rails--rails/guides/source/active_support_instrumentation.md

Active Support Instrumentation

Active Support is a part of core Rails that provides Ruby language extensions, utilities and other things. One of the things it includes is an instrumentation API that can be used inside an application to measure certain actions that occur within Ruby code, such as that inside a Rails application or the framework itself. It is not limited to Rails, however. It can be used independently in other Ruby scripts if it is so desired.

In this guide, you will learn how to use the instrumentation API inside of Active Support to measure events inside of Rails and other Ruby code.

After reading this guide, you will know:

• What instrumentation can provide.
• The hooks inside the Rails framework for instrumentation.
• Adding a subscriber to a hook.
• Building a custom instrumentation implementation.

---

Introduction to instrumentation

The instrumentation API provided by Active Support allows developers to provide hooks which other developers may hook into. There are several of these within the Rails framework, as described below in (TODO: link to section detailing each hook point). With this API, developers can choose to be notified when certain events occur inside their application or another piece of Ruby code.

For example, there is a hook provided within Active Record that is called every time Active Record uses an SQL query on a database. This hook could be subscribed to, and used to track the number of queries during a certain action. There's another hook around the processing of an action of a controller. This could be used, for instance, to track how long a specific action has taken.

You are even able to create your own events inside your application which you can later subscribe to.

Rails framework hooks

Within the Ruby on Rails framework, there are a number of hooks provided for common events. These are detailed below.

Action Controller

write_fragment.action_controller

Key	Value
:key	The complete key

{
  key: 'posts/1-dashboard-view'
}

read_fragment.action_controller

Key	Value
:key	The complete key

{
  key: 'posts/1-dashboard-view'
}

expire_fragment.action_controller

Key	Value
:key	The complete key

{
  key: 'posts/1-dashboard-view'
}

exist_fragment?.action_controller

Key	Value
:key	The complete key

{
  key: 'posts/1-dashboard-view'
}

write_page.action_controller

Key	Value
:path	The complete path

{
  path: '/users/1'
}

expire_page.action_controller

Key	Value
:path	The complete path

{
  path: '/users/1'
}

start_processing.action_controller

Key	Value
:controller	The controller name
:action	The action
:params	Hash of request parameters without any filtered parameter
:format	html/js/json/xml etc
:method	HTTP request verb
:path	Request path

{
  controller: "PostsController",
  action: "new",
  params: { "action" => "new", "controller" => "posts" },
  format: :html,
  method: "GET",
  path: "/posts/new"
}

process_action.action_controller

Key	Value
:controller	The controller name
:action	The action
:params	Hash of request parameters without any filtered parameter
:format	html/js/json/xml etc
:method	HTTP request verb
:path	Request path
:status	HTTP status code
:view_runtime	Amount spent in view in ms
:db_runtime	Amount spent executing database queries in ms

{
  controller: "PostsController",
  action: "index",
  params: {"action" => "index", "controller" => "posts"},
  format: :html,
  method: "GET",
  path: "/posts",
  status: 200,
  view_runtime: 46.848,
  db_runtime: 0.157
}

send_file.action_controller

Key	Value
:path	Complete path to the file

INFO. Additional keys may be added by the caller.

send_data.action_controller

ActionController does not had any specific information to the payload. All options are passed through to the payload.

redirect_to.action_controller

Key	Value
:status	HTTP response code
:location	URL to redirect to

{
  status: 302,
  location: "http://localhost:3000/posts/new"
}

halted_callback.action_controller

Key	Value
:filter	Filter that halted the action

{
  filter: ":halting_filter"
}

Action View

render_template.action_view

Key	Value
:identifier	Full path to template
:layout	Applicable layout

{
  identifier: "/Users/adam/projects/notifications/app/views/posts/index.html.erb",
  layout: "layouts/application"
}

render_partial.action_view

Key	Value
:identifier	Full path to template

{
  identifier: "/Users/adam/projects/notifications/app/views/posts/_form.html.erb",
}

Active Record

sql.active_record

Key	Value
:sql	SQL statement
:name	Name of the operation
:connection_id	self.object_id

INFO. The adapters will add their own data as well.

{
  sql: "SELECT \"posts\".* FROM \"posts\" ",
  name: "Post Load",
  connection_id: 70307250813140,
  binds: []
}

identity.active_record

Key	Value
:line	Primary Key of object in the identity map
:name	Record's class
:connection_id	self.object_id

Action Mailer

receive.action_mailer

Key	Value
:mailer	Name of the mailer class
:message_id	ID of the message, generated by the Mail gem
:subject	Subject of the mail
:to	To address(es) of the mail
:from	From address of the mail
:bcc	BCC addresses of the mail
:cc	CC addresses of the mail
:date	Date of the mail
:mail	The encoded form of the mail

{
  mailer: "Notification",
  message_id: "4f5b5491f1774_181b23fc3d4434d38138e5@mba.local.mail",
  subject: "Rails Guides",
  to: ["users@rails.com", "ddh@rails.com"],
  from: ["me@rails.com"],
  date: Sat, 10 Mar 2012 14:18:09 +0100,
  mail: "..." # omitted for brevity
}

deliver.action_mailer

Key	Value
:mailer	Name of the mailer class
:message_id	ID of the message, generated by the Mail gem
:subject	Subject of the mail
:to	To address(es) of the mail
:from	From address of the mail
:bcc	BCC addresses of the mail
:cc	CC addresses of the mail
:date	Date of the mail
:mail	The encoded form of the mail

{
  mailer: "Notification",
  message_id: "4f5b5491f1774_181b23fc3d4434d38138e5@mba.local.mail",
  subject: "Rails Guides",
  to: ["users@rails.com", "ddh@rails.com"],
  from: ["me@rails.com"],
  date: Sat, 10 Mar 2012 14:18:09 +0100,
  mail: "..." # omitted for brevity
}

ActiveResource

request.active_resource

Key	Value
:method	HTTP method
:request_uri	Complete URI
:result	HTTP response object

Active Support

cache_read.active_support

Key	Value
:key	Key used in the store
:hit	If this read is a hit
:super_operation	:fetch is added when a read is used with #fetch

cache_generate.active_support

This event is only used when #fetch is called with a block.

Key	Value
:key	Key used in the store

INFO. Options passed to fetch will be merged with the payload when writing to the store

{
  key: 'name-of-complicated-computation'
}

cache_fetch_hit.active_support

This event is only used when #fetch is called with a block.

Key	Value
:key	Key used in the store

INFO. Options passed to fetch will be merged with the payload.

{
  key: 'name-of-complicated-computation'
}

cache_write.active_support

Key	Value
:key	Key used in the store

INFO. Cache stores may add their own keys

{
  key: 'name-of-complicated-computation'
}

cache_delete.active_support

Key	Value
:key	Key used in the store

{
  key: 'name-of-complicated-computation'
}

cache_exist?.active_support

Key	Value
:key	Key used in the store

{
  key: 'name-of-complicated-computation'
}

Railties

load_config_initializer.railties

Key	Value
:initializer	Path to loaded initializer from config/initializers

Rails

deprecation.rails

Key	Value
:message	The deprecation warning
:callstack	Where the deprecation came from

Subscribing to an event

Subscribing to an event is easy. Use ActiveSupport::Notifications.subscribe with a block to listen to any notification.

The block receives the following arguments:

• The name of the event
• Time when it started
• Time when it finished
• An unique ID for this event
• The payload (described in previous sections)

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |name, started, finished, unique_id, data|
  # your own custom stuff
  Rails.logger.info "#{name} Received!"
end

Defining all those block arguments each time can be tedious. You can easily create an ActiveSupport::Notifications::Event from block arguments like this:

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |*args|
  event = ActiveSupport::Notifications::Event.new *args

  event.name      # => "process_action.action_controller"
  event.duration  # => 10 (in milliseconds)
  event.payload   # => {:extra=>information}

  Rails.logger.info "#{event} Received!"
end

Most times you only care about the data itself. Here is a shortcut to just get the data.

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |*args|
  data = args.extract_options!
  data # { extra: :information }
end

You may also subscribe to events matching a regular expression. This enables you to subscribe to multiple events at once. Here's you could subscribe to everything from ActionController.

ActiveSupport::Notifications.subscribe /action_controller/ do |*args|
  # inspect all ActionController events
end

Creating custom events

Adding your own events is easy as well. ActiveSupport::Notifications will take care of all the heavy lifting for you. Simply call instrument with a name, payload and a block. The notification will be sent after the block returns. ActiveSupport will generate the start and end times as well as the unique ID. All data passed into the instrument call will make it into the payload.

Here's an example:

ActiveSupport::Notifications.instrument "my.custom.event", this: :data do
  # do your custom stuff here
end

Now you can listen to this event with:

ActiveSupport::Notifications.subscribe "my.custom.event" do |name, started, finished, unique_id, data|
  puts data.inspect # {:this=>:data}
end

You should follow Rails conventions when defining your own events. The format is: event.library. If you application is sending Tweets, you should create an event named tweet.twitter.