15.2 @Silberschatz, Korth and SudarshanDatabase System Concepts 15.2 ©Silberschatz, Korth and Sudarshan 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 inconsistent. When the transaction is committed, the database must be consistent. Two main issues to deal with:  Failures of various kinds, such as hardware failures and system crashes  Concurrent execution of multiple transactions