consensus

Three Phase Commit (3PC)

Non-blocking extension of 2PC: adds pre-commit phase to avoid coordinator-failure blocking.

phaseIDLE
transactions0
Phase 1: can-commit
Phase 2: pre-commit
Phase 3: do-commit
COORDINATOR
○
P1
idle
✓ ok
○
P2
idle
✓ ok
○
P3
idle
✓ ok
Click participants to toggle failure

// transaction log

No transactions yet

How It Works

  • Phase 1: CAN-COMMIT? (voting round)
  • Phase 2: PRE-COMMIT (prepare to commit)
  • Phase 3: DO-COMMIT (finalize)
  • If coordinator fails after pre-commit, participants can recover

vs 2PC

  • + Non-blocking on coordinator failure
  • + Participants can timeout and decide
  • − Higher latency (3 round trips)
  • − More complex, rarely used in practice