Page 163 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

X(A)
R(A)
A: = A-100; T1 T2
W(A)
X (B)
R(B)
Database Management Systems/Managing Database
B: = B + 100;
W (B)
Commit
Notes X(A)
R(A)
A: = A + 0.06 A;
W(A)
X(B)
R(B)
B: = B + 0.06B;
W(B)
Commit

Schedule Illustrating Strict 2PL with Serial Execution

Moreover, the following figure illustrates a schedule following Strict 2PL with interleaved
actions, where shared lock on A can only read A, but cannot write A and both T and T can hold
1 2
shared lock at the same time whereas an exclusive lock on B and C can both read and write the
respective B and C but an exclusive lock cannot be held at the same time, instead an exclusive
lock must hold each data item serially one after the other.

T1 T2
S(A)
R(A)
S(A)
R(A)
X(B)
R(B)
W(B)
Commit
X(C)
R(C)
W(C)
Commit

Task Process to test serializability.

9.10 Timestamp based Protocol

In lock-based concurrency control, conflicting actions of different transactions are ordered by
the order in which locks are obtained, and the lock protocol extends this ordering on actions to
transactions, thereby ensuring serializability. In optimistic concurrency control, a timestamp
ordering is imposed on transactions, and validation checks that all conflicting actions occurred
in the same order.

Timestamps can also be used in another way: each transaction can be assigned a timestamp at
startup, and we can ensure, at execution time, that if action a of transaction T conflicts with
i i
action a of transaction T , ai occurs before a if TS(T ) < TS(T ). If an action violates this ordering,
j j j i j
the transaction is aborted and restarted.

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