Fastify Best Practices

3. Missing Schemas

• Avoid: Defining routes without JSON schemas for request/response.
• Why: Schemas provide faster serialization (Fastify optimizes JSON parsing) and automatic validation/documentation (Swagger).
• Fix: Always define schema in route options.

4. Global State Mutation

• Avoid: Modifying global variables or fastify instance properties directly inside request handlers.
• Why: Causes race conditions and cross-request pollution.
• Fix: Use decorateRequest or request-scoped context.

5. Incorrect Plugin Encapsulation

• Avoid: Registering plugins that accidentally affect parent or sibling contexts.
• Fix: Use fastify-plugin (fp) to break encapsulation when you want to share decorators, but understand Fastify's encapsulation model (DAG) to keep plugins isolated when needed.

6. Using Arrow Functions for Handlers (Context Binding)

• Warning: Be careful when using arrow functions if you rely on this context in Fastify (though standard usage often relies on request/reply args).
• Best Practice: Prefer explicit arguments (request, reply) => ... over relying on this.

Best Practices

1. Dependency Injection

• Register services, db connections, and configs as plugins or decorators early in the boot sequence.

2. Error Handling

• Use fastify.setErrorHandler to centralize error formatting.
• Do not let promises hang; always await or return the promise.

3. Graceful Shutdown

• Handle SIGINT / SIGTERM.
• Close database connections and fastify.close().