2PL(two-phase locking)
2PL is a concurrency control method that guarantees serializability.
There are two major types of locks used in 2PL:
- Write-lock(exclusive lock)
- Read-lock(shared lock)
The compatibilities of the two kinds of locks are as follows:
| Lock type | Shared | Exclusive |
|---|---|---|
| Read(Shared) | Compatible | Not Compatible |
| Write(Exclusive) | Not Compatible | Not Compatible |
By the 2PL protocol, locks are applied and removed in two phases:
- Expanding phase: locks are acquired and no locks are released.
- Shrinking phase: locks are released and no locks are acquired.
Pay attention to the second phase, when a transaction releases a lock, it can't acquire neither Shared Locks nor Exclusive Locks anymore.
This rule ensures the serializability property between conflict transactions.