fastify/docs/Reference/Logging.md

Fast and low overhead web framework, for Node.js

<h1 align="center">Fastify</h1>

## Logging

### Enable logging
Logging is disabled by default, and you can enable it by passing `{ logger: true
}` or `{ logger: { level: 'info' } }` when you create a Fastify instance. Note
that if the logger is disabled, it is impossible to enable it at runtime. We use
[abstract-logging](https://www.npmjs.com/package/abstract-logging) for this
purpose.

As Fastify is focused on performance, it uses
[pino](https://github.com/pinojs/pino) as its logger, with the default log
level, when enabled, set to `'info'`.

Enabling the production JSON logger:

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

Enabling the logger with appropriate configuration for both local development
and production and test environment requires a bit more configuration:

```js
const envToLogger = {
  development: {
    transport: {
      target: 'pino-pretty',
      options: {
        translateTime: 'HH:MM:ss Z',
        ignore: 'pid,hostname',
      },
    },
  },
  production: true,
  test: false,
}
const fastify = require('fastify')({
  logger: envToLogger[environment] ?? true // defaults to true if no entry matches in the map
})
```
⚠️ `pino-pretty` needs to be installed as a dev dependency, it is not included
by default for performance reasons.

### Usage
You can use the logger like this in your route handlers:

```js
fastify.get('/', options, function (request, reply) {
  request.log.info('Some info about the current request')
  reply.send({ hello: 'world' })
})
```

You can trigger new logs outside route handlers by using the Pino instance from
the Fastify instance:
```js
fastify.log.info('Something important happened!');
```

If you want to pass some options to the logger, just pass them to Fastify.
You can find all available options in the
[Pino documentation](https://github.com/pinojs/pino/blob/master/docs/api.md#options).
If you want to specify a file destination, use:

```js
const fastify = require('fastify')({
  logger: {
    level: 'info',
    file: '/path/to/file' // Will use pino.destination()
  }
})

fastify.get('/', options, function (request, reply) {
  request.log.info('Some info about the current request')
  reply.send({ hello: 'world' })
})
```

If you want to pass a custom stream to the Pino instance, just add a stream
field to the logger object.

```js
const split = require('split2')
const stream = split(JSON.parse)

const fastify = require('fastify')({
  logger: {
    level: 'info',
    stream: stream
  }
})
```

<a id="logging-request-id"></a>

By default, Fastify adds an ID to every request for easier tracking. If the
"request-id" header is present its value is used, otherwise a new incremental ID
is generated. See Fastify Factory
[`requestIdHeader`](./Server.md#factory-request-id-header) and Fastify Factory
[`genReqId`](./Server.md#genreqid) for customization options.

The default logger is configured with a set of standard serializers that
serialize objects with `req`, `res`, and `err` properties. The object received
by `req` is the Fastify [`Request`](./Request.md) object, while the object
received by `res` is the Fastify [`Reply`](./Reply.md) object. This behavior
can be customized by specifying custom serializers.

```js
const fastify = require('fastify')({
  logger: {
    serializers: {
      req (request) {
        return { url: request.url }
      }
    }
  }
})
```
For example, the response payload and headers could be logged using the approach
below (even if it is *not recommended*):

```js
const fastify = require('fastify')({
  logger: {
    transport: {
      target: 'pino-pretty'
    },
    serializers: {
      res (reply) {
        // The default
        return {
          statusCode: reply.statusCode
        }
      },
      req (request) {
        return {
          method: request.method,
          url: request.url,
          path: request.routerPath,
          parameters: request.params,
          // Including the headers in the log could be in violation
          // of privacy laws, e.g. GDPR. You should use the "redact" option to
          // remove sensitive fields. It could also leak authentication data in
          // the logs.
          headers: request.headers
        };
      }
    }
  }
});
```

**Note**: In certain cases, the [`Reply`](./Reply.md) object passed to the `res`
serializer cannot be fully constructed. When writing a custom `res` serializer,
it is necessary to check for the existence of any properties on `reply` aside
from `statusCode`, which is always present. For example, the existence of
`getHeaders` must be verified before it can be called:

```js
const fastify = require('fastify')({
  logger: {
    transport: {
      target: 'pino-pretty'
    },
    serializers: {
      res (reply) {
        // The default
        return {
          statusCode: reply.statusCode
          headers: typeof reply.getHeaders === 'function'
            ? reply.getHeaders()
            : {}
        }
      },
    }
  }
});
```

**Note**: The body cannot be serialized inside a `req` method because the
request is serialized when we create the child logger. At that time, the body is
not yet parsed.

See an approach to log `req.body`

```js
app.addHook('preHandler', function (req, reply, done) {
  if (req.body) {
    req.log.info({ body: req.body }, 'parsed body')
  }
  done()
})
```

**Note**: Care should be taken to ensure serializers never throw, as an error
thrown from a serializer has the potential to cause the Node process to exit.
See the [Pino documentation](https://getpino.io/#/docs/api?id=opt-serializers)
on serializers for more information.

*Any logger other than Pino will ignore this option.*

You can also supply your own logger instance. Instead of passing configuration
options, pass the instance. The logger you supply must conform to the Pino
interface; that is, it must have the following methods: `info`, `error`,
`debug`, `fatal`, `warn`, `trace`, `silent`, `child` and a string property `level`.

Example:

```js
const log = require('pino')({ level: 'info' })
const fastify = require('fastify')({ logger: log })

log.info('does not have request information')

fastify.get('/', function (request, reply) {
  request.log.info('includes request information, but is the same logger instance as `log`')
  reply.send({ hello: 'world' })
})
```

*The logger instance for the current request is available in every part of the
[lifecycle](./Lifecycle.md).*

## Log Redaction

[Pino](https://getpino.io) supports low-overhead log redaction for obscuring
values of specific properties in recorded logs. As an example, we might want to
log all the HTTP headers minus the `Authorization` header for security concerns:

```js
const fastify = Fastify({
  logger: {
    stream: stream,
    redact: ['req.headers.authorization'],
    level: 'info',
    serializers: {
      req (request) {
        return {
          method: request.method,
          url: request.url,
          headers: request.headers,
          hostname: request.hostname,
          remoteAddress: request.ip,
          remotePort: request.socket.remotePort
        }
      }
    }
  }
})
```

See https://getpino.io/#/docs/redaction for more details.