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Two-Phase Locking 2PL
Last Updated : 21 May, 2026
Two-Phase Locking 2PL is a concurrency control protocol that guarantees conflict serializability by organizing all lock operations into exactly two phases:
Unit 4103 words9 headingsExamples included
Recap
Two-Phase Locking (2PL) is a concurrency control protocol that guarantees conflict serializability by organizing all lock operations into exactly two phases:
Phase 1 — Growing Phase: Acquire any locks, release none
Phase 2 — Shrinking Phase: Release any locks, acquire none
Why 2PL Ensures Serializability
The lock point of a transaction determines its position in the equivalent serial order.
Theorem: Any schedule produced by transactions following 2PL
is conflict serializable.
Proof sketch:
If T1 reaches its lock point before T2, then in any conflict
between T1 and T2, T1's operation comes before T2's.
→ T1 precedes T2 in the equivalent serial order.
→ No cycles in precedence graph → serializable.
Worked Example — 2PL Schedule
| T1 | Transfer ₹1000 from A to B |
| T2 | Transfer ₹500 from B to C |
| T1 | T2: |
Problems with Basic 2PL
Cascading Rollbacks
| T1 ROLLBACK | X must revert to old value |
| T2 used dirty value | T2 must also ROLLBACK! |
| If T3 read T2's write | T3 must rollback too! |
Strict 2PL solves this by holding X-locks until commit.
Strict 2PL (Most Used in Practice)
Rule: Hold all EXCLUSIVE locks until COMMIT or ROLLBACK.
Shared locks may still be released early.
Benefits:
1. No cascading rollbacks (no transaction reads dirty data)
2. Conflict serializable
3. Simpler recovery
Most commercial DBMS (Oracle, MySQL InnoDB, PostgreSQL) use Strict 2PL.
Strict 2PL Example
T1 (Strict 2PL):
lock-X(A) ← exclusive lock acquired
read(A)
write(A)
← X-lock on A held throughout ─────────────────────┐
lock-X(B) │
read(B) │ Cannot release
write(B) │ until commit
← X-lock on B held ────────────────────────────────┤
COMMIT ← unlock(A), unlock(B) released here only ┘
Deadlock with 2PL
2PL can lead to deadlocks — two or more transactions waiting for each other's locks indefinitely.
T1 holds lock-X(A), waits for lock-X(B)
T2 holds lock-X(B), waits for lock-X(A)
T1 ──► waiting for B ──► T2
T2 ──► waiting for A ──► T1
CIRCULAR WAIT → DEADLOCK
(Deadlocks are handled in the Deadlock topic)
2PL Summary
| Basic 2PL | Strict 2PL | Rigorous 2PL |
|---|---|---|
| S+X released | X locked till | S+X locked till |
| in shrink phase | commit/abort | commit/abort |
| Serializable ✓ | Serializable ✓ | Serializable ✓ |
| Cascading ✗ | No cascading ✓ | No cascading ✓ |
| Dirty reads ✗ | No dirty reads ✓ | No dirty reads ✓ |
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Two-Phase Locking 2PL.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this DBMS topic.
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