felix-metrics

This is an alternative port of Coda Hale's metrics library.

I created this despite the existing metrics port for node.js because I wanted to fully understand the underlaying Math and algorithms.

Install

npm install felix-metrics

Usage

Step 1: Add metrics to your code. For example, lets track the requests/sec of a http server:

var metrics    = require('felix-metrics');
var collection = new metrics.Collection('http');
var http       = require('http');
 
var rps = collection.meter('requestsPerSecond');
http.createServer(function(req, res) {
  meter.mark();
  res.end('Thanks');
}).listen(3000);

Step 2: Show the collected metrics (more advanced examples follow later):

setInterval(function() {
  console.log(collection.toJSON());
}, 1000);

This will output something like this every second:

{ requestsPerSecond:
   { mean: 1710.2180279856818,
     count: 10511,
     '1MinuteRate': 168.08263156623656,
     '5MinuteRate': 34.74630977619571,
     '15MinuteRate': 11.646507524106095 } }

Step 3: Aggregate the data into your backend of choice. I recommend graphite.

Metrics

The following metrics are available (both standalone and on the Collection API):

Gauge

Values that can be read instantly. Example:

var gauge = new metrics.Gauge({read: function() {
  return process.memoryUsage().rss;
});

There is currently no callback support for Gauges because otherwise it would be very difficult to report the metrics inside a collection within a regular interval.

Options:

• read A function that returns the current value of the Gauge.

Methods:

None.

Counter

Things that increment or decrement. Example:

var activeUploads = new metrics.Counter();
http.createServer(function(req, res) {
  activeUploads.inc();
  req.on('end', function() {
    activeUploads.dec();
  });
});

Options:

• count An initial count for the counter. Defaults to 0.

Methods:

• inc(n) Increment the counter by n. Defaults to 1.
• dec(n) Decrement the counter by n. Defaults to 1.

Meter

Things that are measured as events / interval. Example:

var meter = new metrics.Meter();
http.createServer(function(req, res) {
  meter.mark();
});

Options:

• rateUnit The rate unit. Defaults to 1000 (1 sec).
• tickInterval The interval in which the averages are updated. Defaults to 5000 (5 sec).

Methods:

• mark(n) Register n events as having just occured. Defaults to `1.

Histogram

Things that are measured as distributions of scalars. Example:

var histogram = new metrics.Histogram();
http.createServer(function(req, res) {
  if (req.headers['content-length']) {
    histogram.update(parseInt(req.headers['content-length'], 10));
  }
});

Options:

• sample The sample resevoir to use. Defaults to an ExponentiallyDecayingSample.

Methods:

• update(value, timestamp) Pushes value into the sample. timestamp defaults to Date.now().

Timers

Timers are a combination of Meters and Histograms. They measure the rate as well as distribution of scalar events. Since they are frequently used for tracking how long certain things take, they expose an API for that:

var timer = new metrics.Timer();
http.createServer(function(req, res) {
  var stopwatch = timer.start();
  req.on('end', function() {
    stopwatch.end();
  });
});

But you can also use them as generic histograms that also track the rate of events:

var timer = new metrics.Timer();
http.createServer(function(req, res) {
  if (req.headers['content-length']) {
    timer.update(parseInt(req.headers['content-length'], 10));
  }
});

Options:

• meter The internal meter to use. Defaults to a new Meter.
• histogram The internal histogram to use. Defaults to a new Histogram.

Methods:

• start() Returns a Stopwatch.
• update(value) Updates the internal histogram with value and marks one event on the internal meter.

Todo

• Implement a graphite reporter and feature it in the usage section.

License

This module is licensed under the MIT license.