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Bunyan -- a JSON Logger for node.js servers.

Server logs should be structured. JSON's a good format. Let's do that: a log record is one line of JSON.stringify'd output. Let's also specify some common names for the requisite and common fields for a log record (see below).

Also: log4j is way more than you need.

Current Status

Just play stuff here. Don't try to use this for realz yet.


The usual. All loggers must provide a "service" name. This is somewhat akin to log4j logger "name", but Bunyan doesn't so hierarchical logger names.

$ cat hi.js
var Logger = require('bunyan');
var log = new Logger({service: "myapp"});"hi");

Log records are JSON. "hostname", "time" and "v" (the Bunyan log format version) are added for you.

$ node hi.js

A bunyan tool is provided for pretty-printing bunyan logs and, eventually, for filtering (e.g. | bunyan -c 'level>3'). This shows the default output (which is fluid right now) and indented-JSON output. More output formats will be added, including support for custom formats.

$ node hi.js | ./bin/bunyan  # CLI tool to filter/pretty-print JSON logs.
[2012-01-31T00:08:11.387Z] INFO: myapp on banana.local: hi

$ node hi.js | ./bin/bunyan -o json
  "service": "myapp",
  "hostname": "banana.local",
  "level": 2,
  "msg": "hi",
  "time": "2012-01-31T00:10:00.676Z",
  "v": 0

By default, log output is to stdout (stream) and at the "info" level. Explicitly that looks like:

var log = new Logger({service: "myapp", stream: process.stdout, 
  level: "info"});

That is an abbreviated form for a single stream. You can defined multiple streams at different levels.

var log = new Logger({
  service: "amon",
  streams: [
      level: "info",
      stream: process.stdout, // log INFO and above to stdout
      level: "error",
      path: "tmp/error.log"   // log ERROR and above to a file

A log.clone(...) is provided to specialize a logger for a sub-component. The following will have log records from "Wuzzle" instances use exactly the same config as its parent, plus include the "component" field.

var log = new Logger(...);


function Wuzzle(options) {
  this.log = options.log;"creating a wuzzle")
Wuzzle.prototype.woos = function () {
  this.log.warn("This wuzzle is woosey.")

var wuzzle = new Wuzzle({log: log.clone({component: "wuzzle"})});
wuzzle.woos();"done with the wuzzle")

Back to the log.{trace|debug|...|fatal}(...) API:;     // returns a boolean: is the "info" level enabled?"hi"); // log a simple string message"hi %s", bob, anotherVar); // uses `util.format` for msg formatting{foo: "bar"}, "hi");       // adds "foo" field to log record

Bunyan has a concept of "serializers" to produce a JSON-able object from a JavaScript object, so your can easily do the following:{req: <request object>}, "something about handling this request");

Association is by log record field name, "req" in this example, so this requires a registered serializer something like this:

function reqSerializer(req) {
  return {
    method: req.method,
    url: req.url,
    headers: req.headers
var log = new Logger({
  serializers: {
    req: reqSerializer

Or this:

var log = new Logger({
  serializers: {req: Logger.stdSerializers.req}

because Buyan includes a small set of standard serializers. To use all the standard serializers you can use:

var log = new Logger({
  serializers: Logger.stdSerializers

Note: Your own serializers should never throw, otherwise you'll get an ugly message on stderr from Bunyan (along with the traceback) and the field in your log record will be replaced with a short error message.


See "", but basically:

  • More std serializers. See

  • Spec'ing and enforcing the fields (from dap's section in eng guide).

  • Syslog support. Ring-buffer support for storing last N debug messages (or whatever) in memory to support debugability without too much log load.

  • More bunyan output formats and filtering features.

  • Think about a bunyan dashboard that supports organizing and viewing logs from multiple hosts and services.


  • "fatal": the service is going to stop or become unusable now
  • "error": fatal for a particular request, but the service continues servicing other requests
  • "warn": a note on something that should probably be looked at by an operator
  • "info": detail on regular operation
  • "debug": anything else, i.e. too verbose to be included in "info" level.
  • "trace": logging from external libraries used by your app

"debug" should be used sparingly. Information that will be useful to debug errors post mortem should usually be included in "info" messages if it's generally relevant or else with the corresponding "error" event. Don't rely on spewing mostly irrelevant debug messages all the time and sifting through them when an error occurs.

Integers are used for the actual level values (1 for "trace", ..., 6 for "fatal") and constants are defined for the (Logger.TRACE ... Logger.DEBUG). The lowercase level names are aliases supported in the API.

Log Record Fields

TODO: from dap and enforce these

  • "request_id" (better name?) can't be required because some things don't happen in a per-request context. Startup and background processing stuff for example. Tho for request-y things, it is strongly encouraged because it allows collating logs from multiple services for the same request.


A "stream" is Bunyan's name for an output for log messages. It expects a Writable Stream interface. See above for some examples of specifying streams. Supported streams are:

  • A writable "stream". Often this is one of the std handles (process.stdout or process.stderr), but it can be anything you want supporting the node writable stream interface, e.g. fs.createWriteStream.
  • A file. Will append to the given "path".



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