Page 3: Two-Phase Locking (2PL)

The Two-Phase Rule

Like a shopping trip - first you collect items (growing phase), then you check out and leave (shrinking phase).

Phase 1 - Growing Phase:

• Can acquire new locks

• Cannot release any locks

Phase 2 - Shrinking Phase:

• Can release locks

• Cannot acquire new locks

2PL Example:

CopyTransaction T5:
Lock-S(A)     ← Growing phase
Lock-X(B)     ← Growing phase  
Unlock(A)     ← Shrinking phase (first unlock = phase change)
Unlock(B)     ← Shrinking phase

1. 2PL does not prevent deadlocks:

2. ensures conflict-serializability.

3. it does not guarantee cascadeless schedules.

2PL Variants

1. Basic 2PL:

• Ensures serializability but allows cascading rollbacks

• Like dominoes falling - one failure causes others to fail

2. Strict 2PL:

• Hold ALL exclusive locks until transaction commits

• Prevents cascading rollbacks

• Most commonly used in practice

3. Rigorous 2PL:

• Hold ALL locks (shared + exclusive) until commit

• Maximum safety, but reduced concurrency