lect(6) Transaction Management (1).pptx

Database Management Systems Transaction Management Overview Chapter 18  Transactions Concurrent execution of user programs is essential for good DBMS performance. A user’s program may carry out many operations, but the DBMS is only concerned about read/written...

Database Management Systems Transaction Management Overview Chapter 18  Transactions Concurrent execution of user programs is essential for good DBMS performance. A user’s program may carry out many operations, but the DBMS is only concerned about read/written from/to the database. A transaction is the DBMS’s abstract view of a user program: a sequence of reads and writes.  Concurrency in a DBMS Users submit transactions, and can think of each transaction as executing by itself. Concurrency is achieved by the DBMS,  interleaves actions (reads/writes ) of various transactions. Each transaction must leave the database in a consistent state if the DB is consistent when the transaction begins.  Atomicity of Transactions A transaction might commit after completing all its actions, or it could abort A very important property guaranteed by the DBMS for all transactions is that they are atomic. DBMS logs all actions so that it can undo the actions of aborted transactions.  Example Consider two transactions (Xacts): T1: BEGIN A=A+100, B=B-100 END T2: BEGIN A=1.06*A, B=1.06*B END There is no guarantee that T1 will execute before T2 or vice-versa. However, the net effect must be equivalent to these two transactions running serially in some order. Example (Contd.)  Scheduling Transactions Serial schedule: Schedule that does not interleave the actions of different transactions. Equivalent schedules: For any database state, the effect of executing the first schedule is identical to the effect of executing the second schedule.  Scheduling Transactions con’d Serializable schedule: A schedule that is equivalent to some serial execution of the transactions.