2PC vs 3PC
2PC vs 3PC: Two-Phase Commit (2PC) and Three-Phase Commit (3PC) are distributed transaction protocols; 2PC ensures atomic commits but can block on coordinator failure, while 3PC adds a pre-commit phase to reduce blocking and improve fault tolerance.
When to Use
- 2PC: Use when strong consistency is essential, and the network is stable (e.g., within a single data center).
- 3PC: Better for distributed or unreliable networks where availability and non-blocking behavior are priorities.
Example
Think of a bank transfer: 2PC either commits both debit and credit or blocks if the coordinator crashes. 3PC prevents indefinite blocking with an extra pre-commit step.
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Why Is It Important
Understanding these commit protocols helps engineers design reliable distributed systems and answer high-value interview questions confidently.
Interview Tips
Explain both protocols clearly:
- 2PC = prepare + commit/abort.
- 3PC = adds pre-commit to avoid blocking. Highlight trade-offs rather than just definitions.
Trade-offs
- 2PC: Simple but blocking.
- 3PC: Non-blocking but adds latency and complexity.
Pitfalls
- Assuming 2PC never fails—its coordinator can halt the system.
- Believing 3PC is flawless—it depends heavily on reliable timeouts and can be slower.
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