Two-Phase and Three-Phase Commit

The classic protocols for atomic commits across multiple databases — and why modern systems have largely moved away from them.

Key Takeaways

1. 2PC coordinates atomic commits across multiple databases — a coordinator asks all participants to prepare, then commit or abort
2. 2PC is blocking — if the coordinator fails after prepare, participants hold locks indefinitely until it recovers
3. 3PC adds a pre-commit phase to reduce blocking — but still can't handle network partitions correctly
4. Modern systems prefer Sagas over 2PC/3PC — Sagas trade atomicity for availability, using compensating transactions instead of distributed locks