Asynchronous Logging

In essence, asynchronous logging enables even faster performance by Pino.

In Pino's standard mode of operation log messages are directly written to the output stream as the messages are generated with a blocking operation. Asynchronous logging works by buffering log messages and writing them in larger chunks.

const pino = require('pino')
const logger = pino(pino.destination({
  dest: './my-file', // omit for stdout
  minLength: 4096, // Buffer before writing
  sync: false // Asynchronous logging
}))

See pino.destination
pino.destination is implemented on sonic-boom ⇗.

Caveats

This has a couple of important caveats:

4KB of spare RAM will be needed for logging
As opposed to the default mode, there is not a one-to-one relationship between calls to logging methods (e.g. logger.info) and writes to a log file
There is a possibility of the most recently buffered log messages being lost (up to 4KB of logs)
- For instance, a power cut will mean up to 4KB of buffered logs will be lost

So in summary, use asynchronous logging only when performing an extreme amount of logging, and it is acceptable to potentially lose the most recent logs.

Pino will register handlers for the following process events/signals so that Pino can flush the asynchronous logger buffer:
- beforeExit
- exit
- uncaughtException
- SIGHUP
- SIGINT
- SIGQUIT
- SIGTERM
In all of these cases, except SIGHUP, the process is in a state that it must terminate. Thus, if an onTerminated function isn't registered when constructing a Pino instance (see pino#constructor), then Pino will invoke process.exit(0) when no error has occurred, or process.exit(1) otherwise. If an onTerminated function is supplied, it is the responsibility of the onTerminated function to manually exit the process.

In the case of SIGHUP, we will look to see if any other handlers are registered for the event. If not, we will proceed as we do with all other signals. If there are more handlers registered than just our own, we will simply flush the asynchronous logging buffer.

AWS Lambda

On AWS Lambda we recommend to call dest.flushSync() at the end of each function execution to avoid losing data.

Usage

The pino.destination({ sync: false }) method will provide an asynchronous destination.

const pino = require('pino')
const dest = pino.destination({ sync: false }) // logs to stdout with no args
const logger = pino(dest)

Log loss prevention

The following strategy can be used to minimize log loss:

const pino = require('pino')
const dest = pino.destination({ sync: false })
const logger = pino(dest)

// asynchronously flush every 10 seconds to keep the buffer empty
// in periods of low activity
setInterval(function () {
  logger.flush()
}, 10000).unref()

// use pino.final to create a special logger that
// guarantees final tick writes
const handler = pino.final(logger, (err, finalLogger, evt) => {
  finalLogger.info(`${evt} caught`)
  if (err) finalLogger.error(err, 'error caused exit')
  process.exit(err ? 1 : 0)
})
// catch all the ways node might exit
process.on('beforeExit', () => handler(null, 'beforeExit'))
process.on('exit', () => handler(null, 'exit'))
process.on('uncaughtException', (err) => handler(err, 'uncaughtException'))
process.on('SIGINT', () => handler(null, 'SIGINT'))
process.on('SIGQUIT', () => handler(null, 'SIGQUIT'))
process.on('SIGTERM', () => handler(null, 'SIGTERM'))

See pino.destination api
See pino.final api
See destination parameter

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