Two-Phase Locking (2PL)

Two-Phase Locking is the go-to concurrency control protocol in database systems.

How They Work

Two-Phase Locking (2PL)

Rule: Two distinct phases

Phase Structure:
1. Growing Phase: Acquire locks, no releases
2. Shrinking Phase: Release locks, no acquisitions

Transition: The first unlock marks the switch to the shrinking phase

Two-Phase Locking diagram

Strict 2PL (S2PL)

Rule: Hold all locks until commit/abort

• No shrinking phase during execution
• All locks released atomically at transaction end

Key Benefit: Prevents cascading aborts

Strict Two-Phase Locking diagram

The Cascading Abort Problem

Kitchen Disaster: The Salad Dressing Cascade

Regular 2PL - The Cascade

Strategy: Release ingredients as soon as done

Timeline:
1. Sous-chef1 starts making vinaigrette dressing
2. Sous-chef1 finishes and releases dressing to shared station
3. Sous-chef2 takes the dressing for Caesar salad
4. Sous-chef2 mixes dressing with lettuce and serves
5. Head chef tastes Sous-chef1's dressing: "Too acidic!"
6. Disaster: Sous-chef1 must redo dressing, BUT Sous-chef2's salad already served with bad dressing!

Result: Both dishes ruined - cascading failure

Strict 2PL - No Cascade

Strategy: Keep everything until approved

Timeline:
1. Sous-chef1 starts making vinaigrette dressing
2. Sous-chef1 finishes but keeps dressing until approved
3. Sous-chef2 wants dressing but waits...
4. Head chef tastes: "Too acidic!"
5. Sous-chef1 fixes dressing privately, then releases
6. Success: Sous-chef2 gets perfect dressing for salad

Result: Only good dressing ever leaves the station

The Point: S2PL ensures others only see your "approved" (committed) work - like waiting for head chef approval before sharing ingredients.

Lock Sequences: 2PL vs S2PL

Regular 2PL - Cascading Abort

Key Insight: Regular 2PL allows dirty reads via early lock release. S2PL holds locks until transaction end, preventing dirty reads and cascading aborts.

2PL and S2PL for Concurrency

✅ Benefits

⚠ Costs