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Explain schedules in 2PL with multiple granularities locking and schedules under tree(DBMS)
Locking and unlocking of the database should be done in such a way that there is no inconsistency, deadlock and no starvation.
2PL locking protocol
Every transaction will lock and unlock the data item in two different phases.
Growing Phase − All the locks are issued in this phase. No locks are released, after all changes to data-items are committed and then the second phase (shrinking phase) starts.
Shrinking phase − No locks are issued in this phase, all the changes to data-items are noted (stored) and then locks are released.
Consider a tree structure database as follows −
A -> B -> C
Let us now learn about the schedules which are not possible under the 2PL with multiple granularities locking and the schedules which are possible under tree protocol.
The first lock by a transaction Ti can be any item on the graph.
Then a data item X can be locked by the transaction Ti given that the parent of X is presently locked by Ti.
Data items which are locked before can be unlocked at any time.
| T1 | T2 |
|---|---|
| lock(A) | |
| lock(B) | |
| Unlock(A) | |
| lock(A) | |
| lock(C) | |
| Unlock(B) | |
| lock(B) | |
| Unlock(A) | |
| Unlock(B) | |
| Unlock(C) |
Here,
In transaction T1
- The Growing Phase is from steps 1-3.
- The Shrinking Phase is from steps 5-6.
- Lock Point at 3.
Now, let us understand the schedules which are possible under the 2PL with multiple granularities locking and the schedules which are not possible under tree protocol.
- It is performed in two phases.
- Locking is done in the growing phase.
- Unlocking is done in the shrinking phase.
- Lock point is the point at which the growing phase ends.
| T1 | T2 |
|---|---|
| lock(A) | |
| lock(B) | |
| lock(C) | |
| Unlock(B) | |
| Unlock(A) | |
| Unlock(B) |
Here,
In transaction T2
- The Growing Phase is on step 2.
- Shrinking Phase is on step 4.
- Lock Point at 2
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