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Give an example where strict 2 phase locking is followed but the resulting schedule leads to deadlock Explain why the 2PL being followed here is strict and explain why there is deadlock?
Wiki User
∙ 7y ago
- T1: Write X // This acquires an exclusive lock: LockX
- T2: Write Y // This acquires an exclusive lock: LockY
- T2: Read X // This attempts to acquire a shared lock on X, but since X is locked already by T1, T2 is subsequently blocked by the system until X is freed.
- T1: Read Y // This attempts to acquire a shared lock on Y, but since Y is locked already by T2, T1 is subsequently blocked by the system until Y is free.
- T1: Commits // This should release all locks held by T1.
- T2: Commits // This should release all locks held by T2.
* Note that "block" means the transaction will not proceed until the specified condition is met.
Now, since T2 is waiting for T1 to free X AND T1 is waiting for T2 to free Y, we get into a cycle of condition checking. These conditions will never be met because both transactions are waiting for the other one to do something, in order to proceed itself. Therefore we have a deadlock between T1 and T2 trying to access the shared resources X and Y.
The Wikipedia article on Two-phase locking may help you understand why this is strict 2-phase locking and what 2-phase locking is.
Wiki User
∙ 7y ago
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