Backup and Recovery

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Backup and Recovery Techniques Need for Recovery: ã Whenever a transaction is submitted to a DBMS for execution, the system is responsible for making sure that either: All the operations in the transaction are completed successfully and their effect is recorded permanently in the database or; The transaction has no effect whatsoever on the database or on any other transactions. The DBMS must not permit some operations of a transaction T to be applied to the database while other operations of T a
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  Backup and Recovery TechniquesNeed for Recovery: ã Whenever a transaction is submitted to a DBMS forexecution, the system is responsible for making sure thateither:1.All the operations in the transaction are completedsuccessfully and their effect is recorded permanently in thedatabase or;2.The transaction has no effect whatsoever on the databaseor on any other transactions.3.The DBMS must not permit some operations of atransaction T to be applied to the database while otheroperations of T are not.4.This may happen if a transaction fails after executing someof its operations but before executing all of them.  Types of Failures: ã There are several possible reasons for a transactionto fail in the middle of the execution. They are: 1. A computer failure (system crash): ã A hardware, software, or network error occurs in thecomputer system during transaction execution. ã Hardware crashes are usually media failures likemain memory failure.  2. A transaction or system error: ã Some operation in the transaction may cause it to fail, such as integer overflow or division by zero. ã Transaction failure may also occur because of erroneous parameter values or because of a logicalprogramming error. ã In addition the user may interrupt the transaction duringits execution.  3 . Local errors or exception conditions detectedby the transaction: ã During transaction execution, certain conditions mayoccur that necessitate cancellation of the transaction. ã For e.g. data for the transaction may not be found. ã Notice that an exception condition, such as insufficientaccount balance in a banking database may cause atransaction, such as money withdrawal to be cancelled. ã This should be programmed in the transaction itself and hence would not be considered a failure.
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