bc-prometheus-ruby - Drop-in Prometheus metrics

gem 'bc-prometheus-ruby'

Then in your application.rb, prior to extending Rails::Application or any initializers:

require 'bigcommerce/prometheus'

Then in your web server config file (e.g. puma.rb)

before_fork do
  Rails.application.config.before_fork_callbacks.each(&:call)
end

You can then view your metrics at: http://0.0.0.0:9394/metrics

For extra Puma metrics, add this to config/puma.rb:

after_worker_fork do
  Rails.application.config.after_fork_callbacks.each(&:call)
end

In your task 'resque:setup' rake task, do:

require 'bigcommerce/prometheus'
Bigcommerce::Prometheus::Instrumentors::Resque.new(app: Rails.application).start

After requiring the main file, you can further configure with:

To create custom metrics and collectors, simply create two files: a collector (the class that runs and collects metrics), and the type collector, which runs on the threaded prometheus server and

First, create a type collector. Note that the "type" of this will be the full name of the class, with TypeCollector stripped. This is important later. Our example here will have a "type" of "app".

class AppTypeCollector < ::Bigcommerce::Prometheus::TypeCollectors::Base
  def build_metrics
    {
      honks: PrometheusExporter::Metric::Counter.new('honks', 'Running counter of honks'),
      points: PrometheusExporter::Metric::Gauge.new('points', 'Current amount of points')
    }
  end

  def collect_metrics(data:, labels: {})
    metric(:points).observe(data.fetch('points', 0))
    metric(:honks).observe(1, labels) if data.fetch('honks', 0).to_i.positive?
  end
end

There are two important methods here: build_metrics, which registers the different metrics you want to measure, and collect_metrics, which actually takes in the metrics and prepares them to be rendered so that Prometheus can scrape them.

Note also in the example the different ways of observing Gauges vs Counters.

Next, create a collector. Your "type" of the Collector must match the type collector above, so that bc-prometheus-ruby knows how to map the metrics to the right TypeCollector. This is inferred from the class name. Here, it is "app":

class AppCollector < ::Bigcommerce::Prometheus::Collectors::Base
  def honk!
    push(
      honks: 1,
      custom_labels: {
        volume: 'loud'
      }
    )
  end

  def collect(metrics)
    metrics[:points] = rand(1..100)
    metrics
  end
end

There are two types of metrics here: on-demand, and polled. Let's look at the first:

To issue an on-demand metric (usually a counter) that then automatically updates, in your application code, you would then run:

app_collector = AppCollector.new
app_collector.honk!

This will "push" the metrics to our AppTypeCollector instance, which will render them as:

# HELP ruby_honks Running counter of honks
# TYPE ruby_honks counter
ruby_honks{volume="loud"} 2

As you can see this will respect any custom labels we push in as well.

Using our same AppCollector, if you note the collect method: this method will run on a 15 second polled basis (the frequency of which is configurable in the initializer of the AppCollector). Here we're just spitting out random points, so it'll look something like this:

# HELP ruby_points Current amount of points
# TYPE ruby_points gauge
ruby_points 42

Each different type of integration will need to have the collectors passed into them, where appropriate. For example, if we want these collectors to run on our web, resque, and hutch processes, we'll need to:

::Bigcommerce::Prometheus.configure do |c|
  c.web_collectors = [AppCollector]
  c.web_type_collectors = [AppTypeCollector.new]
  c.resque_collectors = [AppCollector]
  c.resque_type_collectors = [AppTypeCollector.new]
  c.hutch_collectors = [AppCollector]
  c.hutch_type_collectors = [AppTypeCollector.new]
end

For custom integrations that initialize their own server, you'll need to pass your TypeCollector instance via the .add_type_collector method on the prometheus server instance before starting it:

server = ::Bigcommerce::Prometheus::Server.new
Bigcommerce::Prometheus.web_type_collectors.each do |tc|
  server.add_type_collector(tc)
end

# and for polling:

AppCollector.start

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