Bunyan -- a JSON Logger for node.js servers.

Server logs should be structured. JSON's a good format. Let's do that: a log
record is one line of `JSON.stringify`'d output. Let's also specify some common
names for the requisite and common fields for a log record (see below).

Also: log4j is way more than you need.


# Current Status

Just play stuff here. Don't try to use this for realz yet.


# Usage

**The usual.** All loggers must provide a "service" name. This is somewhat akin
to log4j logger "name", but Bunyan doesn't so hierarchical logger names.

    $ cat hi.js
    var Logger = require('bunyan');
    var log = new Logger({service: "myapp"});
    log.info("hi");

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

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

A **`bunyan` tool is provided for pretty-printing** bunyan logs and, eventually,
for filtering (e.g. `| bunyan -c 'level>3'`). This shows the default output
(which is fluid right now) and indented-JSON output. More output formats will
be added, including support for custom formats.

    $ node hi.js | ./bin/bunyan  # CLI tool to filter/pretty-print JSON logs.
    [2012-01-31T00:08:11.387Z] INFO: myapp on banana.local: hi
    
    $ node hi.js | ./bin/bunyan -o json
    {
      "service": "myapp",
      "hostname": "banana.local",
      "level": 2,
      "msg": "hi",
      "time": "2012-01-31T00:10:00.676Z",
      "v": 0
    }

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

    var log = new Logger({service: "myapp", stream: process.stdout, 
      level: "info"});

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

    var log = new Logger({
      service: "amon",
      streams: [
        {
          level: "info",
          stream: process.stdout, // log INFO and above to stdout
        },
        {
          level: "error",
          path: "tmp/error.log"   // log ERROR and above to a file
        }
      ]
    });

A `log.clone(...)` is provided to specialize a logger for a sub-component.
The following will have log records from "Wuzzle" instances use exactly the
same config as its parent, plus include the "component" field.

    var log = new Logger(...);

    ...
    
    function Wuzzle(options) {
      this.log = options.log;
      this.log.info("creating a wuzzle")
    }
    Wuzzle.prototype.woos = function () {
      this.log.warn("This wuzzle is woosey.")
    }
    
    var wuzzle = new Wuzzle({log: log.clone({component: "wuzzle"})});
    wuzzle.woos();
    log.info("done with the wuzzle")


# Future

See "TODO.md", but basically:

- "Serializer" support to handle extracting a JSON object for a log record
  for particular object types, e.g. an HTTP request. So for example we
  could do:
  
        log.info({req: req}, "something about handling this request")

  And the "req" serializer would extract a reasonable JSON object for that
  request object -- presumably a subset of all attributes on the request
  object.
  
  This will key off the field name, IOW by convention, rather than getting
  into `instanceof` grossness.

- Spec'ing and enforcing the fields (from dap's section in eng guide).

- Syslog support. Ring-buffer support for storing last N debug messages
  (or whatever) in memory to support debugability without too much log load.

- More `bunyan` output formats and filtering features.

- Think about a bunyan dashboard that supports organizing and viewing logs
  from multiple hosts and services.



# Levels

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

"debug" should be used sparingly. Information that will be useful to debug
errors *post mortem* should usually be included in "info" messages if it's
generally relevant or else with the corresponding "error" event. Don't rely on
spewing mostly irrelevant debug messages all the time and sifting through them
when an error occurs.

Integers are used for the actual level values (1 for "trace", ..., 6 for "fatal") and
constants are defined for the (Logger.TRACE ... Logger.DEBUG). The lowercase
level names are aliases supported in the API.


# Log Record Fields

TODO: from dap and enforce these

- "request\_id" (better name?) can't be required because some things don't
  happen in a per-request context. Startup and background processing stuff
  for example. Tho for request-y things, it is strongly encouraged because it
  allows collating logs from multiple services for the same request.


# Streams

A "stream" is Bunyan's name for an output for log messages. It expects a
[Writable Stream](http://nodejs.org/docs/latest/api/all.html#writable_Stream)
interface. See above for some examples of specifying streams. Supported streams
are:

- A writable "stream". Often this is one of the std handles (`process.stdout` or
  `process.stderr`), but it can be anything you want supporting the node
  writable stream interface, e.g. `fs.createWriteStream`.
- A file. Will append to the given "path". 


# License

MIT.