Metrix

An Elixir library to log custom application metrics (for use by other downstream systems such as Librato, Riemann etc...)

Metrix subscribes to the Twelve Factor App notion that logs are streams of time-ordered events and that events should be captured and recorded in the l2met logging convention.

What this means in pragmatic terms is that this library provides a few convenience methods to help you capture data from your application to stdout in a well-structured key-value format:

measure#api.request.service=23.43ms path=/v1/user.json response_status=200
sample#api.response.size=1.34kb path=/v1/user.json
count#user.login user_id=32 other="data with spaces"

Note: Metrix does not do any calculations itself. It merely logs the data its given in a specific format. If you are looking for an in-app instrumentation library, you may want to look at exometer or folsom instead.

Benefits

Treating "logs as data" in this manner has several advantages, including:

• Logs are machine parseable and human readable, providing the best of both worlds
• It's a low-overhead way of getting data out of your app, not requiring any additional in-app processing or dependencies
• The ability to pipe app data to one or more downstream processors, such as Librato for visualization and Reimann for alerting, without requiring modification to the app
• Data is output via stdout and can be manipulated with a multitude of POSIX utilities

If you are looking for a more substantive justification for this style of logging, please see 5 Steps to Better Application Logging.

Install

Add metrix to your applications in mix.exs:

def application do
  [mod: {YourApp, []},
   applications: [..., :metrix]]
end

And declare it as a dependency:

defp deps do
  [
    # ...
    {:metrix, "~> 0.2.0"}
  ]
end

Then update your dependencies:

$ mix deps.get

Usage

There are three types of metrics natively supported by Metrix: counts, samples and measurements.

Count

When you want to count the occurrences of an event in your app, use count:

import Metrix

count "app.event"
count "app.event", 3

Which will output:

count#event.name=1
count#event.name=3

Event metadata can be attached by passing in a map as the first argument:

%{"path" => "/users/1"} |> count "app.event"

Which outputs:

count#event.name=1 path=/users/1

metadata can be a map or keyword list:

[path: "/users/1"] |> count "app.event"

When passed to log processors like Librato, counts can be min, max, summed, stacked etc...

Sample

Samples are used to take periodic, point-in-time, measurements such as CPU load, hard disk space etc...

Samples are logged in the same fashion as count:

import Metrix

sample "file.size", "12.3kb"
[file: "/images/hi.png"] |> sample "file.size", "12.3kb"

Which will output:

sample#file.size=12.3kb
sample#file.size=12.3kb file=/images/hi.png

Samples are captured in Librato with the units used in the measurement value (kb in this case) and can be averaged, p50, p95, and p99d.

Measure

Measurements are measures of time, most often used to track execution time. As such, they wrap a block of code whose execution time is to be measured:

import Metrix

measure "api.request", fn -> HTTPotion.get "httpbin.org/get" end

[path: "/get"]
|> measure "api.request", fn -> HTTPotion.get "httpbin.org/get" end

Measurements are taken in ms:

measure#api.request=142ms
measure#api.request=131ms path=/get

It's common to want to add metadata to a measurement that is used within the function call (the path above being a good example). Instead of providing a no-arg function to measure, you can provide a 1-arity function that accepts the metrics metadata (and can pattern match against it). For instance:

%{"path" => "/get", "client" => "elixir"}
|> measure "api.request", fn(%{"path" => path}) -> HTTPotion.get "httpbin.org#{path}" end

measure#api.request=131ms path=/get client=elixir

Measurements in Librato are collected into median, p95 and p99 series:

Global context

Often times there is metadata you want applied to every measurement. For instance a source element indicating which server the output originated from or app which differentiates output from multiple components going to the same downstream processor. This type of universally applicable metadata can be set once using the global context:

Metrix.add_context %{"source" => System.get_env("NODE_NAME")}
Metrix.count "event.name"

count#event.name=1 source=node.us-east.1a

The context can be cleared with Metrix.clear_context, though be aware it is global context and will be cleared for all output.

Heroku & Librato

Librato is my preferred choice for metrics visualization and long-term storage. It also plays very well with apps deployed to Heroku. Follow these instructions to get your Heroku app's Metrix log output streaming to Librato for processing.

Librato add-on

If your app is deployed to Heroku, just add the Librato add-on and all custom counts, samples and measurements will automatically be sent to Librato which will apply median, p95, p99 and a host of other real-time aggregations. In addition, Heroku's native logging will also be piped to Librato, giving you both platform and app metrics in one place.

External Librato account

If you already have a Librato account, you can still stream your data to from Heroku by setting up a custom log drain.

Todo

There are a few known missing pieces, including:

• Multiple metrics per log line
• Scoped contexts (e.g., request ids)
• Logger/log level integration
• k/v pair ordering

Acknowledgements

This library was built as an Elixir alternative to the Ruby-based Scrolls library, which I've found to be indispensable. It was also built on top of Logfmt, which handles the mundane but critical task of actually formatting the output as correctly escaped key/value pairs.