What API-first actually means
API-first development means designing and documenting your API before writing any implementation code. The API contract — the endpoints, request/response schemas, authentication model, and error codes — is the primary deliverable, and implementation is secondary.
This sounds counterintuitive if you are used to building the backend and then figuring out the API surface. But it has significant advantages: - Frontend and backend teams can work in parallel using the API spec as a contract - Integration partners can begin building against your API before the implementation is complete - Edge cases and missing requirements surface in the design phase rather than mid-implementation - The API becomes a consistent, well-documented interface that remains stable as internal implementation evolves
Designing the API contract
Resource modelling
Start by identifying the resources your API exposes. Resources are the nouns of your API: users, courses, orders, invoices, reports. Model them around the consumer's mental model, not your database schema. A consumer who wants a course summary with its author information does not want to make three API calls — they want a single endpoint that returns the composed resource.
RESTful conventions
For CRUD APIs, follow REST conventions consistently. Use plural nouns for collections (GET /courses), HTTP verbs correctly (GET for reads, POST for creates, PUT/PATCH for updates, DELETE for deletes), and meaningful HTTP status codes. Inconsistency in REST conventions is one of the most common sources of API usability complaints.
OpenAPI specification
Write your API contract as an OpenAPI 3.1 specification. OpenAPI is the industry standard for REST API documentation and enables tooling that generates client SDKs, validates request/response schemas against the spec, and produces interactive documentation (Swagger UI, Redoc).
Writing the OpenAPI spec first — before implementation — enforces the API-first discipline and gives you immediate feedback on the coherence of your design.
Authentication and security
JWT vs API keys vs OAuth
For machine-to-machine integrations, API keys are appropriate: simple, easy to rotate, and easy to scope. For user-facing applications, JWT with short expiry is the standard. For third-party integrations where you need delegated access ("allow this app to access my data"), OAuth 2.0 with PKCE is correct.
Do not roll your own authentication. Use a proven implementation or an authentication service (Auth0, Clerk, Supabase Auth). Authentication bugs have serious security consequences.
Rate limiting and quotas
Design rate limiting into the API contract, not as an afterthought. Define rate limits per tier (free, professional, enterprise) and include rate limit headers in every response (X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset). Consumers need to know when they are approaching limits and how to back off gracefully.
Versioning strategy
APIs need a versioning strategy from the first release. Once clients build against your API, breaking changes are costly to coordinate. The two main approaches:
URL versioning
/v1/courses, /v2/courses — the most visible and explicit approach. Easy to route, easy to document, easy to deprecate. The downside: major version increments require clients to update their base URLs.
Header versioning
Clients specify the API version in an Accept-Version header. The URL stays stable; the behaviour changes based on the version header. Cleaner URLs but harder to test in a browser and less visible in logs.
For most cases, URL versioning is the pragmatic choice. Commit to a backwards-compatibility policy (no breaking changes within a major version) and give clients at least 12 months notice before deprecating a version.
Documentation as a product
API documentation is not a deliverable — it is a product that your API consumers use every day. Good API documentation includes: - An overview of authentication, error handling, and pagination patterns (do not make developers read the full reference to find this) - Quickstart guides with complete, runnable code examples - An interactive API explorer (Swagger UI or Redoc) - Changelog that clearly marks breaking changes and deprecations - Code samples in the languages your primary consumers use
Bad API documentation is one of the leading causes of developer frustration and integration abandonment. Treat documentation with the same priority as the API itself.