Harness

Harness connects measurements coming from ActiveSupport::Notifications to external metric tracking services. Counters are stored locally with redis before being sent to the service.

Currently Supported Services:

• Librato

Current Features:

• Track counters over time (# of registered users)
• Read time specific values (# time to cache something)
• Build meters on top of counters (# requests per second)
• Sidekiq integration
• Resque integration
• Rails integration
• Capture and log all measurements coming out of Rails

Crash Course

class ComplicatedClass
  def hard_work
    # Automatically track how long each of these calls takes so they can
    # tracked and compared over time.
    ActiveSupport::Notifications.instrument "hard_work", :gauge => true do
      # do hard_work
    end
  end

  def register_user
    # Automatically track the total # of registered users you have.
    # As well, as be able to take measurements of users created in a
    # specific interval
    ActiveSupport::Notifications.instrument "register_user", :counter => true do
      # register_user
    end
  end
end

Installation

Add this line to your application's Gemfile:

gem 'harness'

And then execute:

$ bundle

Or install it yourself as:

$ gem install harness

Usage

In the metrics world there are two types of things: Gauges and Counters. Gauges are time senstive and represent something at a specific point in time. Counters keep track of things and should be increasing. Counters can be reset back to zero. You can combine counters and/or gauges to correlate data about your application. Meters monitor counters. They allow you look at rates of counters (read: counters per second).

Harness makes this process easily. Harness' primary goal it make it dead simple to start measuring different parts of your application. ActiveSupport::Notifications makes this very easy because it provides measurements and implements the observer pattern.

Tracking Things

I guess you read the ActiveSupport::Notifications documentation before going any further or this will seems like php to you. Harness hooks into your notifications and looks for :gauge or :counter options. If either is present, it will be sent to the external service. For example, you can track how long it's taking to do a specific thing:

class MyClass
  def important_method(stuff)
    ActiveSupport::Notifications.instrument "important_method.my_class", :gauge => true do
      do_important_stuff
    end
  end
end

You can do the same with a counter. Counter values are automatically stored in redis and incremented. This means you can simply pass :counter => true in instrumentations if you'd like to count it. You may also pass :counter => 5 if you'd like to provide your own value. This value is stored in redis so the next time :counter => true will work correctly. You can reset all the counters back to zero by calling: Harness.reset_counters!.

class MyClass
  def important_method(stuff)
    ActiveSupport::Notifications.instrument "important_method.my_class", :counter => true do
      do_important_stuff
    end
  end
end

The instuments name will be sent as the name (important_method.my_class) for that gauge or counter.

Harness will do all the extra work in sending these metrics to whatever service you're using.

Once you the counters are you are instrumented, then you can meter them. Meters allow you take arbitary readings of counter rates. The results return a gauge so they can be logged as well.

# Define a counter
class MyClass
  def important_method(stuff)
    ActiveSupport::Notifications.instrument "important_method.my_class", :counter => true do
      do_important_stuff
    end
  end
end

# Now you can meter it
meter = Harnes::Meter.new('important_method.my_class')
meter.per_second # returns a gauge
meter.per_second.value # if you just want the number
meter.per(1.hour).value # You can use your own interval
meter.per_minute
meter.per_hour

Customizing

You can pash a hash to :counter or :gauge to initialize the measurement your own way.

class MyClass
  def important_method(stuff)
    ActiveSupport::Notifications.instrument "important_method.my_class", :gauge => { :id => 'custom-id', :name => "My Measurement" } do
      do_important_stuff
    end
  end
end

One Off Gauges and Counters

You can instantiate Harness::Counter and Harness::Guage wherever you want. Events from ActiveSupport are just converted to these classes under the covers anyways. You can use these class if you want to take peridocial measurements or tracking something that happens outside the application.

gauge = Harness::Gauge.new
gauge.id = "foo.bar"
gauge.name = "Foo's Bar"
gauge.time # defaults to Time.now
gauge.value = readings_from_my_server
gauge.units = 'bytes'
gauge.log

counter = Harness::Counter.new
counter.id = "foo.bar"
counter.name = "# of Foo bars"
counter.time # defaults to Time.now
counter.value = read_total_users_in_database
counter.log

# Both class take an option hash

gauge = Harness::Guage.new :time => Time.now, :id => 'foo.bar'
counter = Harness::Counter.new :time => Time.now, :id => 'foo.bar'

Configuration

Harness.config.adapter = :librato

Harness.config.librato.email = 'example@example.com'
Harness.config.librato.token = 'your-api-key'

Harness.redis = Redis.new

Rails Integration

Harness will automatically log metrics coming from ActionPack, ActiveRecord, ActiveSupport and ActionMailer.

You can configure Harness from application.rb

config.harness.adapter = :librato
config.librato.email = 'example@example.com'
config.librato.token = 'your-api-key'

Redis will be automatically configured if you REDISTOGO_URL or REDIS_URL environment variables at set. They are wrapped in a namespace so there will be no conflicts. If they are not present, the default values are used. You can customize this in an initializer:

# config/initializers/harness.rb
require 'erb'

file = Rails.root.join 'config', 'resque.yml'
config = YAML.load(ERB.new(File.read(Rails.root.join('config', 'redis.yml'))).result)

Harness.redis = Redis.new(:url => config[Rails.env])

rake harness:reset_counters is also added.

Rails Environments

Measurements are completely ignored in the test env. They are processed in development mode, but not sent to the external service. Everything is logged in production.

Background Processing

Harness integrates automatically with Resque or Sidekiq. This is because reporting measurements can take time and add unncessary overhead to the response time. If neither of these libraries are present, measurements will be posted in realtime. You can set your own queue by specifiying a class like so:

Harness.config.queue = MyCustomQueue

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request