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Exclusive lock
Let us explain the above two modes with the help of an instance. We will once again consider the transactions T1 and T2 but in addition a transaction T11 that search the total of accounts Y and Z.
Schedule
T1
T2
T11
S_Lock X
S_Lock Y
Read X
S_Lock Z
Read Y
Read Z
X_Lock X
X_Lock X. The exclusive lock request on X is denied as T2 holds the Read lock. The transaction T1 Waits.
Display X+Y
Unlock X
X_Lock Y
X_Lock Y. The previous exclusive lock request on X is granted as X is unlocked. But the new exclusive lock request on Y is not granted as Y is locked by T2 and T11 in read mode. Thus T1 waits till both T2 and T11 will release the read lock on Y.
Display Y+Z
Unlock Y
Unlock Z
Subtract 100
Write X
Add 100
Write Y
Figure: Example of Locking in multiple-modes
Therefore, the locking as above results in a serialisable schedule. Now the query is can we release locks a bit early and have still no concurrency related problem?Yes, we can do it if we lock using two-phase locking protocol.
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