System Design Article

Distributed Transactions (2PC, Saga Pattern)

Difficulty: Hard

When a single business operation spans multiple services or databases, you cannot rely on a single ACID transaction. This lesson covers the two dominant patterns for keeping consistency across services: Two-Phase Commit (2PC) for synchronous, atomic, blocking transactions, and the Saga pattern (orchestration vs choreography) for long-running asynchronous workflows with compensating actions. We also cover Three-Phase Commit, idempotency keys, the outbox pattern, and the trade-offs that explain why 2PC is rare in microservices and Sagas are everywhere. By the end you can pick the right pattern for an order checkout, a money transfer, or a multi-step booking flow.