Recovery Algorithms Suppose transaction T;transfers $50 from account A to account B Two updates:subtract 50 from A and add 50 to B Transaction 7;requires updates to A and B to be output to the database. A failure may occur after one of these modifications have been made but before both of them are made. Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent state Not modifying the database may result in lost updates if failure occurs just after transaction commits Recovery algorithms have two parts 1.Actions taken during normal transaction processing to ensure enough information exists to recover from failures 2.Actions taken after a failure to recover the database contents to a state that ensures atomicity,consistency and durability Database System Concepts-7th Edition 19.4 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 19.4 ©Silberschatz, Korth and Sudarshan th Edition Recovery Algorithms ▪ Suppose transaction Ti transfers $50 from account A to account B • Two updates: subtract 50 from A and add 50 to B ▪ Transaction Ti requires updates to A and B to be output to the database. • A failure may occur after one of these modifications have been made but before both of them are made. • Modifying the database without ensuring that the transaction will commit may leave the database in an inconsistent state • Not modifying the database may result in lost updates if failure occurs just after transaction commits ▪ Recovery algorithms have two parts 1. Actions taken during normal transaction processing to ensure enough information exists to recover from failures 2. Actions taken after a failure to recover the database contents to a state that ensures atomicity, consistency and durability