only run Skylight for 20% of the dynos (rounding up if there are less than five)

lib/conditional_skylight.rb

@@ -0,0 +1,38 @@
+module ConditionalSkylight
+  module DummyMixin
+    def self.included(object)
+      object.extend(self)
+      super
+    end
+
+    def instrument_method(*)
+    end
+  end
+
+  extend self
+
+  def enabled?
+    authenticated? and lucky_dyno?
+  end
+
+  def authenticated?
+    ENV['SKYLIGHT_AUTHENTICATION'.freeze]
+  end
+
+  def lucky_dyno?
+    return @lucky_dyno if instance_variable_defined? :@lucky_dyno
+    if ENV['DYNO'.freeze] and ENV['DYNO_COUNT'.freeze]
+      dyno        = Integer ENV['DYNO'.freeze][/\d+/]
+      @lucky_dyno = dyno % 5 == 1
+    else
+      @lucky_dyno = true
+    end
+  end
+
+  if enabled?
+    require 'skylight'
+    Mixin = Skylight::Helpers
+  else
+    Mixin = DummyMixin
+  end
+end

lib/travis/api/app.rb

@@ -1,3 +1,4 @@
+require 'conditional_skylight'
 require 'travis'
 require 'travis/model'
 require 'travis/support/amqp'

lib/travis/api/app/access_token.rb

@@ -3,7 +3,7 @@ require 'securerandom'
 
 class Travis::Api::App
   class AccessToken
-    include Skylight::Helpers
+    include ConditionalSkylight::Mixin
 
     DEFAULT_SCOPES = [:public, :private]
     attr_reader :token, :scopes, :user_id, :app_id, :expires_in, :extra

lib/travis/api/app/middleware/skylight.rb

@@ -1,4 +1,6 @@
-if ENV['SKYLIGHT_AUTHENTICATION']
+require 'conditional_skylight'
+
+if ConditionalSkylight.enabled?
   require_relative 'skylight/actual'
 else
   require_relative 'skylight/dummy'

lib/travis/api/app/responders/base.rb

@@ -1,6 +1,6 @@
 module Travis::Api::App::Responders
   class Base
-    include Skylight::Helpers
+    include ConditionalSkylight::Mixin
     attr_reader :endpoint, :resource, :options
 
     def initialize(endpoint, resource, options = {})

lib/travis/api/app/stack_instrumentation.rb

@@ -18,7 +18,7 @@ class Travis::Api::App
       end
 
       def instrument?
-        defined? ::Skylight
+        ConditionalSkylight.enabled?
       end
     end
 

lib/travis/api/instruments.rb

@@ -1,6 +1,8 @@
-require 'skylight'
+require 'conditional_skylight'
 
-Travis.services.send(:services).each_value do |service|
-  service.send(:include, Skylight::Helpers)
+if ConditionalSkylight.enabled?
+  Travis.services.send(:services).each_value do |service|
+    service.send(:include, ConditionalSkylight::Mixin)
     service.send(:instrument_method, :run)
+  end
 end

travis-api.gemspec

@@ -17,10 +17,11 @@ Gem::Specification.new do |s|
     "Josh Kalderimis",
     "Henrik Hodne",
     "Andre Arko",
-    "Erik Michaels-Ober",
     "Dan Buch",
+    "Erik Michaels-Ober",
     "Steve Richert",
     "Brian Ford",
+    "Patrick Williams",
     "Bryan Goldstein",
     "Puneeth Chaganti",
     "Thais Camilo and Konstantin Haase",
@@ -29,8 +30,7 @@ Gem::Specification.new do |s|
     "James Dennes",
     "rainsun",
     "Dan Rice",
-    "Nick Schonning",
-    "Patrick Williams"
+    "Nick Schonning"
   ]
 
   s.email = [
@@ -45,21 +45,22 @@ Gem::Specification.new do |s|
     "konstantin.haase@gmail.com",
     "andre@arko.net",
     "svenfuchs@artweb-design.de",
-    "sferik@gmail.com",
     "dan@travis-ci.org",
+    "sferik@gmail.com",
     "steve.richert@gmail.com",
     "bford@engineyard.com",
     "henrik@travis-ci.com",
-    "punchagan@muse-amuse.in",
+    "brysgo@gmail.com",
+    "jdennes@gmail.com",
     "rainsuner@gmail.com",
     "dev+narwen+rkh@rkh.im",
     "tim@spork.in",
     "e@zzak.io",
-    "jdennes@gmail.com",
+    "punchagan@muse-amuse.in",
     "dan@zoombody.com",
+    "dan@meatballhat.com",
     "nschonni@gmail.com",
-    "patrick@bittorrent.com",
-    "brysgo@gmail.com"
+    "patrick@bittorrent.com"
   ]
 
   s.files = [
@@ -116,6 +117,9 @@ Gem::Specification.new do |s|
     "lib/travis/api/app/middleware/metriks.rb",
     "lib/travis/api/app/middleware/rewrite.rb",
     "lib/travis/api/app/middleware/scope_check.rb",
+    "lib/travis/api/app/middleware/skylight.rb",
+    "lib/travis/api/app/middleware/skylight/actual.rb",
+    "lib/travis/api/app/middleware/skylight/dummy.rb",
     "lib/travis/api/app/middleware/user_agent_tracker.rb",
     "lib/travis/api/app/responders.rb",
     "lib/travis/api/app/responders/atom.rb",
@@ -127,6 +131,8 @@ Gem::Specification.new do |s|
     "lib/travis/api/app/responders/service.rb",
     "lib/travis/api/app/responders/xml.rb",
     "lib/travis/api/app/services/schedule_request.rb",
+    "lib/travis/api/app/stack_instrumentation.rb",
+    "lib/travis/api/instruments.rb",
     "lib/travis/api/serializer.rb",
     "lib/travis/api/v2.rb",
     "lib/travis/api/v2/http.rb",