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Rollback and cascading rollback

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• Rollback
  • Rollback:

    Rollback = undo all the updates performed by a transaction

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• Cascading rollback
  • Important fact:

    If a transaction T1 writes a DB element A (and transaction T1 has not committed) and later the transaction T2 reads the value written by transaction T1, then: The validity of the value read by transaction T2 will depend on whether transaction T1 will commit

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  • Cascading rollback:

    If transaction T1 has read (used) dirty data written by the uncommitted transaction T2, then: When transaction T1 aborts, then: We must also abort transaction T2 !!!

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  • Cascading rollback procedure:

    T = the transaction that has aborted RollBack( T ) { Undo the updates performed by transaction T; // This depends on the logging method // See below... Let S = set of transactions that has read (dirty) data from T for ( each Ti ∈ S ) do { Abort( Ti ); RollBack( Ti ); // This will abort and rollback // ALL transactions that have read // from T indirectly } }

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• How to rollback a transaction
  • Fact:

    The rollback procedure is different for different logging methods !!!

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  • Rollback an update when using an undo log:

    In undo logging, we write a undo log record when we update the database element: The dirty data can be found in these places: Dirty data can be stored: In memory buffer            On disk Rollback procedure: Find all the updated values by an aborted transaction T in the undo log Use the (old) value in the undo log to: Restore the value of the database element on disk and in the memory buffer

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  • Rollback an update when using a redo log:

    In redo logging, we will only perform an OUTPUT operation (and update the database element on disk) when the transaction has committed: The dirty data can be found in these places: Dirty data can be stored: In memory buffer            Rollback procedure: Find all the updated values by an aborted transaction T in the redo log Use the value in the database element stored on disk to: Restore the value of the database element in the memory buffer

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  • Rollback an update when using an undo/redo log:

    In undo/redo logging, we write a undo log record when we update the database element: The dirty data can be found in these places: Dirty data can be stored: In memory buffer            On disk Rollback procedure: Find all the updated values by an aborted transaction T in the undo/redo log Use the (old) value in the undo/redo log to: Restore the value of the database element on disk and in the memory buffer