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, but I'm using it for some production services.

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

Usage

The usual. All loggers must provide a "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({name: "myapp"});
log.info("hi");

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

$ node hi.js
{"name":"myapp","hostname":"banana.local","level":2,"msg":"hi","time":"2012-01-31T00:07:44.216Z","v":0}

The full log.{trace|debug|...|fatal}(...) API is:

log.info();     // Returns a boolean: is the "info" level enabled?

log.info('hi');                     // Log a simple string message.
log.info('hi %s', bob, anotherVar); // Uses `util.format` for msg formatting.

log.info({foo: 'bar'}, 'hi');       // Adds "foo" field to log record.

log.info(err);  // Special case to log an `Error` instance, adds "err"
                // key with exception details (including the stack) and
                // sets "msg" to the exception message.
log.info(err, 'more on this: %s', more);
                // ... or you can specify the "msg".

bunyan tool

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
{
  "name": "myapp",
  "hostname": "banana.local",
  "level": 2,
  "msg": "hi",
  "time": "2012-01-31T00:10:00.676Z",
  "v": 0
}

streams

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

var log = new Logger({name: "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({
  name: "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
    }
  ]
});

More on streams in the "Streams" section below.

log.child

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;
  this.log.info("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();
log.info("done with the wuzzle")

A little log.child hack.

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:

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.

The hack is that , true. It is a small bunyan performance hack by which you can assert that you're just adding simple fields to the child logger -- i.e. not special fields like "level", "serializers", "streams". 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.

serializers

Bunyan has a concept of "serializers" to produce a JSON-able object from a JavaScript object, so you can easily do the following:

log.info({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.

src

The source file, line and function of the log call site can be added to log records by using the src: true config option:

var log = new Logger({src: true, ...});

This adds the call source info with the 'src' field, like this:

{
  "name": "src-example",
  "hostname": "banana.local",
  "component": "wuzzle",
  "level": 4,
  "msg": "This wuzzle is woosey.",
  "time": "2012-02-06T04:19:35.605Z",
  "src": {
    "file": "/Users/trentm/tm/node-bunyan/examples/src.js",
    "line": 20,
    "func": "Wuzzle.woos"
  },
  "v": 0
}

WARNING: Determining the call source info is slow. Never use this option in production.

Levels

The log levels in bunyan are:

• "fatal" (60): the service/app is going to stop or become unusable now
• "error" (50): fatal for a particular request, but the service/app continues servicing other requests
• "warn" (40): a note on something that should probably be looked at by an operator
• "info" (30): detail on regular operation
• "debug" (20): anything else, i.e. too verbose to be included in "info" level.
• "trace" (10): logging from external libraries used by your app

General level usage suggestions: "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 (10 for "trace", ..., 60 for "fatal") and constants are defined for the (Logger.TRACE ... Logger.DEBUG). The lowercase level names are aliases supported in the API.

Here is the API for changing levels in an existing logger:

log.level() -> INFO   // gets current level (lowest level of all streams)

log.level(INFO)       // set all streams to level INFO
log.level("info")     // set all streams to level INFO

log.levels() -> [DEBUG, INFO]   // get array of levels of all streams
log.levels(0) -> DEBUG          // get level of stream at index 0
log.levels("foo")               // get level of stream with name "foo"

log.levels(0, INFO)             // set level of stream 0 to INFO
log.levels(0, "info")           // can use "info" et al aliases
log.levels("foo", WARN)         // set stream named "foo" to WARN

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 log.info(...) or equivalent (other than for calling with the incorrect signature) is always a bug in Bunyan.

A typical Bunyan log record looks like this:

{"name":"myserver","hostname":"banana.local","pid":123,"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}

Pretty-printed:

{
  "name": "myserver",
  "hostname": "banana.local",
  "pid": 123,
  "req": {
    "method": "GET",
    "url": "/path?q=1#anchor",
    "headers": {
      "x-hi": "Mom",
      "connection": "close"
    },
    "remoteAddress": "120.0.0.1",
    "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 "CHANGES.md" (the change log).
• level: Required. Integer. Added by Bunion. Cannot be overriden. See the "Levels" section.
• name: Required. String. Provided at Logger creation. You must specify a name for your logger when creating it. Typically this is the name of the service/app using Bunyan for logging.
• 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().
• pid: Required. Integer. Filled in automatically at Logger creation.
• 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.
• src: Optional. Object giving log call source info. This is added automatically by Bunyan if the "src: true" config option is given to the Logger. Never use in production as this is really slow.

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.info(err) to get:

  ...
  "err": {
    "message": "boom",
    "name": "TypeError",
    "stack": "TypeError: boom\n    at Object.<anonymous> ..."
  },
  "msg": "boom",
  ...
  

  Or use the Logger.stdSerializers.err serializer in your Logger and do this log.error({err: err}, "oops"). 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": "120.0.0.1",
      "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.

Streams

A "stream" is Bunyan's name for an output for log messages (the equivalent to a log4j Appender). Ultimately Bunyan uses a Writable Stream interface, but there are some additional attributes used to create and manage the stream. A Bunyan Logger instance has one or more streams. General streams are specified with the "streams" option:

var Logger = require('bunyan');
var log = new Logger({
  name: "foo",
  streams: [
    {
        stream: process.stderr,
        level: "debug"
    },
    ...
  ]
})

For convenience, if there is only one stream, it can specified with the "stream" and "level" options (internal converted to a Logger.streams):

var log = new Logger({
  name: "foo",
  stream: process.stderr,
  level: "debug"
})

If none are specified, the default is a stream on process.stdout at the "info" level.

Logger.streams is an array of stream objects with the following attributes:

• type: Typically implied. E.g. "stream" or "file". See supported types below.
• stream: This is the "Writable Stream", e.g. a std handle or an open file write stream.
• level: The level at which logging to this stream is enabled. If not specified it defaults to INFO.

Supported stream types are:

• stream: A "stream" argument is given.

• file: A "path" argument is given. Bunyan will open this file for appending. E.g.:

    {
      "path": "/var/log/foo.log",
      "level": "warn"
    }
  

License

MIT.

Future

See "TODO.md", but basically:

• 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.

• Syslog support.

• Some speed comparisons with others to get a feel for Bunyan's speed.