How do you evolve a public API without breaking existing clients?
Reviewed by Gurusankar M.
⚡ Short Answer
Change additively: add optional fields/endpoints, never remove or repurpose existing ones, keep defaults stable, and tolerate unknown fields. For unavoidable breaks, introduce a new version, support both during a published deprecation window, and communicate via docs + Deprecation/Sunset headers.
☕Coffee Chat Question
Concept Made Simple
“How do you evolve a public API without breaking existing clients?”
🧠Mind Map Answer
Remember It Faster
🔥What If?
Think Beyond the Expected
You must remove a field thousands of clients still use — what's the rollout?
Don't remove it abruptly. Mark it deprecated in docs and via Deprecation/Sunset headers, monitor who still uses it (telemetry), reach out to those consumers, keep it working through the announced window, then remove only after usage drops — ideally behind a new version.
😂Real World
Stripe/GitHub-style API longevity comes from additive evolution + clear deprecation policies; the costliest API mistakes are silent breaking changes that take down integrations.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗Removing/renaming fields in place
- ✗Repurposing an existing field's meaning
- ✗No deprecation communication
✅Best Practices
- ✓Evolve additively; tolerate unknown fields
- ✓Publish + signal deprecations (Sunset header)
- ✓Use contract tests + usage telemetry
🔁Follow-up Questions
- 1What is the 'tolerant reader' principle?
- 2How do consumer-driven contract tests help?
- 3How do you measure who still uses a deprecated field?
🧩Related Technologies
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Plain-language foundations
I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: Versioning (REST APIs) Interview question: "How do you evolve a public API without breaking existing clients?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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