Two-Phase Locking (2PL)

A concurrency control protocol that guarantees serializability using locks.

Core Idea: A transaction must acquire ALL locks before releasing ANY lock.

The Two Phases

Phase 1 — Growing Phase

Transaction can ONLY acquire locks. Cannot release any lock yet.

Phase 2 — Shrinking Phase

Transaction can ONLY release locks. Cannot acquire any new lock.

-- GROWING PHASE
T1: Lock(A)   T1: Lock(B)   T1: Read(A)   T1: Write(B)

-- LOCK POINT (peak — no more acquisitions after this)

-- SHRINKING PHASE
T1: Unlock(A)   T1: Unlock(B)
-- T1 cannot acquire any new lock after first unlock

3 Variants of 2PL

Variant	Release Write Locks	Release Read Locks	Cascading Rollback
Basic 2PL	Anytime in shrinking	Anytime in shrinking	Possible
Strict 2PL	Only at COMMIT	Anytime in shrinking	Prevented
Rigorous 2PL	Only at COMMIT	Only at COMMIT	Prevented

The Big Problem with 2PL — Deadlock

T1 holds Lock(A) → wants Lock(B)
T2 holds Lock(B) → wants Lock(A)
Both waiting forever → DEADLOCK

How DBs handle deadlocks

• Deadlock Detection → Build a wait-for graph; if cycle detected → abort one transaction
• Deadlock Prevention → Assign priorities by timestamp; younger transaction always aborts (wound-wait)
• Timeout → If waiting too long → assume deadlock → abort

2PL vs MVCC

	2PL	MVCC
Mechanism	Locks	Versions
Readers block writers?	Yes	No
Deadlocks possible?	Yes	Rare
Serializability	Yes	Needs SSI
Used in	Older systems	Modern DBs