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[rbdr/cologne] / README.md

# Cologne

Cologne is a logger multiplexer that uses a JSON log format inspired in gelf.
It can be instantiated with several loggers, or they can be changed after
the fact.

## Usage

Install from npm

```
$ npm install --save cologne
```

Create an instance

```javascript
const { Cologne, Loggers, Formatters } = require('cologne');

const co = new Cologne({
  from: 'Special Worker Logger',
  loggers: [
    new Loggers.Console({
      formatter: new Formatters.Token({
        formatString: '[{{_timestamp}}]{{_from}}: {{message}}'
      })
    })
  ]
});
```

This example would create a cologne instance with a console logger that
uses a Token formatter. (More on loggers and formatters below.);

## Quick API Reference

* **addLogger(logger)**: Adds a logger to the cologne instance.
* **removeLogger(logger)**: Removes a logger from the cologne instance.
* **buildLog(item, level, [meta])**: Generates a cologne log object as if you had
  logged an item (it will do this automatically when you log anything.)
  level defaults to 6. You can optionally send it an object to extend
  the object with.
* **log, info, notice, warn, error**: Generates a log object with the
  appropriate severity level and sends it to all loggers.

## Loggers

Cologne loggers are any object that responds to the `#log()` method.
This methoud should be able to receive any number of arguments and
log them independently. Similar to how you can send multiple arguments
to the browser console.

`#log()` will receive any number of `Cologne Log Objects`. For a detailed
reference of this format, see further below.

Cologne includes two loggers out of the box:

* `Loggers.Console` logs to the JS console
* `Loggers.File` appends to a file

### Loggers.Console

This logger communicates with the Javascript console. It uses the log level
to trigger the appropriate method, so `error` logs would go to stderr
as expected when calling `console.error`.

This logger can be sent a `formatter`, which is an object that responds
to the `#format()` method: it should receive a cologne log object and respond
with a string.

```javascript
new Loggers.Console({
  formatter : new Formatters.Token({
    formatString: '[{{_timestamp}}]{{_from}}: {{message}}'
  })
});
```

### Loggers.File

This logger opens a writable stream to a file, to which it will append
everything. Like the Console logger it supports a `formatter` property
that will respond to the `#format()` method.

It MUST include a `file` property on initialization, otherwise it will throw
an exception.

```javascript
new Loggers.File({
  file: '/var/log/server_log.log',
  formatter : new Formatters.Token({
    formatString: '[{{_ansi:_level}}{{_timestamp}}{{_ansi:reset}}]{{_from}}: {{message}}'
  })
});
```

### More Loggers?

We're working on a socket logger. It's separate so you don't have to
install the socket dependencies if you don't want to.

You can build your own logger easily for any method of transport you find
necessary (e.g. mail, database, twitter, etc). Any object that responds
to `#log()` is a valid logger:

```javascript
// A valid, very minimalistic logger
const simpleLogger = {
  log: function(...logs) {

    for (const log of logs) {
      this._doSomeMagic(logs);
    }
  },

  _doSomeMagic: function(log) {

    console.log(log + "... but magical!");
  }
};

logger.addLogger(simpleLogger);
```


## Formatters

Cologne doesn't need formatters to work, and in fact they're optional in
the included loggers. But if you would like to make your logs prettier,
then you can use one of the included formatters or roll your own.

Formatters are objects that respond to the `#format()` method. It will
receive a single cologne log object (see fields it includes below), and
it should return a string.

We include some formatters so you can get running real quicklike:

* `Formatters.Simple` a simple predefined formatter
* `Formatters.Token` a formatter that lets you define format
  strings that it will use to build your final log.

### Formatters.Simple

This is the lazy formatter, it just outputs the string in the following
format:

```
'[{{_timestamp}}][{{_levelString}}]{{_from}}: {{message}}'
```

Where `_timestamp` is converted to ISO.

#### Accepted Options

* `colorize` <Boolean>: whether or not to add color. False by default.

By default we don't colorize the output, but if you enable the flag this
formatter will add a bit of color in the level string. Red for error, crit,
alert, and emerg; yellow for warn; blue for info; green for debug; and white
for everything else.

#### Usage

```javascript
new Formatters.Simple({
  colorize: true
});
```

### Example Output

```
co.log("hello world");
// -> [2016-01-21T05:50:36.505Z][INFO] Server Logger: hello world
```

### Formatters.Token

The token formatter lets you build strings with simple tokens. When
instantiating, you can specify a `formatString` to interpolate
properties from the logObject. The default version looks for tokens
inside double curly braces like `{{message}}` or `{{_level}}`. If
you don't like it, you can specify your own.

#### Accepted Options

* `formatString` <String>: The string used to replace. Defaults to `"{{message}}"`
* `replaceRule` <String>: The regex rule to use for replacement of tokens in the
  formatString. Defaults to `/{{(.*?)}}/g`
* `isoDate` <Boolean> : Whether or not to convert `_timestamp` to ISO
  date. Defaults to true. Otherwise it'll use the raw timestamp.

#### Usage

```javascript
new Formatters.Token({
  formatString: '[{{_timestamp}}]{{_from}}: {{message}}'
});
```

#### ANSI tokens

If you want to add color to your logs, you can use the special \_ansi
token. It has several options which you can call like `{{_ansi:red}}`
and `{{_ansi:reset}}`. Here's a list of all the ansi stuff you can use:

* `bold`: makes text bold
* `italics`: makes text italics
* `underline`: makes text underlined
* `inverse`: inverts foreground and background
* `strikethrough`: strikethrough text
* `bold_off`, `italics_off`, `underline_off`, `inverse_off`, and
  `strikethrough_off`: turn off the specified effect.
* `black`, `red`, `green`, `yellow`, `blue`, `magenta`, `cyan`, `white`,
  and `default`: change the foreground color of your text.
* `black_bg`, `red_bg`, `green_bg`, `yellow_bg`, `blue_bg`, `magenta_bg`,
  `cyan_bg`, `white_bg`, and `default_bg`: change the background color of your
  text.
* `reset`: makes everything normal again.
* `_level`: this is a special code that will set a color depending on
  the level of the log: debug gets green, info and notice blue, warn is
  yellow, and anything worse is red.

### More Formatters?

You can create your own formatters by creating an object that responds
to the `#format()` method, knows how to handle cologne log objects and
returns a string.

Here's an example of a logger that surrounds a log with sparkles:

```javascript
const sparkleFormatter = {
  format: function(logObject) {

    return `✨${logObject.message}✨`;
  }
}

logger.addLogger(new Loggers.Console({
  formatter: sparkleFormatter
}));
```

## The Cologne Log Format

The cologne log format is a JSON based log format, based on the cobalt
log format, which is in turn based on Graylog's GELF. However, where GELF
treats all internal fields without a prefix, and all user fields with a
prefix, we do it backwards so it's easier to extend the object with
metadata from existing objects.

You could try to build it on your own, but you can use `#buildLog()`
to build it without logging.

### Fields

* **\_timestamp** : A bigint timestamp in nanoseconds
* **\_cologneLog** <String> : This is how we know if the log is already
  formatted and ready to go. This field is a string containing the
  version of cologne log format it's using. It's `2.0.0` right now.
* **\_from**: The sender of the log (Defaults to Generic Cologne Logger)
* **\_level**: The level of the log (Defaults to 6)
* **\_levelString**: The string corresponding to the log level (e.g. 7 ->
  debug, 3 -> error, 0 -> emerg)

### A word on Log Levels

The log levels in cologne correspond to the syslog levels, and the
levelStrings correspond to the priority keywords:

* `0 -> emerg`
* `1 -> alert`
* `2 -> crit`
* `3 -> error`
* `4 -> warning`
* `5 -> notice`
* `6 -> info`
* `7 -> debug`

This is useful when deciding how to log. You could even have a logger
filter out unnecessary levels (eg. If you have a reporting logger that
only reports error or worse.)

## Further Improvements

* Improve the API for buildLog
* More loggers & formatters (will not be distributed in core cologne)
* Improve tests