fastify-api-design

Fastify API Design

Overview

Keep the Fastify app small, testable, and schema-first. Build on the exported createApp() factory so tests can inject the server without binding a fixed port.

Process

1. Register routes on the app returned by createApp() in src/app.js.
2. Define JSON schemas for request/response when the payload shape matters.
3. Return correct status codes (200/201 success, 400 client error, 404 not found).
4. Use Fastify's built-in error handler; do not leak stack traces in production.
5. Add a test/ case that starts the app on port 0 and asserts responses.

Plugin Order

1. Core plugins (cors, sensible) if needed
2. Route registration
3. Global error handler

Testing

• Integration tests: app.listen({ port: 0 }), fetch against 127.0.0.1.
• Extract pure helpers for unit tests when logic grows beyond route handlers.
• Run npm test before every commit.

Guidelines

• Prefer route-level schemas over ad-hoc validation in handlers.
• Keep handlers thin; move business logic to testable functions.
• Read secrets from environment variables at startup, not from requests.
• Prefer explicit routes over catch-all handlers.

Anti-patterns

• Bypassing Fastify validation with unvalidated request.body access.
• Starting the server inside createApp() — keep listen in server.js only.
• Global mutable state shared across requests.