How do you approach API versioning and backward compatibility?

Long Answer

Treat API versioning and backward compatibility as a product promise backed by engineering guardrails. Your aim is to let teams ship new capabilities quickly while legacy clients keep working—no surprises, no “gotchas,” and no midnight firefights. The playbook blends strategy (how you version and deprecate), mechanics (tests, tooling), and operations (safe rollouts, observability, communications).

1) Versioning strategy. Choose explicit negotiation and stick to it. Path versions like /v1 are simple, cache-friendly, and easy to document; media-type or header versions (e.g., Accept: application/vnd.acme.v2+json) help when multiple representations must coexist. Pair the API approach with semantic versioning in SDKs so “major = break,” “minor = feature,” “patch = fix.” Keep resource semantics stable: never change meanings; instead add optional fields, new endpoints, or new media types. Document defaults and ranges so tolerant readers can ignore unknowns while strict writers validate inputs.

2) Deprecation policy. Publish SLAs (e.g., 12–18 months). Mark deprecated fields in OpenAPI; emit Deprecation and Sunset headers with dates; expose a changelog/status endpoint. Ship migration guides and code-mod scripts for official SDKs. Track who uses what with per-version telemetry so you can target outreach. Deprecations aren’t vibes; they’re contracts with clocks.

3) Compatibility techniques. Prefer additive evolution—new fields with safe defaults, query params behind feature flags, or separate endpoints for fundamentally different behavior. For pagination, keep cursors backward-compatible and never repurpose tokens. Treat enums as open: accept unknown values, map them to “other,” and never overload an existing symbol. Be explicit about time zones, locales, and number formats; use durable IDs rather than positional meaning. Where behavior must differ, keep compatibility shims or feature toggles server-side so old clients still see the world they expect.

4) Contracts and tests. Your schema is a contract—treat it like code. Run OpenAPI/AsyncAPI diffing in CI to block accidental breaking changes. Add consumer-driven contract testing (e.g., Pact) so producers must satisfy the expectations of real clients. Spin up synthetic legacy clients that exercise deprecated paths and quirky edge cases. Shadow real production traffic to a new version, compare responses and error codes, and only then ramp live traffic.

5) Safe rollout mechanics. Route by version in an API gateway (or service mesh), enabling per-route canaries and rolling upgrades. Keep at least N-2 versions live during migration and pin partner traffic to known versions. Automate progressive delivery: 1% → 10% → 25% → 50% → 100%, with automatic rollback on SLO breaches (error ratio, p95 latency, schema-validation failures). For data shape changes, dual-write, backfill, and ensure idempotency so retries don’t double-charge. Blue-green or canary deploys keep the blast radius tiny when something goes sideways.

6) Observability and governance. Tag logs/metrics with api.version, client.id, and sdk.version. Build dashboards that show adoption curves, remaining legacy load, and the cost of keeping old code paths alive. Set per-version error budgets so you know when legacy pain outweighs value. Establish a review board and lightweight ADRs for changes; proposals include a compatibility analysis, rollout plan, and customer comms. Governance shouldn’t be red tape; it’s the guardrail that keeps ships off the rocks.

7) Communication and developer experience. Ship migration guides with before/after payloads, diff tables, and “copy-paste” examples. Provide sandboxes and preview environments so clients can test early. Announce deprecations via email, status pages, RSS, and SDK release notes. During the window, maintain the old behavior with shims and support office hours for high-value integrators. When the sunset date arrives, enforce it—gracefully—backed by months of evidence and outreach.

Do this well and your API versioning and backward compatibility become boring, predictable infrastructure—leaving teams free to chase features instead of firefighting.

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Table

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Common Mistakes

Sneaking breaks into “minor” releases and calling them bug fixes. Renaming fields or flipping semantics instead of additive changes with defaults. Spinning up /v2 without a deprecation policy, usage analytics, or outreach—then deprecating blind. No schema diffing or consumer contract testing, so regressions ship to prod. Forcing hard-cut switchoffs before partners are ready. Repurposing pagination tokens or enum values, breaking tolerant readers. Overloading a single field with multiple meanings. Fuzzy time/locale formats that differ by version. No per-version telemetry, so teams can’t see who still depends on legacy paths and end up guessing—then breaking critical clients during a promo.

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Sample Answers (Junior / Mid / Senior)

Junior:
“I expose /v1 and evolve through additive fields and endpoints. When I deprecate, I mark it in OpenAPI, publish notes, and keep a compat path so clients migrate safely.”

Mid:
“I support path or header negotiation, use semantic versioning in SDKs, and run OpenAPI diff checks in CI. We add Sunset/Deprecation headers and give a 12–18-month window. Rollouts go through the API gateway with canaries and rolling upgrades; we track adoption per version.”

Senior:
“Our API versioning and backward compatibility policy keeps N-2 versions live, with shims for legacy behavior. We default to additive change; majors land only with migration guides, sandboxes, and office hours. CI blocks breaking diffs; Pact tests guard consumers; shadow traffic precedes ramp. Telemetry tagged by api.version and sdk.version drives deprecation and retirement.”

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Evaluation Criteria

Strong answers treat API versioning and backward compatibility as policy + tooling. Look for explicit negotiation (path or header), additive evolution, and a published deprecation policy with Sunset/Deprecation headers. Expect semantic versioning for SDKs, OpenAPI diffing, and consumer contract testing. Safe rollout mechanics include canaries, rolling upgrades, blue-green, and gateway routing with auto-rollback on SLO breaches. They mention dual-writes/backfills for data, idempotency for retries, and per-version telemetry showing adoption and risk. Communication matters: migration guides, examples, calendars, and office hours. Weak answers hand-wave with “just ship /v2” and skip governance, tests, and comms.

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Preparation Tips

Build a demo with /v1 and /v2 behind an API gateway. Support path and header negotiation; generate separate OpenAPI specs. Wire CI to fail on breaking schema diffs; add consumer contract testing for two mock clients. Emit Sunset/Deprecation headers and host a deprecation page with dates and examples. Create a progressive delivery pipeline with canaries and rolling upgrades; enable auto-rollback on error/latency spikes. Tag logs/metrics with api.version, client.id, sdk.version, and chart adoption curves. Practice a data migration using dual-writes and idempotent request IDs, then backfill safely. Write a crisp migration guide and comms template aligned to your deprecation policy and semantic versioning rules.

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Real-world Context

A payments platform ran N-2 versions and emitted Sunset headers; dashboards showed /v1 falling under 2%, enabling a calm retirement. A SaaS vendor used OpenAPI diffing plus Pact to catch a breaking enum change hours before release. A marketplace shadowed /v2 for a week; response deltas exposed a tax rounding issue, fixed before partners noticed. Another team launched a major with sandboxes, code-mods, and office hours; migrations finished early, support tickets dropped, and the deprecation policy gained credibility. Across these rollouts, API versioning and backward compatibility—backed by semantic versioning, contracts, and gateway canaries—turned a risky rewrite into a boring, repeatable upgrade.

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Key Takeaways

• Default to additive change; reserve majors for real breaks.
  
• Use clear version negotiation and semantic versioning in SDKs.
  
• Enforce schemas with diffing and consumer contract testing.
  
• Ship via gateway canaries and rolling upgrades, with auto-rollback.
  
• Publish a firm deprecation policy and track adoption per version.
  
  

Practice Exercise

Scenario:
You own a high-traffic Orders API used by partners, mobile apps, and internal services. A pricing overhaul requires breaking response fields and a new pagination scheme. You must ship /v2 without torching existing integrations.

Tasks:

1. Define /v2 with additive fields first; isolate true breaks. Publish OpenAPI and a migration guide showing before/after payloads.
   
2. Enable version negotiation via path and header. Add Sunset/Deprecation headers to /v1 with a 12-month window and a deprecation page.
   
3. Wire CI with OpenAPI diff checks; add consumer contract testing for two key partners and the mobile client.
   
4. Roll out through the API gateway with canaries and rolling upgrades: 1%→10%→25%→50%→100%, auto-rollback on SLO breaches.
   
5. Shadow production traffic to /v2, compare responses/errors, and fix deltas before ramp.
   
6. For data changes, dual-write to new tables, backfill history, and enforce idempotent request IDs to survive retries.
   
7. Tag telemetry with api.version, client.id, sdk.version; build dashboards for adoption, p95 latency, and error ratios per version.
   
8. Execute your deprecation policy: send notices, host office hours, and ship SDK updates with code-mods.
   

Deliverable:
Dashboards + a brief postmortem proving safe /v2 adoption, stable latency, and a clean retirement plan for /v1, demonstrating disciplined API versioning and backward compatibility.