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ACR schedules: Avoiding Cascading Rollback

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• Review
  • Recall: 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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  • The cause of cascading rollback:

    Allowing transactions to read dirty data

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• Avoid Cascading Rollbacks (ACR) schedules
  • Avoid Cascading Rollback (ACR) schedule:

    A schedule is an ACR schedule if: Every read operation performed by a transaction must read (= use) values written by (already) committed transactions I.e.: At the moment that the read operation happens, the transaction that has written the data has already committed

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  • In other words:

    An ACR schedule will forbid the use (= reading) of dirty data !!!!

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• Example of non-ACR and ACR schedules
  • Example of a non-ACR schedule:

    The following schedule is not ACR: Because: Transaction T2 has read dirty data written by uncommitted transaction T1:            Therefore: If transaction T1 aborts, we must also abort transaction T2

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  • Example of a ACR schedule:

    The following schedule is ACR: Because: Transaction T2 has read "guaranteed" data written by a committed transaction T1 Notice that: When T2 reads the database element B, we already know the value written by (committed transaction) T1 will persist in the database !!!