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Preventing deadlock with timestamps: wound-wait scheme

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• Recall: timestamping transaction
  • Timestamped transactions:

    Each transaction is assigned a unique increasing timestamp: T1, T2, T3, ...             Earlier transactions receives a smaller timestamp

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  • Purpose of the time stamp:

    To give older transactions (= transactions with smaller timestamps) a higher priority

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• Wound(kill)-wait locking rule for transactions

  • The Wound(kill)-wait locking rule:

    When an older transaction tries to lock a DB element that has been locked by a younger transaction: The older transaction will wound (= kill/abort) the younger transaction        When a younger transaction tries to lock a DB element that has been locked by a older transaction: The younger transaction wait until the lock is released (by the older transaction)

    Graphically:

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  • The wound-wait rule will prevent (forbid) the development of deadlock because:

    T1 will wound (= kill/abort) the (younger) Tn: (The wait direction in wound-wait is opposite from a wait-die locking rule !!!)

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  • Note:

    In both schemes (wait-die and wound-wait), the younger transaction will get aborted The older transaction is not be aborted !!!

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• Example: wound-wait locking
  • Transactions:

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  • Example execution that obeys the wound-wait rule:

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  • Note:

    When a transaction is aborted and restarts, the transaction will retain its transaction timestamp !!!