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, transaction boundaries, integration tests, race conditions, contract testing, 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
One API request needs a clear database commit boundary.
Database Transaction Boundaries in Backend APIs
Tests pass, but API behavior breaks when real persistence or auth is involved.
How to Write API Integration Tests
A response, error, schema, or request contract changed and broke existing clients.
API Contract Testing: Prevent Breaking Clients Before Release
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, redeliveries, or out-of-order events can repeat the same business effect.
Webhook Idempotency and Retries in Production
These articles cover the main places API behavior becomes unsafe: retries, concurrency, transaction boundaries, testing gaps, contract drift, 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, contract checks, and race-condition thinking to verify behavior across HTTP, persistence, retries, clients, and shared state.
Build integration tests that exercise real request boundaries, persistence, auth, failure paths, and concurrency.
A practical guide to API integration testing with real request boundaries, auth, persistence checks, failure cases, idempotency, concurrency, and response contracts.
Define the API write boundary so one request commits the right state without mixing external calls, retries, or slow work into the transaction.
How to choose database transaction boundaries in backend APIs so writes stay atomic without holding locks across external calls, slow work, retries, or post-commit side effects.
Catch breaking request, response, error, and schema changes before release with OpenAPI diffs, consumer examples, and CI compatibility gates.
How API contract testing catches breaking request, response, error, and schema changes before release with OpenAPI diffs, consumer examples, and CI compatibility checks.
Understand where overlapping requests, duplicate delivery, and shared records break otherwise reasonable code.
How to prevent race conditions in backend systems by naming the invariant, moving correctness to durable boundaries, and testing overlapping requests, retries, and jobs.
Keep clients and integrations safe when APIs evolve, providers retry, and downstream work happens outside the request.
Plan incompatible API changes with compatibility rules, side-by-side versions, telemetry, deprecation signals, and migration gates.
How to version APIs without breaking existing clients, including compatibility policy, breaking-change examples, staged v1 to v2 rollout, deprecation headers, telemetry, and release gates.
Receive duplicate and out-of-order webhook deliveries safely with signed raw-body verification, durable receipts, fast acknowledgement, and idempotent workers.
How to handle webhook idempotency and retries in production with durable receipt records, atomic deduplication, fast acknowledgements, idempotent workers, and replay-safe side effects.
Idempotency protects a retried write from becoming a duplicate side effect. Transaction boundaries decide which state one API request commits together. Integration tests prove that behavior through the real request and persistence boundary. Contract tests protect the request, response, error, and schema promises clients depend on. 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.