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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

Basic functionality there. Still a fair amount of planned work... most importantly the clarifying of required and suggested fields.

Currently supports node 0.4+, but I'll probably make the jump to node 0.6+ as a base soonish.


The usual. All loggers must provide a "service" name. This is somewhat akin to log4j logger "name", but Bunyan doesn't do 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.child(...) 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.child({component: "wuzzle"})});
wuzzle.woos();"done with the wuzzle")

An example and a hack: The node-restify framework integrates bunyan. One feature is that each restify request handler includes a req.log logger that is a:

log.child({req_id: <unique request id>}, true)

Apps using restify can then use req.log and have all such log records include the unique request id (as "req_id"). Handy. What is that true? It is a small bunyan hack by which you can assert that you're just adding simple fields to the child logger. This makes log.child 10x faster and, hence, never a worry for slowing down HTTP request handling. See the changelog for node-bunyan 0.3.0 for details.

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" (6): the service is going to stop or become unusable now
  • "error" (5): fatal for a particular request, but the service continues servicing other requests
  • "warn" (4): a note on something that should probably be looked at by an operator
  • "info" (3): detail on regular operation
  • "debug" (2): anything else, i.e. too verbose to be included in "info" level.
  • "trace" (1): 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

This section will describe rules for the Bunyan log format: field names, field meanings, required fields, etc. However, a Bunyan library doesn't strictly enforce all these rules while records are being emitted. For example, Bunyan will add a time field with the correct format to your log records, but you can specify your own. It is the caller's responsibility to specify the appropriate format.

The reason for the above leniency is because IMO logging a message should never break your app. This leads to this rule of logging: a thrown exception from or equivalent (other than for calling with the incorrect signature) is always a bug in Bunyan.

A typical Bunyan log record looks like this:

{"service":"myserver","hostname":"banana.local","req":{"method":"GET","url":"/path?q=1#anchor","headers":{"x-hi":"Mom","connection":"close"}},"level":3,"msg":"start request","time":"2012-02-03T19:02:46.178Z","v":0}


  "service": "myserver",
  "hostname": "banana.local",
  "req": {
    "method": "GET",
    "url": "/path?q=1#anchor",
    "headers": {
      "x-hi": "Mom",
      "connection": "close"
    "remoteAddress": "",
    "remotePort": 51244
  "level": 3,
  "msg": "start request",
  "time": "2012-02-03T19:02:57.534Z",
  "v": 0

Core fields:

  • v: Required. Integer. Added by Bunion. Cannot be overriden. This is the Bunyan log format version (require('bunyan').LOG_VERSION). The log version is a single integer. 0 is until I release a version "1.0.0" of node-bunyan. Thereafter, starting with 1, this will be incremented if there is any backward incompatible change to the log record format. Details will be in "" (the change log).
  • level: Required. Integer. Added by Bunion. Cannot be overriden. See the "Levels" section.
  • service: Required. String. Provided at Logger creation. You must specify a service name for your logger when creating it.
  • hostname: Required. String. Provided or determined at Logger creation. You can specify your hostname at Logger creation or it will be retrieved vi os.hostname().
  • time: Required. String. Added by Bunion. Can be overriden. The date and time of the event in ISO 8601 Extended Format format and in UTC, as from Date.toISOString().
  • msg: Required. String. Every log.debug(...) et al call must provide a log message.

Go ahead and add more fields, and nest ones are fine (and recommended) as well. This is why we're using JSON. Some suggestions and best practices follow (feedback from actual users welcome).

Recommended/Best Practice Fields:

  • err: Object. A caught JS exception. Log that thing with log.error({err: err}, "oops") and use the Logger.stdSerializers.err serializer for it. See "examples/err.js".
  • req_id: String. A request identifier. Including this field in all logging tied to handling a particular request to your server is strongly suggested. This allows post analysis of logs to easily collate all related logging for a request. This really shines when you have a SOA with multiple services and you carry a single request ID from the top API down through all APIs (as node-restify facilitates with its 'X-Request-Id' header).
  • req: An HTTP server request. Bunyan provides Logger.stdSerializers.req to serialize a request with a suggested set of keys. Example:

        "method": "GET",
        "url": "/path?q=1#anchor",
        "headers": {
          "x-hi": "Mom",
          "connection": "close"
        "remoteAddress": "",
        "remotePort": 51244
  • res: An HTTP server response. Bunyan provides Logger.stdSerializers.res to serialize a response with a suggested set of keys. Example:

        "statusCode": 200,
        "header": "HTTP/1.1 200 OK\r\nContent-Type: text/plain\r\nConnection: keep-alive\r\nTransfer-Encoding: chunked\r\n\r\n"

Other fields to consider:

  • req.username: Authenticated user (or for a 401, the user attempting to auth).
  • Some mechanism to calculate response latency. "restify" users will have a "X-Response-Time" header. A latency custom field would be fine.
  • req.body: If you know that request bodies are small (common in APIs, for example), then logging the request body is good.


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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