Airline retailing APIs often run into the same problems: duplicate bookings from retries, breaking changes that blindside partners, and debugging sessions with no correlation IDs. Idempotency and versioning are not glamorous, but they are what make integrations trustworthy.
Idempotency basics
Any endpoint that changes state-create offer, commit order, apply payment-must accept an idempotency key. The client generates it per logical action; the server stores the result and returns the same response if the request is retried. Persist the key alongside request payload hashes and timestamps. Expire entries only after you are certain downstream processing cannot still act on a duplicate.
- Use a composite key of
{client_id, idempotency_key}. - Store the HTTP status, response body, and any side effects identifiers (order IDs, payment references).
- Return a clear error when the payload changes for the same key-do not silently accept mismatched requests.
Versioning strategy
Versioning starts with telling partners how long you support a schema. Choose between URI versioning (/v1/orders) or header-based negotiation, but be consistent. Publish a calendar: when a new version appears, how long v1 remains supported, and what migration support exists. Provide change logs, sample payloads, and contract tests partners can run.
Backwards compatibility guidelines
- Additive changes (new optional fields) stay within the same version.
- Behavioral changes or mandatory fields trigger a new version.
- Run compatibility suites that replay historical requests against the new implementation to detect surprises.
Observability and diagnostics
Idempotency and versioning only help if you can trace issues. Include correlation IDs in every response and require clients to echo them in support tickets. Emit structured logs that tie the idempotency record to downstream actions (inventory holds, payment calls, event publications).
With disciplined idempotency and a transparent versioning roadmap, partners stop fearing API upgrades. You spend less time firefighting duplicates and more time adding features travelers notice.