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Preventing deadlock with timestamps: the wait-die method

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• Timestamping transaction
  • Timestamped transactions:

    Each transaction is assigned a unique increasing timestamp: T1, T2, T3, ...             Important fact: An 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 over "younger" transactions

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• Wait-die locking rule for deadlock avoidance

  • The Wait-die locking rule:

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

    Graphically:

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  • The wait-die rule will prevent the development of deadlock because:

    Tn cannot wait for T1: So there cannot be a cycle in the wait-for graph: No deadlock possible !!!

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• Example: wait-die locking
  • Transactions:

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  • Example execution that obeys the wait-die rule:

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  • Important note:

    When a transaction is aborted and restarts: The transaction will retain its (old) transaction timestamp !!!           Therefore: Eventually, a transaction will become the "oldest" transaction and will complete execution !!!