Report on queued connections in any Rack-based server using Raindrops and the Raindrops::Linux features.

First, add rack-queue-metrics to your Gemfile:

gem 'rack-queue-metrics'

You're done! If you want to instrument queue metrics beyond the default output, you can subscribe to the unicorn.metrics.queue notifcation in your Rails app. For example, to print queue information to your logs, add the following to `config/initializers/notifcations.rb:

# config/initializers/notifications.rb
ActiveSupport::Notifications.subscribe(/unicorn.metrics.queue/) do |*args|
  event = ActiveSupport::Notifications::Event.new(*args)
  payload = event.payload

  addr = payload[:addr]
  active = payload[:requests][:active]
  queued = payload[:requests][:queued]
  queue_time = payload[:queue_time]

  puts "STATS addr=#{addr} active=#{active} queued=#{queued} queue_time=#{queue_time} "
end

For more information, see the ActiveSupport::Notification docs.

Include the Raindrops and Rack::QueueMetrics middleware in your application's config.ru:

  # config.ru
  use Raindrops::Middleware
  use Rack::QueueMetrics::Middleware

With every request, rack-queue-metrics will output a log line with the the following format:

at=metric measure=rack.queue-metrics addr=10.10.10.90:5000 queue_time=0 queue_depth=0

The following information is sent in the notification payload:

• requests[:active]: Number of requests currently being processed by Unicorn at the start of the request
• requests[:queued]: Number of requests waiting to be processed at the start of the request
• queue_time: Amount of time the current request spent in the queue
• addr: Address of the dyno processing the request