node-bunyan-lite/README.md

Bunyan is **a simple and fast JSON logging library** for node.js services:

    var bunyan = require('bunyan');
    var log = bunyan.createLogger({name: "myapp"});
    log.info("hi");

and **a `bunyan` CLI tool** for nicely viewing those logs:

![bunyan CLI screenshot](https://raw.github.com/trentm/node-bunyan/master/tools/screenshot1.png)

Manifesto: 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

Solid core functionality is there. Joyent is using this for a number of
production services. Bunyan supports node 0.6 and greater. Follow
<a href="https://twitter.com/intent/user?screen_name=trentmick" target="_blank">@trentmick</a>
for updates to Bunyan.

There is an email discussion list
[bunyan-logging@googlegroups.com](mailto:bunyan-logging@googlegroups.com),
also [as a forum in the
browser](https://groups.google.com/forum/?fromgroups#!forum/bunyan-logging).


# Installation

    npm install bunyan

**Tip**: The `bunyan` CLI tool is written to be compatible (within reason) with
all versions of Bunyan logs. Therefore you might want to `npm install -g bunyan`
to get the bunyan CLI on your PATH, then use local bunyan installs for
node.js library usage of bunyan in your apps.


# Features

- elegant [log method API](#log-method-api)
- extensible [streams](#streams) system for controlling where log records
  go (to a stream, to a file, [log file rotation](#stream-type-rotating-file),
  etc.)
- [`bunyan` CLI](#cli-usage) for pretty-printing and filtering of Bunyan logs
- simple include of log call source location (file, line, function) with
  [`src: true`](#src)
- light-weight specialization of Logger instances with [`log.child`](#logchild)
- custom rendering of logged objects with ["serializers"](#serializers)
- [Runtime log snooping via Dtrace support](#dtrace-support)


# Introduction

Like most logging libraries you create a Logger instance and call methods
named after the logging levels:

    $ cat hi.js
    var bunyan = require('bunyan');
    var log = bunyan.createLogger({name: 'myapp'});
    log.info('hi');
    log.warn({lang: 'fr'}, 'au revoir');

All loggers must provide a "name". This is somewhat akin to the log4j logger
"name", but Bunyan doesn't do hierarchical logger names.

**Bunyan log records are JSON.** A few fields are added automatically:
"pid", "hostname", "time" and "v".

    $ node hi.js
    {"name":"myapp","hostname":"banana.local","pid":40161,"level":30,"msg":"hi","time":"2013-01-04T18:46:23.851Z","v":0}
    {"name":"myapp","hostname":"banana.local","pid":40161,"level":40,"lang":"fr","msg":"au revoir","time":"2013-01-04T18:46:23.853Z","v":0}


## Log Method API

The example above shows two different ways to call `log.info(...)`. The
full API is:

    log.info();     // Returns a boolean: is the "info" level enabled?
                    // This is equivalent to `log.isInfoEnabled()` or
                    // `log.isEnabledFor(INFO)` in log4j.

    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. You can add any number
                    // of additional fields here.

    log.info(err);  // Special case to log an `Error` instance to the record.
                    // This adds an "err" field 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".

Note that this implies **you cannot pass any object as the first argument
to log it**. IOW, `log.info(mywidget)` may not be what you expect. Instead
of a string representation of `mywidget` that other logging libraries may
give you, Bunyan will try to JSON-ify your object. It is a Bunyan best
practice to always give a field name to included objects, e.g.:

    log.info({widget: mywidget}, ...)

This will dove-tail with [Bunyan serializer support](#serializers), discussed
later.

The same goes for all of Bunyan's log levels: `log.trace`, `log.debug`,
`log.info`, `log.warn`, and `log.fatal`. See the [levels section](#levels)
below for details and suggestions.


## CLI Usage

Bunyan log output is a stream of JSON objects. This is great for processing,
but not for reading directly. A **`bunyan` tool** is provided **for
pretty-printing bunyan logs** and for **filtering** (e.g.
`| bunyan -c 'this.foo == "bar"'`). Using our example above:

    $ node hi.js | ./bin/bunyan
    [2013-01-04T19:01:18.241Z]  INFO: myapp/40208 on banana.local: hi
    [2013-01-04T19:01:18.242Z]  WARN: myapp/40208 on banana.local: au revoir (lang=fr)

See the screenshot above for an example of the default coloring of rendered
log output. That example also shows the nice formatting automatically done for
some well-known log record fields (e.g. `req` is formatted like an HTTP request,
`res` like an HTTP response, `err` like an error stack trace).

One interesting feature is **filtering** of log content, which can be useful
for digging through large log files or for analysis. We can filter only
records above a certain level:

    $ node hi.js | bunyan -l warn
    [2013-01-04T19:08:37.182Z]  WARN: myapp/40353 on banana.local: au revoir (lang=fr)

Or filter on the JSON fields in the records (e.g. only showing the French
records in our contrived example):

    $ node hi.js | bunyan -c 'this.lang == "fr"'
    [2013-01-04T19:08:26.411Z]  WARN: myapp/40342 on banana.local: au revoir (lang=fr)

See `bunyan --help` for other facilities.


## Streams Introduction

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

    var log = bunyan.createLogger({
        name: 'myapp',
        stream: process.stdout,
        level: 'info'
    });

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

    var log = bunyan.createLogger({
      name: 'myapp',
      streams: [
        {
          level: 'info',
          stream: process.stdout,           // log INFO and above to stdout
        },
        {
          level: 'error',
          path: '/var/log/myapp-error.log'  // log ERROR and above to a file
        }
      ]
    });

More on streams in the [Streams section](#streams) below.


## log.child

Bunyan has a concept of a child logger to **specialize a logger for a
sub-component of your application**, i.e. to create a new logger with
additional bound fields that will be included in its log records. A child
logger is created with `log.child(...)`.

In the following example, logging on a "Wuzzle" instance's `this.log` will
be exactly as on the parent logger with the addition of the `widget_type`
field:

    var bunyan = require('bunyan');
    var log = bunyan.createLogger({name: 'myapp'});

    function Wuzzle(options) {
        this.log = options.log.child({widget_type: 'wuzzle'});
        this.log.info('creating a wuzzle')
    }
    Wuzzle.prototype.woos = function () {
        this.log.warn('This wuzzle is woosey.')
    }

    log.info('start');
    var wuzzle = new Wuzzle({log: log});
    wuzzle.woos();
    log.info('done');

Running that looks like (raw):

    $ node myapp.js
    {"name":"myapp","hostname":"myhost","pid":34572,"level":30,"msg":"start","time":"2013-01-04T07:47:25.814Z","v":0}
    {"name":"myapp","hostname":"myhost","pid":34572,"widget_type":"wuzzle","level":30,"msg":"creating a wuzzle","time":"2013-01-04T07:47:25.815Z","v":0}
    {"name":"myapp","hostname":"myhost","pid":34572,"widget_type":"wuzzle","level":40,"msg":"This wuzzle is woosey.","time":"2013-01-04T07:47:25.815Z","v":0}
    {"name":"myapp","hostname":"myhost","pid":34572,"level":30,"msg":"done","time":"2013-01-04T07:47:25.816Z","v":0}

And with the `bunyan` CLI (using the "short" output mode):

    $ node myapp.js  | bunyan -o short
    07:46:42.707Z  INFO myapp: start
    07:46:42.709Z  INFO myapp: creating a wuzzle (widget_type=wuzzle)
    07:46:42.709Z  WARN myapp: This wuzzle is woosey. (widget_type=wuzzle)
    07:46:42.709Z  INFO myapp: done


A more practical example is in the
[node-restify](https://github.com/mcavage/node-restify) web framework.
Restify uses Bunyan for its logging. One feature of its integration, is that
if `server.use(restify.requestLogger())` is used, 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.


## 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');

Serializers is a mapping of log record field name, "req" in this example, to
a serializer function. That looks like this:

    function reqSerializer(req) {
        return {
            method: req.method,
            url: req.url,
            headers: req.headers
        }
    }
    var log = bunyan.createLogger({
        name: 'myapp',
        serializers: {
            req: reqSerializer
        }
    });

Or this:

    var log = bunyan.createLogger({
        name: 'myapp',
        serializers: {req: bunyan.stdSerializers.req}
    });

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

    var log = bunyan.createLogger({
      ...
      serializers: bunyan.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 = bunyan.createLogger({src: true, ...});

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

    {
      "name": "src-example",
      "hostname": "banana.local",
      "pid": 123,
      "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 as follows. The level descriptions are best
practice *opinions*.

- "fatal" (60): The service/app is going to stop or become unusable now.
  An operator should definitely look into this soon.
- "error" (50): Fatal for a particular request, but the service/app continues
  servicing other requests. An operator should look at this soon(ish).
- "warn" (40): A note on something that should probably be looked at by an
  operator eventually.
- "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 or *very*
  detailed application logging.

Suggestions: Use "debug" 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 (bunyan.TRACE ... bunyan.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 Bunyan. 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 Bunyan. 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 Bunyan. Can be overriden.
  The date and time of the event in [ISO 8601
  Extended Format](http://en.wikipedia.org/wiki/ISO_8601) format and in UTC,
  as from
  [`Date.toISOString()`](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/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 nested 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 `bunyan.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](https://github.com/mcavage/node-restify) facilitates
  with its 'Request-Id' header).

- `req`: An HTTP server request. Bunyan provides `bunyan.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 `bunyan.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](http://nodejs.org/docs/latest/api/all.html#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.
In general streams are specified with the "streams" option:

    var bunyan = require('bunyan');
    var log = bunyan.createLogger({
        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 (internally converted to a `Logger.streams`).

    var log = bunyan.createLogger({
        name: "foo",
        stream: process.stderr,
        level: "debug"
    });

Note that "file" streams do not support this shortcut (partly for historical
reasons and partly to not make it difficult to add a literal "path" field
on log records).

If neither "streams" nor "stream" are specified, the default is a stream of
type "stream" emitting to `process.stdout` at the "info" level.


## stream errors

Bunyan re-emits error events from the created `WriteStream`. So you can
do this:

    var log = bunyan.createLogger({name: 'mylog', streams: [{path: LOG_PATH}]});
    log.on('error', function (err, stream) {
        // Handle stream write or create error here.
    });


## stream type: `stream`

A `type === 'stream'` is a plain ol' node.js [Writable
Stream](http://nodejs.org/docs/latest/api/all.html#writable_Stream). A
"stream" (the writeable stream) field is required. E.g.: `process.stdout`,
`process.stderr`.

    var log = bunyan.createLogger({
        name: 'foo',
        streams: [{
            stream: process.stderr
            // `type: 'stream'` is implied
        }]
    });

<table>
<tr>
<th>Field</th>
<th>Required?</th>
<th>Default</th>
<th>Description</th>
</tr>
<tr>
<td>stream</td>
<td>Yes</td>
<td>-</td>
<td>A "Writable Stream", e.g. a std handle or an open file write stream.</td>
</tr>
<tr>
<td>type</td>
<td>No</td>
<td>n/a</td>
<td>`type == 'stream'` is implied if the `stream` field is given.</td>
</tr>
<tr>
<td>level</td>
<td>No</td>
<td>info</td>
<td>The level at which logging to this stream is enabled. If not
specified it defaults to "info". If specified this can be one of the
level strings ("trace", "debug", ...) or constants (`bunyan.TRACE`,
`bunyan.DEBUG`, ...).</td>
</tr>
<tr>
<td>name</td>
<td>No</td>
<td>-</td>
<td>A name for this stream. This may be useful for usage of `log.level(NAME,
LEVEL)`. See the [Levels section](#levels) for details. A stream "name" isn't
used for anything else.</td>
</tr>
</table>


## stream type: `file`

A `type === 'file'` stream requires a "path" field. Bunyan will open this
file for appending. E.g.:

    var log = bunyan.createLogger({
        name: 'foo',
        streams: [{
            path: '/var/log/foo.log',
            // `type: 'file'` is implied
        }]
    });

<table>
<tr>
<th>Field</th>
<th>Required?</th>
<th>Default</th>
<th>Description</th>
</tr>
<tr>
<td>path</td>
<td>Yes</td>
<td>-</td>
<td>A file path to which to log.</td>
</tr>
<tr>
<td>type</td>
<td>No</td>
<td>n/a</td>
<td>`type == 'file'` is implied if the `path` field is given.</td>
</tr>
<tr>
<td>level</td>
<td>No</td>
<td>info</td>
<td>The level at which logging to this stream is enabled. If not
specified it defaults to "info". If specified this can be one of the
level strings ("trace", "debug", ...) or constants (`bunyan.TRACE`,
`bunyan.DEBUG`, ...).</td>
</tr>
<tr>
<td>name</td>
<td>No</td>
<td>-</td>
<td>A name for this stream. This may be useful for usage of `log.level(NAME,
LEVEL)`. See the [Levels section](#levels) for details. A stream "name" isn't
used for anything else.</td>
</tr>
</table>


## stream type: `rotating-file`

A `type === 'rotating-file'` is a file stream that handles file automatic
rotation.

    var log = bunyan.createLogger({
        name: 'foo',
        streams: [{
            type: 'rotating-file',
            path: '/var/log/foo.log',
            period: '1d',   // daily rotation
            count: 3        // keep 3 back copies
        }]
    });

This will rotate '/var/log/foo.log' every day (at midnight) to:

    /var/log/foo.log.0     # yesterday
    /var/log/foo.log.1     # 1 day ago
    /var/log/foo.log.2     # 2 days ago

*Currently*, there is no support for providing a template for the rotated
files, or for rotating when the log reaches a threshold size.

<table>
<tr>
<th>Field</th>
<th>Required?</th>
<th>Default</th>
<th>Description</th>
</tr>
<tr>
<td>type</td>
<td>Yes</td>
<td>-</td>
<td>"rotating-file"</td>
</tr>
<tr>
<td>path</td>
<td>Yes</td>
<td>-</td>
<td>A file path to which to log. Rotated files will be "$path.0",
"$path.1", ...</td>
</tr>
<tr>
<td>period</td>
<td>No</td>
<td>1d</td>
<td>The period at which to rotate. This is a string of the format
"$number$scope" where "$scope" is one of "h" (hours), "d" (days), "w" (weeks),
"m" (months), "y" (years). Or one of the following names can be used
"hourly" (means 1h), "daily" (1d), "weekly" (1w), "monthly" (1m),
"yearly" (1y). Rotation is done at the start of the scope: top of the hour (h),
midnight (d), start of Sunday (w), start of the 1st of the month (m),
start of Jan 1st (y).</td>
</tr>
<tr>
<td>count</td>
<td>No</td>
<td>10</td>
<td>The number of rotated files to keep.</td>
</tr>
<tr>
<td>level</td>
<td>No</td>
<td>info</td>
<td>The level at which logging to this stream is enabled. If not
specified it defaults to "info". If specified this can be one of the
level strings ("trace", "debug", ...) or constants (`bunyan.TRACE`,
`bunyan.DEBUG`, ...).</td>
</tr>
<tr>
<td>name</td>
<td>No</td>
<td>-</td>
<td>A name for this stream. This may be useful for usage of `log.level(NAME,
LEVEL)`. See the [Levels section](#levels) for details. A stream "name" isn't
used for anything else.</td>
</tr>
</table>



## stream type: `raw`

- `raw`: Similar to a "stream" writeable stream, except that the write method
  is given raw log record *Object*s instead of a JSON-stringified string.
  This can be useful for hooking on further processing to all Bunyan logging:
  pushing to an external service, a RingBuffer (see below), etc.



## `raw` + RingBuffer Stream

Bunyan comes with a special stream called a RingBuffer which keeps the last N
records in memory and does *not* write the data anywhere else.  One common
strategy is to log 'info' and higher to a normal log file but log all records
(including 'trace') to a ringbuffer that you can access via a debugger, or your
own HTTP interface, or a post-mortem facility like MDB or node-panic.

To use a RingBuffer:

    /* Create a ring buffer that stores the last 100 records. */
    var bunyan = require('bunyan');
    var ringbuffer = new bunyan.RingBuffer({ limit: 100 });
    var log = bunyan.createLogger({
        name: 'foo',
        streams: [
            {
                level: 'info',
                stream: process.stdout
            },
            {
                level: 'trace',
                type: 'raw',    // use 'raw' to get raw log record objects
                stream: ringbuffer
            }
        ]
    });

    log.info('hello world');
    console.log(ringbuffer.records);

This example emits:

    [ { name: 'foo',
        hostname: '912d2b29',
        pid: 50346,
        level: 30,
        msg: 'hello world',
        time: '2012-06-19T21:34:19.906Z',
        v: 0 } ]


## third-party streams

- syslog:
  [mcavage/node-bunyan-syslog](https://github.com/mcavage/node-bunyan-syslog)
  provides support for directing bunyan logging to a syslog server.

- TODO: eventually https://github.com/trentm/node-bunyan-winston



# Runtime log snooping via DTrace

On systems that support DTrace (e.g., MacOS, FreeBSD, illumos derivatives
like SmartOS and OmniOS), Bunyan will create a DTrace provider (`bunyan`)
that makes available the following probes:

    log-trace
    log-debug
    log-info
    log-warn
    log-error
    log-fatal

Each of these probes has a single argument: the string that would be
written to the log.  Note that when a probe is enabled, it will
fire whenever the corresponding function is called, even if the level of
the log message is less than that of any stream.


## DTrace examples

Trace all log messages coming from any Bunyan module on the system.
(The `-x strsize=4k` is to raise dtrace's default 256 byte buffer size
because log messages are longer than typical dtrace probes.)

    dtrace -x strsize=4k -qn 'bunyan*:::log-*{printf("%d: %s: %s", pid, probefunc, copyinstr(arg0))}'

Trace all log messages coming from the "wuzzle" component:

    dtrace -x strsize=4k -qn 'bunyan*:::log-*/strstr(this->str = copyinstr(arg0), "\"component\":\"wuzzle\"") != NULL/{printf("%s", this->str)}'

Aggregate debug messages from process 1234, by message:

    dtrace -x strsize=4k -n 'bunyan1234:::log-debug{@[copyinstr(arg0)] = count()}'

Have the bunyan CLI pretty-print the traced logs:

    dtrace -x strsize=4k -qn 'bunyan1234:::log-*{printf("%s", copyinstr(arg0))}' | bunyan

A convenience handle has been made for this:

    bunyan -p 1234


On systems that support the
[`jstack`](http://dtrace.org/blogs/dap/2012/04/25/profiling-node-js/) action
via a node.js helper, get a stack backtrace for any debug message that
includes the string "danger!":

    dtrace -x strsize=4k -qn 'log-debug/strstr(copyinstr(arg0), "danger!") != NULL/{printf("\n%s", copyinstr(arg0)); jstack()}'

Output of the above might be:

    {"name":"foo","hostname":"763bf293-d65c-42d5-872b-4abe25d5c4c7.local","pid":12747,"level":20,"msg":"danger!","time":"2012-10-30T18:28:57.115Z","v":0}

              node`0x87e2010
              DTraceProviderBindings.node`usdt_fire_probe+0x32
              DTraceProviderBindings.node`_ZN4node11DTraceProbe5_fireEN2v85LocalINS1_5ValueEEE+0x32d
              DTraceProviderBindings.node`_ZN4node11DTraceProbe4FireERKN2v89ArgumentsE+0x77
              << internal code >>
              (anon) as (anon) at /root/node-bunyan/lib/bunyan.js position 40484
              << adaptor >>
              (anon) as doit at /root/my-prog.js position 360
              (anon) as list.ontimeout at timers.js position 4960
              << adaptor >>
              << internal >>
              << entry >>
              node`_ZN2v88internalL6InvokeEbNS0_6HandleINS0_10JSFunctionEEENS1_INS0_6ObjectEEEiPS5_Pb+0x101
              node`_ZN2v88internal9Execution4CallENS0_6HandleINS0_6ObjectEEES4_iPS4_Pbb+0xcb
              node`_ZN2v88Function4CallENS_6HandleINS_6ObjectEEEiPNS1_INS_5ValueEEE+0xf0
              node`_ZN4node12MakeCallbackEN2v86HandleINS0_6ObjectEEENS1_INS0_8FunctionEEEiPNS1_INS0_5ValueEEE+0x11f
              node`_ZN4node12MakeCallbackEN2v86HandleINS0_6ObjectEEENS1_INS0_6StringEEEiPNS1_INS0_5ValueEEE+0x66
              node`_ZN4node9TimerWrap9OnTimeoutEP10uv_timer_si+0x63
              node`uv__run_timers+0x66
              node`uv__run+0x1b
              node`uv_run+0x17
              node`_ZN4node5StartEiPPc+0x1d0
              node`main+0x1b
              node`_start+0x83

              node`0x87e2010
              DTraceProviderBindings.node`usdt_fire_probe+0x32
              DTraceProviderBindings.node`_ZN4node11DTraceProbe5_fireEN2v85LocalINS1_5ValueEEE+0x32d
              DTraceProviderBindings.node`_ZN4node11DTraceProbe4FireERKN2v89ArgumentsE+0x77
              << internal code >>
              (anon) as (anon) at /root/node-bunyan/lib/bunyan.js position 40484
              << adaptor >>
              (anon) as doit at /root/my-prog.js position 360
              (anon) as list.ontimeout at timers.js position 4960
              << adaptor >>
              << internal >>
              << entry >>
              node`_ZN2v88internalL6InvokeEbNS0_6HandleINS0_10JSFunctionEEENS1_INS0_6ObjectEEEiPS5_Pb+0x101
              node`_ZN2v88internal9Execution4CallENS0_6HandleINS0_6ObjectEEES4_iPS4_Pbb+0xcb
              node`_ZN2v88Function4CallENS_6HandleINS_6ObjectEEEiPNS1_INS_5ValueEEE+0xf0
              node`_ZN4node12MakeCallbackEN2v86HandleINS0_6ObjectEEENS1_INS0_8FunctionEEEiPNS1_INS0_5ValueEEE+0x11f
              node`_ZN4node12MakeCallbackEN2v86HandleINS0_6ObjectEEENS1_INS0_6StringEEEiPNS1_INS0_5ValueEEE+0x66
              node`_ZN4node9TimerWrap9OnTimeoutEP10uv_timer_si+0x63
              node`uv__run_timers+0x66
              node`uv__run+0x1b
              node`uv_run+0x17
              node`_ZN4node5StartEiPPc+0x1d0
              node`main+0x1b
              node`_start+0x83


# Versioning

The scheme I follow is most succintly described by the bootstrap guys
[here](https://github.com/twitter/bootstrap#versioning).

tl;dr: All versions are `<major>.<minor>.<patch>` which will be incremented for
breaking backward compat and major reworks, new features without breaking
change, and bug fixes, respectively.


# License

MIT. See "LICENSE.txt".


# See Also

- Bunyan syslog support: <https://github.com/mcavage/node-bunyan-syslog>.
- Bunyan + Graylog2: <https://github.com/mhart/gelf-stream>.
- An example of a Bunyan shim to the Winston logging system:
  <https://github.com/trentm/node-bunyan-winston>.
- [Bunyan for Bash](https://github.com/trevoro/bash-bunyan).
- TODO: `RequestCaptureStream` example from restify.