API design

REST

Resource, not action

URLs name nouns. Actions are verbs.

Good: POST /users/:id/password-reset Bad: POST /resetUserPassword?id=123

Status codes mean things

• 200 success with body
• 201 created (include Location)
• 204 success, no body
• 400 client sent something invalid (bad syntax, bad shape)
• 401 not authenticated
• 403 authenticated but not authorized
• 404 resource doesn't exist (or the caller isn't allowed to know it does)
• 409 conflict (version conflict, duplicate, precondition failed)
• 422 semantic validation failed (body parses but rules reject)
• 429 rate limited
• 5xx server fault — internal detail logged, opaque to caller

Error envelope

Pick one. Use it everywhere.

{
  "error": {
    "code": "resource_not_found",
    "message": "User alice@example.com not found.",
    "requestId": "abc123"
  }
}

• code is machine-readable, stable, lowercase_snake.
• message is human-readable. Safe to surface.
• requestId lets the caller tell you what went wrong without you reading prod logs.

Pagination

Cursor-based for anything you'll scale. Offset paging breaks under concurrent writes.

GET /messages?cursor=abc&limit=50
→ { items: [...], nextCursor: "xyz" }

Idempotency

Any POST that has side-effects and will be retried needs an idempotency key:

POST /payments
Idempotency-Key: <uuid-v4>

Server stores the (key → response) for a reasonable TTL. Retries return the cached response, not a second charge.

Versioning

Prefer additive changes. Add fields; don't remove or rename. When you must break, version:

• URL (/v2/...) — simple, explicit, slightly ugly
• Accept header (Accept: application/vnd.foo.v2+json) — prettier URLs, slightly opaque

Pick one per service. Never mix.

GraphQL

Nullability is a contract

String! says "this WILL be present". If you're not sure, it's String (nullable). A lie here cascades.

Relay-style pagination

Connections, edges, pageInfo, cursors. Don't invent your own.

One mutation per intent

updateUser(input: { name, email, password }) is three different operations glued together. Split them.

Errors as types

Return { user: User | UserNotFoundError | ValidationError } unions where it matters. Generic errors[] at the top is for transport-level fault, not business rules.

Across both

• Typed shapes. Schemas (OpenAPI / GraphQL / proto). Generate clients from them.
• Rate limit expensive or abuse-prone endpoints.
• Observability. Every request has a requestId. Log it. Return it on errors. Tie logs, metrics, and traces together with it.
• Auth at the edge — check in the handler or middleware the handler trusts. Never rely on the gateway alone.