Types of Locks
There are two necessary types of locks:
- Binary lock: This locking mechanism has two states for to a data item: locked or unlocked
- Multiple-mode locks: In this locking type every data item can be in 3 states read locked or shared locked, write locked or exclusive locked or unlocked.
Let us first take an instance for binary locking and describes how it solves the concurrency related troubles. Let us reconsider the transactions T1 and T2 for this purpose; though we will add to needed binary locks to them.
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Schedule
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T1
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T2
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Lock X
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Lock X
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Read X
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Read X
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Subtract 100
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Subtract 100
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Write X
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Write X
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Unlock X
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Unlock X
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Lock X
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Lock X
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Lock Y
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Lock Y
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Read X
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Read X
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Read Y
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Read Y
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Display X+Y
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Display X+Y
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Unlock X
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Unlock X
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Unlock Y
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Unlock Y
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Lock Y
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Lock Y
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Read Y
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Read Y
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Add 100
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Add 100
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Write Y
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Write Y
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Unlock Y
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Unlock Y
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Figure: An incorrect locking implementation
Does the locking as completed above solve the problem of concurrent transactions? No the similar problems still remains. Try working with the old value. Therefore, locking should be completed with some logic in order to make sure that locking results in no concurrency related problem. One such answer is given below:
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Schedule
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T1
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T2
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Lock X
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Lock X
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Lock Y
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Lock Y
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Read X
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Read X
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Subtract 100
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Subtract 100
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Write X
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Write X
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Lock X (issued by
T2)
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Lock X: denied as T1 holds the lock.
The transaction T2 Waits and T1 continues.
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Read Y
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Read Y
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Add 100
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Add 100
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Write Y
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Write Y
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Unlock X
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Unlock X
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The lock request of T2 on X can now
be granted it can resumes by locking X
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Unlock Y
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Unlock Y
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Lock Y
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Lock Y
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Read X
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Read X
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Read Y
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Read Y
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Display X+Y
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Display X+Y
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Unlock X
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Unlock X
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Unlock Y
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Unlock Y
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Figure: A correct but restrictive locking implementation
Therefore, the locking as above when you get all the locks at the starting of the transaction and release them at the end ensures that transactions are implemented with no concurrency related problems. Though, such a scheme limits the concurrency. We will talk about a two-phase locking method in the next subsection that gives sufficient concurrency. Though, let us first discuss multiple mode locks.