Topic Hub
API correctness is the difference between an endpoint that works during a clean demo and an endpoint that still behaves predictably when clients retry, requests overlap, providers redeliver events, and older clients keep using yesterday's contract.
This hub collects CodeNotes articles about the parts of API design that usually fail at the boundaries: duplicate writes, idempotency keys, integration tests, race conditions, webhook delivery, and versioning. The common thread is simple: an API is correct only when its behavior stays safe under real timing, real storage, and real client behavior.
Start from the failure mode you are trying to prevent, then follow the article that matches that boundary.
A client retried a POST request and created duplicate writes.
API Idempotency Keys: Prevent Duplicate Requests Safely
Tests pass, but API behavior breaks when real persistence or auth is involved.
How to Write API Integration Tests
Two requests can update the same resource at the same time.
How to Prevent Race Conditions in Backend Systems
Changing the API response might break clients you do not control.
API Versioning Without Breaking Clients
Webhook retries can process the same event more than once.
Webhook Idempotency and Retries in Production
These articles cover the main places API behavior becomes unsafe: retries, concurrency, testing gaps, external delivery, and contract evolution.
Begin with the duplicate-request boundary. It is one of the clearest places where ordinary retries become correctness bugs.
Design idempotency keys, request fingerprints, TTL, in-progress states, and safe response replay.
How to design API idempotency keys for retries, duplicate POST requests, payload mismatch, TTL, in-progress requests, and safe response replay.
Use tests and race-condition thinking to verify behavior across HTTP, persistence, retries, and shared state.
Build integration tests that exercise real request boundaries, persistence, auth, failure paths, and concurrency.
How to write API integration tests that verify real persistence, auth rules, contracts, idempotency, concurrency, and failure handling across request boundaries.
Understand where overlapping requests, duplicate delivery, and shared records break otherwise reasonable code.
How to prevent race conditions in backend systems using constraints, locking, idempotency, and concurrency-aware testing instead of lucky timing.
Keep clients and integrations safe when APIs evolve, providers retry, and downstream work happens outside the request.
Evolve API contracts without breaking clients through compatibility rules, staged rollout, and deprecation policy.
How to evolve APIs without breaking clients through explicit compatibility rules, staged rollouts, and deprecation workflows that teams can enforce.
Receive webhook retries safely with durable deduplication, fast acknowledgement, and idempotent downstream work.
How to handle webhook retries safely in production by verifying signatures, deduplicating atomically, acknowledging quickly, and making downstream processing idempotent.
Idempotency protects a retried write from becoming a duplicate side effect. Integration tests prove that behavior through the real request and persistence boundary. Race condition handling protects shared state when different requests compete. Versioning protects clients when contracts evolve. Webhook deduplication extends the same correctness model to external event delivery.
These are not separate best-practice checklists. They are different views of the same engineering problem: API behavior must remain reliable when the clean request-response story stops being clean.