API Documentation
An undocumented API is an unusable API. Even brilliant design means nothing if developers can't figure out how to use it. Good documentation shows what's possible, explains how to get started, and provides reference material for every endpoint. Modern tools make creating and maintaining documentation much easier.
OpenAPI Specification
OpenAPI (formerly Swagger) is the industry standard for describing REST APIs. It's a machine-readable format that tools can use to generate documentation, client libraries, and even mock servers:
openapi: 3.0.0
info:
title: User API
version: 1.0.0
description: API for managing users
paths:
/users:
get:
summary: List all users
description: Returns a paginated list of users
parameters:
- name: status
in: query
description: Filter by user status
schema:
type: string
enum: [active, inactive]
- name: limit
in: query
schema:
type: integer
default: 20
responses:
'200':
description: Successful response
content:
application/json:
schema:
type: object
properties:
data:
type: array
items:
$ref: '#/components/schemas/User'
components:
schemas:
User:
type: object
properties:
id:
type: integer
email:
type: string
format: email
name:
type: string
Documentation Tools
Several tools transform OpenAPI specs into beautiful, interactive documentation:
Swagger UI: The original interactive documentation. Developers can try API calls directly from the docs.
Redoc: Clean, three-panel documentation that's easy to navigate. Great for reference documentation.
Postman: Combines API testing with documentation. Collections can be published as documentation.
What Good Docs Include
Beyond endpoint references, effective documentation provides:
- Getting started guide: How to authenticate and make your first request
- Authentication details: How to obtain and use credentials
- Code examples: In multiple languages when possible
- Error reference: What errors mean and how to handle them
- Rate limiting info: Limits and how to handle throttling
- Changelog: What changed between versions
Keeping Docs Updated
Documentation that's out of sync with the API is worse than no documentation — it actively misleads developers.
Generate from code: Tools can extract OpenAPI specs from code annotations, keeping docs automatically synchronized.
Include in code review: Documentation changes should be part of pull requests for API changes.
Test examples: Automated tests can verify that documentation examples actually work.