Labs 9 - Recovery algorithm.
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Exercises
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(1) With the starting values A=100, B=200, C=300, write the log file for the
    following schedule. What happens during the recovery according to the 
    recovery algorithm? Specify the resulting compensation log records.
    
    T1:         T2:         T3:
                start
    start
                            start
                            read(C)
                read(B)
    read(A)
    A:=A-20
                            C:=C+40
                            write(C)
                B:=B-80
    write(A)
                write(B)
    commit
    ------------CRASH---------------

(2) [Ex. 16.18 from textbook] Consider the following log. What happens during
    the recovery according to the recovery algorithm? Mark the start and end
    points of the redo and undo phases. Specify the transactions in the undo
    list.

    <T0, start>
    <T0, B, 2000, 2050>
    <T1, start>
    <checkpoint, {T0, T1}>
    <T1, commit>
    <T2, start>
    <T2, A, 500, 400>
    <T0, B, 2000>
    --------CRASH---------

(3) With the starting values A=3, B=100, write the log file for the following 
    schedule. What happens during the recovery according to the recovery 
    algorithm when the system crashes (a) after commit of T3, (b) after write(B)
    of T1? Mark the start and end points of the redo and undo phases. Specify
    the resulting compensation log records.
    
    T1:         T2:         T3:
                            start
                            read(A)
                            A:=A+1
    start
    read(B)
    B:=B+10
                            write(A)
                            commit
                start
                read(A)
                read(B)
                A:=A+B
                write(A)
                commit
    ------------CHECKPOINT----------
    read(A)
    A:=A+B
    write(B)
    commit