Design distributed transactions protocol

Q: Design distributed transactions protocol

This is a System Design interview question from Google for Software Engineer roles. View the full question and solution on PracHub.

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Question

System Design: Distributed Transactions Across Multiple Services

Context

You must design a distributed transactions protocol to coordinate updates across multiple services or databases in a microservices environment. The network is unreliable (messages can be delayed, duplicated, or lost), services can crash and recover, and latency and availability matter.

Assume services communicate over RPC and/or a message bus. Some services may support a "prepare/commit" primitive; others may not.

Task

Propose a protocol to coordinate multi-service updates.
Compare and contrast the following approaches:
- Two-Phase Commit (2PC)
- Three-Phase Commit (3PC)
- Saga (orchestration and/or choreography)
For each approach, specify:
- Message flow (step-by-step)
- Failure handling (coordinator crash, participant crash, message loss/duplication, partition)
- Idempotency mechanisms
- Timeouts and retries
- Delivery guarantees (exactly-once vs at-least-once) and what you can realistically achieve
Conclude with a recommendation for when to use each approach and how to implement it safely.

Design distributed transactions protocol

System Design: Distributed Transactions Across Multiple Services

Context

Task

Solution (Locked)

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