Fastify Adapter

Introduction

Express is battle-tested but not the fastest. Fastify can handle significantly more requests per second with lower latency. NestJS's platform-agnostic design lets you switch from Express to Fastify for performance-critical applications.

Key Concepts

• Fastify: High-performance web framework for Node.js
• FastifyAdapter: NestJS adapter for Fastify platform
• JSON Schema: Fastify uses JSON Schema for validation (optional)
• Plugins: Fastify's extension mechanism (instead of middleware)

Real World Context

Benchmark-dependent, but Fastify often handles 2x the requests per second compared to Express. For high-traffic APIs, this can mean significant cost savings and improved user experience.

Deep Dive

Installation

bash
npm install @nestjs/platform-fastify

Switching to Fastify

typescript
import { NestFactory } from '@nestjs/core';
import { FastifyAdapter, NestFastifyApplication } from '@nestjs/platform-fastify';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create<NestFastifyApplication>(
    AppModule,
    new FastifyAdapter(),
  );
  
  // IMPORTANT: Fastify requires explicit host for Docker/K8s
  await app.listen(3000, '0.0.0.0');
}
bootstrap();

Fastify Options

typescript
new FastifyAdapter({
  logger: true,
  trustProxy: true,
  bodyLimit: 10485760, // 10MB
})

Using Fastify Plugins

typescript
async function bootstrap() {
  const app = await NestFactory.create<NestFastifyApplication>(
    AppModule,
    new FastifyAdapter(),
  );
  
  await app.register(fastifyHelmet);
  await app.register(fastifyCompress);
  await app.register(fastifyCors);
  
  await app.listen(3000, '0.0.0.0');
}

Accessing Fastify Instance

typescript
const fastifyInstance = app.getHttpAdapter().getInstance();
fastifyInstance.addHook('onRequest', (request, reply, done) => {
  console.log('Request:', request.url);
  done();
});

Migration Considerations

NestJS 11 Platform Changes

NestJS 11 requires Node.js v20+ and ships with Fastify v5 and Express v5 support.

Middleware wildcards changed in both platforms:

typescript
// Fastify v5: named wildcard
consumer.apply(LoggerMiddleware).forRoutes('*splat');

// Express v5: named wildcard with braces
consumer.apply(LoggerMiddleware).forRoutes('{*splat}');

// Previously (NestJS 10): .forRoutes('(.*)');

Fastify v5 CORS: By default, only CORS-safelisted methods (GET, HEAD, POST) are allowed. You must explicitly configure methods like PUT, PATCH, and DELETE:

typescript
const app = await NestFactory.create<NestFastifyApplication>(
  AppModule,
  new FastifyAdapter(),
);
app.enableCors({
  origin: 'https://your-frontend.com',
  methods: ['GET', 'POST', 'PUT', 'PATCH', 'DELETE'],
});

Common Pitfalls

1. Express middleware incompatibility: Most Express middleware won't work with Fastify. Use Fastify alternatives.
2. Forgetting '0.0.0.0': Fastify binds to localhost by default—breaks containers.
3. @Res() decorator: Using @Res() directly bypasses Fastify's optimizations.
4. Fastify v5 CORS: Only safelisted HTTP methods are allowed by default — configure explicitly.

Best Practices

• Test thoroughly after migration
• Use Fastify plugins instead of Express middleware
• Avoid @Res() when possible—let NestJS handle responses
• Profile actual endpoints, not just synthetic benchmarks

Summary

Fastify provides significant performance improvements over Express. Use FastifyAdapter for the switch, register Fastify plugins instead of Express middleware, and remember to bind to 0.0.0.0 for containerized deployments.

import { NestFactory } from '@nestjs/core';
import {
  FastifyAdapter,
  NestFastifyApplication,
} from '@nestjs/platform-fastify';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create<NestFastifyApplication>(
    AppModule,
    new FastifyAdapter(),
  );
  // Fastify requires explicit host for Docker/K8s
  await app.listen(3000, '0.0.0.0');
}
bootstrap();