How to handle multiple domains and multiple routes in Fastify?

How to handle multiple domains and multiple routes in Fastify?

Key idea:

Fastify runs on a single port, so it distinguishes incoming requests by the Host header (the domain the client is requesting).

You can inspect the domain in each request and route or respond differently based on that.

Minimal example with Fastify for multiple domains + routes:

js
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const fastify = require('fastify')({ logger: true });

fastify.addHook('onRequest', async (request, reply) => {
  const host = request.headers.host; // e.g. "example1.com" or "example2.com"
  request.domain = host; // attach domain to request for routing
});

// Route for example1.com
fastify.get('/app1/*', (request, reply) => {
  if (request.domain && request.domain.startsWith('example1.com')) {
    return reply.send({ message: 'Hello from example1.com app1 route!' });
  }
  reply.status(404).send({ error: 'Not found on this domain' });
});

// Route for example2.com
fastify.get('/app2/*', (request, reply) => {
  if (request.domain && request.domain.startsWith('example2.com')) {
    return reply.send({ message: 'Hello from example2.com app2 route!' });
  }
  reply.status(404).send({ error: 'Not found on this domain' });
});

// Default catch-all route for unknown domains or routes
fastify.all('*', (request, reply) => {
  reply.status(404).send({ error: 'Domain or route not configured' });
});

fastify.listen({ port: 3000 }, (err, address) => {
  if (err) {
    fastify.log.error(err);
    process.exit(1);
  }
  console.log(`Server listening at ${address}`);
});

Explanation:

• The server listens on port 3000 for all domains pointing to this server.

• It checks request.headers.host to determine the domain.

• Then routes are handled conditionally by domain + path.

• You can extend logic to serve different content or proxy requests internally.

More scalable pattern: Use Fastify's plugin and routing

If you want better separation and cleaner code, you can register plugins conditionally:

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const fastify = require('fastify')({ logger: true });

// Plugin for example1.com
async function example1Routes(fastify, opts) {
  fastify.get('/app1/*', (request, reply) => {
    reply.send({ message: 'Example1 app1 response' });
  });
}

// Plugin for example2.com
async function example2Routes(fastify, opts) {
  fastify.get('/app2/*', (request, reply) => {
    reply.send({ message: 'Example2 app2 response' });
  });
}

// Main request hook
fastify.addHook('onRequest', async (request, reply) => {
  const host = request.headers.host;
  request.domain = host;
});

// Middleware to route by domain
fastify.addHook('preHandler', async (request, reply) => {
  if (request.domain && request.domain.startsWith('example1.com')) {
    // Register example1 routes dynamically if not registered yet
    if (!fastify.hasExample1) {
      await fastify.register(example1Routes);
      fastify.hasExample1 = true;
    }
  } else if (request.domain && request.domain.startsWith('example2.com')) {
    if (!fastify.hasExample2) {
      await fastify.register(example2Routes);
      fastify.hasExample2 = true;
    }
  }
});

fastify.listen({ port: 3000 });

Note: The above dynamic plugin registration inside hooks is not very typical and may cause some issues, but illustrates domain-based modular routing.

Alternative: Use reverse proxy in front (e.g. Nginx) + one Fastify app per domain

• Use Nginx to route domain requests to different ports on the same server.

• Run multiple Fastify instances on different ports for each domain or route group.

If you want, I can help you write full production-ready config or with reverse proxy setup. What fits your case?
Do you want Fastify to handle everything, or use Nginx as proxy?