Logging

Proper logging is an essential practice for any application. Logs are crucial, and Fastify provides an almost perfect implementation along with Pino.

Logs are JSON-formatted objects written to a stream, typically stdout.

To enable Fastify logging, you need to activate the logger property of Fastify.

src/index.ts

import fastify from 'fastify';

const app = fastify({
  logger: true
});

By doing this, you will automatically see logs in the console:

JSON Log

{"level":30,"time":1712271794784,"pid":1026864,"hostname":"kitajs","msg":"Server listening at http://127.0.0.1:1227"}

All logs will follow the above format. Having the JSON format is super important for servers in production, where these logs can be collected and analyzed by tools like Elasticsearch, Logstash, and Kibana.

Pretty logs

JSON is not human-readable, and during development, we need friendlier feedback. To achieve this, it is recommended to use pino-pretty.

Terminal

npm install -D pino-pretty

With/Without pino-pretty

// Without pino-pretty
{"level":30,"time":1522431328992,"msg":"hello world","pid":42,"hostname":"foo","myProperty":1}

// With pino-pretty
[14:35:28.992] INFO (42): hello world
    myProperty: 1

Formatting logs requires processing, and one thing we want to avoid is diverting processing power that could be used to serve requests to format logs.

Therefore, it is recommended to use pino-pretty via pipelines.

In your development script, after using the node command, you can use the pino-pretty command to format the logs.

package.json

{
  "scripts": {
    // Example of a dev script.
    "dev:server": "node --watch dist/index.js | pino-pretty"
  }
}

In production, it is still recommended to store logs as JSON, and if necessary, you can use pino-pretty later to format logs in a post processing step.

Read more about Pino and Pino Pretty.

Using in a route

The FastifyRequest interface has a log method that automatically includes the req.id in the log to correlate logs from the same request.

src/routes/index.ts

import type { FastifyRequest } from 'fastify';

export function get({ log, ip }: FastifyRequest) {
  log.info({ ip }, 'I am a log message');

  return { hello: 'Arthur!' };
}

There you go, you automatically have correlated logs for each request:

Log

[20:25:48.786] INFO (1060221): incoming request
    reqId: "req-1"
    req: {
      "method": "GET",
      "url": "/",
      "hostname": "localhost:1227",
      "remoteAddress": "127.0.0.1",
      "remotePort": 52670
    }
[20:25:48.787] INFO (1060221): I am a log message
    reqId: "req-1"
    ip: "127.0.0.1"
[20:25:48.788] INFO (1060221): request completed
    reqId: "req-1"
    res: {
      "statusCode": 200
    }
    responseTime: 1.7987719997763634

Filtering logs

By default, when using logger: true, only logs with a level of info or higher are displayed.

To display logs with lower levels, such as trace or debug, you need to change the logger level.

index.ts

import fastify from 'fastify';

const app = fastify({
  logger: {
    level: process.env.LOG_LEVEL || 'trace'
  }
});

It is recommended to filter logs in production to avoid unnecessary logs. You can do this by either changing the logger.level as demonstrated above, or by using the -L flag of pino pretty:

package.json

{
  "scripts": {
    // Shows debug+ logs
    "dev:server": "node --watch dist/index.js | pino-pretty -L debug"
  }
}