consensus

Two Phase Commit (2PC)

Atomic distributed transactions: Phase 1 (Prepare/Vote) → Phase 2 (Commit or Abort).

phaseIDLE
transactions0
participants3
Phase 1: prepare
Phase 2: commit/abort
COORDINATOR
waiting
↓ ... ↓
○
P1
vote: pending
✓ will vote YES
○
P2
vote: pending
✓ will vote YES
○
P3
vote: pending
✓ will vote YES
Click participants to toggle failure (before starting)

// transaction log

No transactions yet

How It Works

  • Phase 1: Coordinator asks all to prepare (vote)
  • If ALL vote YES → Phase 2: COMMIT
  • If ANY votes NO → Phase 2: ABORT
  • Blocking protocol: coordinator failure blocks all

Tradeoffs

  • + Atomic: all-or-nothing guarantee
  • − Blocking: coordinator is single point of failure
  • − Latency: 2 round trips minimum
  • Used in: distributed databases, XA transactions