node-bunyan-lite/README.md

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

```js
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).


## Table of Contents

<!-- toc -->

- [Current Status](#current-status)
- [Installation](#installation)
- [Features](#features)
- [Introduction](#introduction)
  * [Constructor API](#constructor-api)
  * [Log Method API](#log-method-api)
  * [CLI Usage](#cli-usage)
  * [Streams Introduction](#streams-introduction)
  * [log.child](#logchild)
  * [Serializers](#serializers)
    + [Requirements for serializers functions](#requirements-for-serializers-functions)
    + [Standard Serializers](#standard-serializers)
  * [src](#src)
- [Levels](#levels)
  * [Level suggestions](#level-suggestions)
- [Log Record Fields](#log-record-fields)
  * [Core fields](#core-fields)
  * [Recommended/Best Practice Fields](#recommendedbest-practice-fields)
  * [Other fields to consider](#other-fields-to-consider)
- [Streams](#streams)
  * [Adding a Stream](#adding-a-stream)
  * [stream errors](#stream-errors)
  * [stream type: `stream`](#stream-type-stream)
  * [stream type: `file`](#stream-type-file)
  * [stream type: `rotating-file`](#stream-type-rotating-file)
  * [stream type: `raw`](#stream-type-raw)
  * [`raw` + RingBuffer Stream](#raw--ringbuffer-stream)
  * [third-party streams](#third-party-streams)
- [Runtime log snooping via DTrace](#runtime-log-snooping-via-dtrace)
  * [DTrace examples](#dtrace-examples)
- [Runtime environments](#runtime-environments)
  * [Browserify](#browserify)
  * [Webpack](#webpack)
- [Versioning](#versioning)
- [License](#license)
- [See Also](#see-also)

<!-- tocstop -->

# Current Status

Solid core functionality is there. Joyent is using this for a number of
production services. Bunyan supports node 0.10 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

```sh
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)
- lightweight specialization of Logger instances with [`log.child`](#logchild)
- custom rendering of logged objects with ["serializers"](#serializers)
- [Runtime log snooping via DTrace support](#runtime-log-snooping-via-dtrace)
- Support for a few [runtime environments](#runtime-environments): Node.js,
  [Browserify](http://browserify.org/), [Webpack](https://webpack.github.io/), [NW.js](http://nwjs.io/).


# Introduction

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

```js
// 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".

```sh
$ 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}
```


## Constructor API

```js
var bunyan = require('bunyan');
var log = bunyan.createLogger({
    name: <string>,                     // Required
    level: <level name or number>,      // Optional, see "Levels" section
    stream: <node.js stream>,           // Optional, see "Streams" section
    streams: [<bunyan streams>, ...],   // Optional, see "Streams" section
    serializers: <serializers mapping>, // Optional, see "Serializers" section
    src: <boolean>,                     // Optional, see "src" section

    // Any other fields are added to all log records as is.
    foo: 'bar',
    ...
});
```


## Log Method API

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

```js
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 (or number).
log.info('hi %s', bob, anotherVar); // Uses `util.format` for msg formatting.

log.info({foo: 'bar'}, 'hi');
                // The first field can optionally be a "fields" object, which
                // is merged into the log record.

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".

log.info({foo: 'bar', err: err}, 'some msg about this error');
                // To pass in an Error *and* other fields, use the `err`
                // field name for the Error instance.
```

Note that this implies **you cannot blindly pass any object as the first
argument to log it** because that object might include fields that collide with
Bunyan's [core record fields](#core-fields). In other words,
`log.info(mywidget)` may not yield 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.:

```js
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`, `log.error`, 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:

```sh
$ node hi.js | ./node_modules/.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:

```sh
$ 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):

```sh
$ 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:

```js
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**.

```js
var log = bunyan.createLogger({
  name: 'myapp',
  streams: [
    {
      level: 'info',
      stream: process.stdout            // log INFO and above to stdout
    },
    {
      level: 'error',
      path: '/var/tmp/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:

```js
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):

```sh
$ 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):

```sh
$ 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:

```js
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 **"serializer" functions to produce a JSON-able object
from a JavaScript object**, so you can easily do the following:

```js
log.info({req: <request object>}, 'something about handling this request');
```

and have the `req` entry in the log record be just a reasonable subset of
`<request object>` fields (or computed data about those fields).


A logger instance can have a `serializers` mapping of log record field name
("req" in this example) to a serializer function. When creating the log
record, Bunyan will call the serializer function for fields of that name.
An example:

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


Typically serializers are added to a logger at creation time via
`bunyan.createLogger({..., serializers: <serializers>})`. However, serializers
can be added after creation via `<logger>.addSerializers(...)`, e.g.:

```js
var log = bunyan.createLogger({name: 'myapp'});
log.addSerializers({req: reqSerializer});
```

### Requirements for serializers functions

A serializer function is passed unprotected objects that are passed to the
`log.info`, `log.debug`, etc. call. This means a poorly written serializer
function can case side-effects. Logging shouldn't do that. Here are a few
rules and best practices for serializer functions:

- A serializer function *should never throw*. The bunyan library *does*
  protect somewhat from this: if the serializer throws an error, then
  bunyan will (a) write an ugly message on stderr (along with the traceback),
  and (b) the field in the log record will be replace with a short error message.
  For example:

    ```
    bunyan: ERROR: Exception thrown from the "foo" Bunyan serializer. This should never happen. This is a bug in that serializer function.
    TypeError: Cannot read property 'not' of undefined
        at Object.fooSerializer [as foo] (/Users/trentm/tm/node-bunyan/bar.js:8:26)
        at /Users/trentm/tm/node-bunyan/lib/bunyan.js:873:50
        at Array.forEach (native)
        at Logger._applySerializers (/Users/trentm/tm/node-bunyan/lib/bunyan.js:865:35)
        at mkRecord (/Users/trentm/tm/node-bunyan/lib/bunyan.js:978:17)
        at Logger.info (/Users/trentm/tm/node-bunyan/lib/bunyan.js:1044:19)
        at Object.<anonymous> (/Users/trentm/tm/node-bunyan/bar.js:13:5)
        at Module._compile (module.js:409:26)
        at Object.Module._extensions..js (module.js:416:10)
        at Module.load (module.js:343:32)
    {"name":"bar","hostname":"danger0.local","pid":47411,"level":30,"foo":"(Error in Bunyan log \"foo\" serializer broke field. See stderr for details.)","msg":"one","time":"2017-03-08T02:53:51.173Z","v":0}
    ```

- A serializer function *should never mutate the given object*. Doing so will
  change the object in your application.

- A serializer function *should be defensive*. In my experience it is common to
  set a serializer in an app, say for field name "foo", and then accidentally
  have a log line that passes a "foo" that is undefined, or null, or of some
  unexpected type. A good start at defensiveness is to start with this:

    ```javascript
    function fooSerializers(foo) {
        // Guard against foo be null/undefined. Check that expected fields
        // are defined.
        if (!foo || !foo.bar)
            return foo;
        var obj = {
            // Create the object to be logged.
            bar: foo.bar
        }
        return obj;
    };
    ```


### Standard Serializers

Bunyan includes a small set of "standard serializers", exported as
`bunyan.stdSerializers`. Their use is completely optional. Example using
all of them:

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

or particular ones:

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

Standard serializers are:

| Field | Description |
| ----- | ----------- |
| err   | Used for serializing JavaScript error objects, including traversing an error's cause chain for error objects with a `.cause()` -- e.g. as from [verror](https://github.com/davepacheco/node-verror). |
| req   | Common fields from a node.js HTTP request object. |
| res   | Common fields from a node.js HTTP response object. |

Note that the `req` and `res` serializers intentionally do not include the
request/response *body*, as that can be prohibitively large. If helpful, the
[restify framework's audit logger plugin](https://github.com/restify/node-restify/blob/ac13902ad9716dcb20aaa62295403983075b1841/lib/plugins/audit.js#L38-L87)
has its own req/res serializers that include more information (optionally
including the body).


## 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:

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

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

```js
{
  "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* of the author.

- "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.

Setting a logger instance (or one of its streams) to a particular level implies
that all log records *at that level and above* are logged. E.g. a logger set to
level "info" will log records at level info and above (warn, error, fatal).

While using log level *names* is preferred, the actual level values are integers
internally (10 for "trace", ..., 60 for "fatal"). Constants are defined for
the levels: `bunyan.TRACE` ... `bunyan.FATAL`. The lowercase level names are
aliases supported in the API, e.g. `log.level("info")`. There is one exception:
DTrace integration uses the level names. The fired DTrace probes are named
'bunyan-$levelName'.

Here is the API for querying and changing levels on an existing logger.
Recall that a logger instance has an array of output "streams":

```js
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
```


## Level suggestions

Trent's biased suggestions for server apps: 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.

Trent's biased suggestions for node.js libraries: IMHO, libraries should only
ever log at `trace`-level. Fine control over log output should be up to the
app using a library. Having a library that spews log output at higher levels
gets in the way of a clear story in the *app* logs.


# 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:

```js
{"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:

```js
{
  "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 overridden.
  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 overridden.
  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 overridden.
  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:

    ```js
    ...
    "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:

    ```js
    {
      "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:

    ```js
    {
      "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 where it outputs log messages (the equivalent
to a log4j Appender). Ultimately Bunyan uses a
[Writable Stream](https://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:

```js
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`).

```js
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.

## Adding a Stream

After a bunyan instance has been initialized, you may add additional streams by
calling the `addStream` function.

```js
var bunyan = require('bunyan');
var log = bunyan.createLogger('myLogger');
log.addStream({
  name: "myNewStream",
  stream: process.stderr,
  level: "debug"
});
```

## stream errors

A Bunyan logger instance can be made to re-emit "error" events from its
streams. Bunyan does so by default for [`type === "file"`
streams](#stream-type-file), so you can do this:

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

As of bunyan@1.7.0, the `reemitErrorEvents` field can be used when adding a
stream to control whether "error" events are re-emitted on the Logger. For
example:

    var EventEmitter = require('events').EventEmitter;
    var util = require('util');

    function MyFlakyStream() {}
    util.inherits(MyFlakyStream, EventEmitter);

    MyFlakyStream.prototype.write = function (rec) {
        this.emit('error', new Error('boom'));
    }

    var log = bunyan.createLogger({
        name: 'this-is-flaky',
        streams: [
            {
                type: 'raw',
                stream: new MyFlakyStream(),
                reemitErrorEvents: true
            }
        ]
    });
    log.info('hi there');

The behaviour is as follows:

- `reemitErrorEvents` not specified: `file` streams will re-emit error events
  on the Logger instance.
- `reemitErrorEvents: true`: error events will be re-emitted on the Logger
  for any stream with a `.on()` function -- which includes file streams,
  process.stdout/stderr, and any object that inherits from EventEmitter.
- `reemitErrorEvents: false`: error events will not be re-emitted for any
  streams.

Note: "error" events are **not** related to log records at the "error" level
as produced by `log.error(...)`. See [the node.js docs on error
events](https://nodejs.org/api/events.html#events_error_events) for details.


## 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 writable stream) field is required. E.g.: `process.stdout`,
`process.stderr`.

```js
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 to 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`, ...). This serves as a severity threshold for that stream
so logs of greater severity will also pass through (i.e. If level="warn",
error and fatal will also pass through this stream).</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.:

```js
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 to 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`, ...). This serves as a severity threshold for that
stream so logs of greater severity will also pass through (i.e. If
level="warn", error and fatal will also pass through this stream).</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`

**WARNING on node 0.8 usage:** Users of Bunyan's `rotating-file` should (a) be
using at least bunyan 0.23.1 (with the fix for [this
issue](https://github.com/trentm/node-bunyan/pull/97)), and (b) should use at
least node 0.10 (node 0.8 does not support the `unref()` method on
`setTimeout(...)` needed for the mentioned fix). The symptom is that process
termination will hang for up to a full rotation period.

**WARNING on [cluster](http://nodejs.org/docs/latest/api/all.html#all_cluster)
usage:** Using Bunyan's `rotating-file` stream with node.js's "cluster" module
can result in unexpected file rotation. You must not have multiple processes
in the cluster logging to the same file path. In other words, you must have
a separate log file path for the master and each worker in the cluster.
Alternatively, consider using a system file rotation facility such as
`logrotate` on Linux or `logadm` on SmartOS/Illumos. See
[this comment on issue #117](https://github.com/trentm/node-bunyan/issues/117#issuecomment-44804938)
for details.

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

```js
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:

```sh
/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 "ms" (milliseconds -- only useful for
testing), "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>


**Note on log rotation**: Often you may be using external log rotation utilities
like `logrotate` on Linux or `logadm` on SmartOS/Illumos. In those cases, unless
your are ensuring "copy and truncate" semantics (via `copytruncate` with
logrotate or `-c` with logadm) then the fd for your 'file' stream will change.
You can tell bunyan to reopen the file stream with code like this in your
app:

```js
var log = bunyan.createLogger(...);
...
process.on('SIGUSR2', function () {
    log.reopenFileStreams();
});
```

where you'd configure your log rotation to send SIGUSR2 (or some other signal)
to your process. Any other mechanism to signal your app to run
`log.reopenFileStreams()` would work as well.


## stream type: `raw`

- `raw`: Similar to a "stream" writable 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:

```js
/* 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:

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

## third-party streams

See the [user-maintained list in the Bunyan
wiki](https://github.com/trentm/node-bunyan/wiki/Awesome-Bunyan).


# Runtime log snooping via DTrace

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

```sh
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.)

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

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

```sh
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:

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

Have the bunyan CLI pretty-print the traced logs:

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

A convenience handle has been made for this:

```sh
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!":

```sh
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
```


# Runtime environments

Node-bunyan supports running in a few runtime environments:

- [Node.js](https://nodejs.org/)
- [Browserify](http://browserify.org/): See the
  [Browserify section](#browserify) below.
- [Webpack](https://webpack.github.io/): See the [Webpack section](#webpack) below.
- [NW.js](http://nwjs.io/)

Support for other runtime environments is welcome. If you have suggestions,
fixes, or mentions that node-bunyan already works in some other JavaScript
runtime, please open an [issue](https://github.com/trentm/node-bunyan/issues/new)
or a pull request.

The primary target is Node.js. It is the only environment in which I
regularly test. If you have suggestions for how to automate testing for other
environments, I'd appreciate feedback on [this automated testing
issue](https://github.com/trentm/node-bunyan/issues/342).

## Browserify

As the [Browserify](http://browserify.org/) site says it "lets you
`require('modules')` in the browser by bundling up all of your dependencies."
It is a build tool to run on your node.js script to bundle up your script and
all its node.js dependencies into a single file that is runnable in the
browser via:

```html
<script src="play.browser.js"></script>
```

As of version 1.1.0, node-bunyan supports being run via Browserify. The
default [stream](#streams) when running in the browser is one that emits
raw log records to `console.log/info/warn/error`.

Here is a quick example showing you how you can get this working for your
script.

1. Get browserify and bunyan installed in your module:

    ```sh
    $ npm install browserify bunyan
    ```

2. An example script using Bunyan, "play.js":

    ```js
    var bunyan = require('bunyan');
    var log = bunyan.createLogger({name: 'play', level: 'debug'});
    log.trace('this one does not emit');
    log.debug('hi on debug');   // console.log
    log.info('hi on info');     // console.info
    log.warn('hi on warn');     // console.warn
    log.error('hi on error');   // console.error
    ```

3. Build this into a bundle to run in the browser, "play.browser.js":

    ```sh
    $ ./node_modules/.bin/browserify play.js -o play.browser.js
    ```

4. Put that into an HTML file, "play.html":

    ```html
    <!DOCTYPE html>
    <html>
    <head>
      <meta charset="utf-8">
      <script src="play.browser.js"></script>
    </head>
    <body>
      <div>hi</div>
    </body>
    </html>
    ```

5. Open that in your browser and open your browser console:

    ```sh
    $ open play.html
    ```

Here is what it looks like in Firefox's console: ![Bunyan + Browserify in the
Firefox console](./docs/img/bunyan.browserify.png)

For some, the raw log records might not be desired. To have a rendered log line
you'll want to add your own stream, starting with something like this:

```js
var bunyan = require('./lib/bunyan');

function MyRawStream() {}
MyRawStream.prototype.write = function (rec) {
    console.log('[%s] %s: %s',
        rec.time.toISOString(),
        bunyan.nameFromLevel[rec.level],
        rec.msg);
}

var log = bunyan.createLogger({
    name: 'play',
    streams: [
        {
            level: 'info',
            stream: new MyRawStream(),
            type: 'raw'
        }
    ]
});

log.info('hi on info');
```

## Webpack
Webpack can work with the same example Browserify above. To do this, we need to make webpack ignore optional files:
Create "empty_shim.js":
```javascript
// This is an empty shim for things that should be not be included in webpack
```
Now tell webpack to use this file for
[optional dependencies](https://webpack.github.io/docs/configuration.html#resolve-alias)
in your "webpack.config.js":
```
resolve: {
    // These shims are needed for bunyan
    alias: {
        'dtrace-provider': '/path/to/shim/empty_shim.js',
        fs: '/path/to/shim/empty_shim.js',
        'safe-json-stringify': '/path/to/shim/empty_shim.js',
        mv: '/path/to/shim/empty_shim.js',
        'source-map-support': '/path/to/shim/empty_shim.js'
    }
}
```
Now webpack builds, ignoring these optional dependencies via shimming in an empty JS file!

# Versioning

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. tl;dr: [Semantic
versioning](http://semver.org/).

# License

[MIT](./LICENSE.txt).

# See Also

See the [user-maintained list of Bunyan-related software in the Bunyan
wiki](https://github.com/trentm/node-bunyan/wiki/Awesome-Bunyan).