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Rollback methods

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• Recall: general procedure to rollback a transaction
  • When using a undo or undo/redo log:

    The before value of the write operation is stored in the log Rollback procedure: Find all the undo values for the transaction T in the log and use the before value to restore the DB element on disk and in the memory buffer

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  • When using a redo log:

    Due to the redo log update rule: The DMBS must first write the log records: <T, X, v> .... (will include other updates by T) <COMMIT T> before writing the DB elements to disk we have that: The copy of the DB element on disk has not been updated by an uncommitted trabsaction Rollback procedure: Read the copy of the DB element and use the value to replace the copy in memory buffer

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• Special techniques for rollback
  • Special techniques for rollback:

    Create a log for rollback purpose If a DB element is equal to a block, we can use a simple trick to perform rollback

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• Rollback log
  • Rollback log:

    Maintain in memory: A log of changes made by each transaction The log will contain only undo values The log records are kept only when the transaction is running !!!

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• Rollback trick for blocks
  • If the lockable DB element is one disk block, then:

    There is a simple trick that we can use to rollback a transaction

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  • Rollback trick for disk block DB elements:

    Blocks written by uncommitted transactions are pinned in memory: Example: We can set a bit in the buffer to indicate that a pinned block can not be written to disk How to rollback the updates made by the uncommitted transaction: Free (= discard) the buffer that contains the block: Result: When a transaction tries to access the DB element: the buffer manager will have to read the DB element from the disk !!!

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  • This trick will work because:

    The block is exclusively locked by the uncommitted transaction (that will get rolled back) Therefore: No other transaction could have locked the same block !!!