bunyan

a JSON Logger library for node.js services

npm install bunyan
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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

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 @trentmick for updates to Bunyan.

There is an email discussion list bunyan-logging@googlegroups.com, also as a forum in the browser.

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

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 (or number).
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, 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 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 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 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 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 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 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 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. 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
    }]
});
Field Required? Default Description
stream Yes - A "Writable Stream", e.g. a std handle or an open file write stream.
type No n/a type == 'stream' is implied if the stream field is given.
level No info 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, ...).
name No - A name for this stream. This may be useful for usage of log.level(NAME, LEVEL). See the Levels section for details. A stream "name" isn't used for anything else.

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
    }]
});
Field Required? Default Description
path Yes - A file path to which to log.
type No n/a type == 'file' is implied if the path field is given.
level No info 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, ...).
name No - A name for this stream. This may be useful for usage of log.level(NAME, LEVEL). See the Levels section for details. A stream "name" isn't used for anything else.

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.

Field Required? Default Description
type Yes - "rotating-file"
path Yes - A file path to which to log. Rotated files will be "$path.0", "$path.1", ...
period No 1d 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).
count No 10 The number of rotated files to keep.
level No info 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, ...).
name No - A name for this stream. This may be useful for usage of log.level(NAME, LEVEL). See the Levels section for details. A stream "name" isn't used for anything else.

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" sematics (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:

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" writeable stream, except that the write method is given raw log record Objects 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

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 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}

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          node`_ZN4node9TimerWrap9OnTimeoutEP10uv_timer_si+0x63
          node`uv__run_timers+0x66
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          node`0x87e2010
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          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.

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

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