Server-Side Development with Fastify — Custom Logging and Route Config

Fastify is a small Node framework for developing back end web apps.

In this article, we’ll look at how to create back end apps with Fastify.

Custom Log Level

We can change the log level of our Fastify app.

For instance, we can write:

index.js

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

fastify.register(require('./routes/v1/users'), { prefix: '/v1' })
fastify.register(require('./routes/v2/users'), { prefix: '/v2' })

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

routes/v1/users.js

module.exports = function (fastify, opts, done) {
  fastify.get('/user', async () => 'v1')
  done()
}

routes/v2/user.js

module.exports = function (fastify, opts, done) {
  fastify.get('/user', async () => 'v2')
  done()
}

We set logger to true to enable logging.

We can also set the logging levels for individual routes.

For instance, we can write:

const fastify = require('fastify')()

fastify.get('/', { logLevel: 'warn' }, (request, reply) => {
  reply.send({ hello: 'world' })
})

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

We set the logLevel to 'warn' on the / route.

Custom Log Serializer

We can customize the log serializer.

For instance, we can write:

const fastify = require('fastify')({
  logger: {
    level: 'info',
    serializers: {
      user (req) {
        return {
          method: req.method,
          url: req.url,
          headers: req.headers,
          hostname: req.hostname,
          remoteAddress: req.ip,
          remotePort: req.socket.remotePort
        }
      }
    }
  }
})

fastify.register(context1, {
  logSerializers: {
    user: value => `My serializer father - ${value}`
  }
})

async function context1 (fastify, opts) {
  fastify.get('/', (req, reply) => {
    req.log.info({ user: 'call father serializer', key: 'another key' })
    reply.send({})
  })
}
const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

to add the logSerializers property.

We configure the logger when we call the Fastify factory function on the first line.

We set the level to set the log level.

serializers has an object with the user function that returns the info from the request.

We return the request method, URL, headers, hostname, IP, and remote port respectively.

Then when we make the request, we see the output in the console.

Route Config

We can pass in a config object when we define our route.

For instance, we can write:

const fastify = require('fastify')()

function handler (req, reply) {
  reply.send(reply.context.config.output)
}

fastify.get('/en', { config: { output: 'hello world!' } }, handler)
fastify.get('/fr', { config: { output: 'bonjour' } }, handler)

const start = async () => {
  try {
    await fastify.listen(3000, '0.0.0.0')
  } catch (err) {
    fastify.log.error(err)
    process.exit(1)
  }
}
start()

We pass in an object with the config property in the 2nd argument.

Then we can get that value from the reply.context property.

Conclusion

We can set the custom logging and set route config with Fastify.