Eel is EventEmitter logging. Simple, extensible, and convenient
Switch branches/tags
Nothing to show
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.

eel - EventEmitter Logging Build Status

eel is a logging "framework" that hopes to embody a few simple principles:

  1. Logging should be easy.
  2. Logs should record structured data.
  3. Logging should be flexible.

It accomplishes these goals by decoupling the logging of events from the writing of logs in the simplest way possible: using an EventEmitter


The eel module exports a function that logs at the "info" level:

log = require('eel')
version = require('./package.json').version
log("Started up", {version: version, port: port})

To log at another level, use the log[level] functions:

process.on('uncaughtException', function (err) {
  log.error("uncaughtException", {err: err, stack: err.stack})

The default levels are debug, info, warning, error, and critical.

Structured Data

Eel uses the logstash JSON event format internally. Every logging method takes a @message as the first parameter, and an object representing the @fields part of a event as the second. Either parameter can be omitted, and if you include any of the @ prefixed "metadata" fields in the field data object they will override the defaults.

Confused? Hopefully these examples will clarify:

log('Something happened', {count: 10})
{ '@message': 'Something happened',
  '@tags': [],
  '@fields': { level: 'info', count: 10 },
  '@timestamp': '2012-12-16T05:44:48.125Z' }

log.warn('Something happened', {count: 10, '@tags': ['bad']})
{ '@message': 'Something happened',
  '@tags': ['bad'],
  '@fields': { level: 'warn', count: 10 },
  '@timestamp': '2012-12-16T05:44:48.125Z' }

log.error('Something happened', {'@message': 'Overridden!'})
{ '@message': 'Overridden!',
  '@tags': [],
  '@fields': { level: 'error' },
  '@timestamp': '2012-12-16T05:44:48.125Z' }

Be careful, Eel doesn't go out of it's way to prevent you from generating garbage log entries!


In addition to the various logging methods, the eel object also acts like an EventEmitter. In fact, none of the above examples produce any output, because nothing is listening to the events being emitted. To rectify this we can attach the simplest possible logging backend to the 'entry' event:

log.on('info', console.log).on('error', console.error)

Now our prepared log entry objects will be printed to the console:

log('Something happened', {relephant: 'data'})
/* Actually prints this to the console:
{ '@message': 'Something happened',
  '@fields': { level: 'info', relephant: 'data' },
  '@timestamp': '2012-05-31T23:49:01.523Z' }

Logging Backends

Using console.log as a backend might suffice for development, but chances are you will want to log to a file or centralized log aggregator (such as Logstash) in production. Currently eel ships with 2 logging backends: files and TCP sockets.

Configuring a backend

require('eel').backends.configure('proto://...', ['warn', 'error', 'critical'])

The first argument should be a string URI describing the logging destination. The way the URI is interpreted depends on the protocol used (see below for examples), while the levels parameter should be an array of log levels this backend should handle.

When it comes time for your program to exit, you can tell a given logging backend to close any resources it holds with logging.backend.unload(uri).

File Backend


The above will log to the file /var/log/my_program.log and rotate the log file every time more than 10 megabytes has been written to it. At most 10 old log files will be kept, named my_program.log.[0-9].

File Backend Options

  • rotateSize - Size at which log files will be rotated. Can be suffixed with kb/mb/gb/tb.
  • rotateSignal - A signal name that cause log files to be rotated.
  • maxFiles - Number of old log files to keep. Beyond this number they will be destroyed. If a file backend is rotated via signal and no maxFiles parameter is given, it will re-open the same log file

TCP backend

log.backends.configure('tcp://localhost:1234', levels)

This backend will write JSON events directly to a socket, separated by new lines. It takes no additional options.

Creating a custom backend

To create your own backend, you simply need to define a factory function that will take a parsed URI and return a log event handler function. For example, a simplified version of the TCP handler might look like this:

var net = require('net')

module.exports = function (uri) {
	var socket = net.connect(uri.hostname, uri.port)
	return function (entry) { socket.write(JSON.stringify(entry) + '\n') }

(The TCP handler that ships with eel also handles reconnecting on socket errors and JSONifying circular structures)

If your logging handler requires any cleanup for a program to cleanly exit, it's good practice to attach an end method to the event handler:

var net = require('net')

module.exports = function (uri) {
	var socket = net.connect(, uri.port)
	var handler = function (entry) { socket.write(JSON.stringify(entry) + '\n') }
	handler.end = socket.end.bind(socket)
	return handler


Investigate using EventEmitter2 for namespacing and pattern matching log events.