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    Recoverability of Schedules

    Consistently the transaction cannot implement entirely directly to a software application problem, framework crash, and hardware failure. During this method, a failed transaction needs to be roll-back. But a few different transactions can also have utilized cost created by way of the failed transaction. Therefore, we further need to roll-back those transactions. This schedule is known as the recoverability of schedules.

    T1 T1’s Buffer Space T2 T2’s Buffer Space Database
    P=5000
    R(P); P=5000 P=5000
    P=P-100; P=4000 P=5000
    R(P); P=4000 P=4000
    P=P+500; P=4500 P=4000
    W(P); P=4500 P=4500
    Commit;
    Failure Point
    Commit;

    The above table displays a schedule with two transactions, T1 reads and writes P, and that value is read and written using T2. T2 commits. But next on, T1 fails. Therefore, we need to be rollback T1. Because T2 has read the value written with the aid of T1, it must also be roll-backed. Hence, we earlier committed that. Therefore, this schedule is called irrecoverable schedules.

    Irrecoverable Schedule

    While Tj is reading the value updated by Ti and Tj is committed earlier than the commit of Ti, the schedule will be irrecoverable.

    T1 T1’s Buffer Space T2 T2’s Buffer Space Database
    P=5000
    R(P); P=5000 P=5000
    P=P-100; P=4000 P=5000
    W(P); P=4000 P=4000
    R(P); P=4000 P=4000
    P=P+500; P=4500 P=4000
    W(P); P=4500 P=4500
    Failure Point
    Commit;
    Commit;

    Table 2 displays a schedule with two transactions, T1 reads and writes P, and that value is read and written with the aid of T2. But next on, T1 fails. So we must rollback T1. Because T2 has reading the value written by using T1, it may still roll-backed. Also, it has no longer committed; we can rollback T2 as well. Therefore, this is recoverable with cascading roll-back.

    Recoverable with Cascading Rollback

    While Tj is read the value restore by Ti and commit of Tj is detain through commit of Ti, this schedule is said to be recoverable with cascading rollback.

    T1 T1’s Buffer Space T2 T2’s Buffer Space Database
    P=5000
    R(P); P=5000 P=5000
    P=P-100; P=4000 P=5000
    W(P); P=4000 P=4000
    Commit;
    R(P); P=4000 P=4000
    P=P+500; P=4500 P=4000
    W(P); P=4500 P=4500
    Commit;

    Table 3 displays a schedule with two transactions, T1 reads and writes P and commits, and that value is read with the aid of T2. Therefore, T1 fails earlier than commit, and no other transaction should read its value. Hence there’s no desire to roll back the new transaction. Therefore, the schedule is said to be a cascadeless recoverable schedule.

    Cascadeless Recoverable Schedule

    While Tj is reading the value updated by way of Ti at most later Ti is committed, the schedule will be cascadeless recoverable.

    T1 T1’s Buffer Space T2 T2’s Buffer Space Database
    P=5000
    R(P); P=5000 P=5000
    P=P-100; P=4000 P=5000
    W(P); P=4000 P=4000
    Commit;
    R(P); P=4000 P=4000
    R(P); P=4000 P=4000
    P=P+500; P=4500 P=4000
    W(P); P=4500 P=4500
    Commit;
    Strict Recoverable Schedule

    In Strict schedule, while the write operation of a transaction anticipate a conflicting operation (read or write operation) of some new transaction later the commit or abort operation of such transaction should further anticipate the conflicting operation of other transaction.

    Example

    Let us say we have two transactions T1 and T2. The write operation of transaction T1 anticipates the read or write operation of transaction T2, therefore the commit or abort operation of transaction T1 must also anticipate the read or write of T2.

    Transaction T1 Transaction T2
    Read(P)
    Read(P)
    Write(P)
    Commit;
    Write(P)
    Read(P)
    Commit;

    Hence the write operation Write (P) of T1 anticipates the conflicting operation (read or write operation) of T2; therefore, the conflicting operation of T2 had to wait for the commit operation of T1.

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