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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:

Chapters

  1. Introduction to instrumentation
  2. Subscribing to an event
  3. Rails framework hooks
  4. Action Controller
  5. Action Dispatch
  6. Action View
  7. Active Record
  8. Action Mailer
  9. Active Support
  10. Active Job
  11. Action Cable
  12. Active Storage
  13. Railties
  14. Rails
  15. Exceptions
  16. Creating custom events

1 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. 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.

2 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
  • A unique ID for the instrumenter that fired the event
  • The payload (described in future sections)
ActiveSupport::Notifications.subscribe "process_action.action_controller" do |name, started, finished, unique_id, data|
  # your own custom stuff
  Rails.logger.info "#{name} Received! (started: #{started}, finished: #{finished})" # process_action.action_controller Received (started: 2019-05-05 13:43:57 -0800, finished: 2019-05-05 13:43:58 -0800)
end

If you are concerned about the accuracy of started and finished to compute a precise elapsed time then use ActiveSupport::Notifications.monotonic_subscribe. The given block would receive the same arguments as above but the started and finished will have values with an accurate monotonic time instead of wall-clock time.

ActiveSupport::Notifications.monotonic_subscribe "process_action.action_controller" do |name, started, finished, unique_id, data|
  # your own custom stuff
  Rails.logger.info "#{name} Received! (started: #{started}, finished: #{finished})" # process_action.action_controller Received (started: 1560978.425334, finished: 1560979.429234)
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

You may also pass a block that accepts only one argument, and it will receive an event object:

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |event|
  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 how to subscribe to everything from ActionController.

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

3 Rails framework hooks

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

4 Action Controller

4.1 write_fragment.action_controller

Key Value
:key The complete key
{
  key: 'posts/1-dashboard-view'
}

4.2 read_fragment.action_controller

Key Value
:key The complete key
{
  key: 'posts/1-dashboard-view'
}

4.3 expire_fragment.action_controller

Key Value
:key The complete key
{
  key: 'posts/1-dashboard-view'
}

4.4 exist_fragment?.action_controller

Key Value
:key The complete key
{
  key: 'posts/1-dashboard-view'
}

4.5 write_page.action_controller

Key Value
:path The complete path
{
  path: '/users/1'
}

4.6 expire_page.action_controller

Key Value
:path The complete path
{
  path: '/users/1'
}

4.7 start_processing.action_controller

Key Value
:controller The controller name
:action The action
:params Hash of request parameters without any filtered parameter
:headers Request headers
:format html/js/json/xml etc
:method HTTP request verb
:path Request path
{
  controller: "PostsController",
  action: "new",
  params: { "action" => "new", "controller" => "posts" },
  headers: #<ActionDispatch::Http::Headers:0x0055a67a519b88>,
  format: :html,
  method: "GET",
  path: "/posts/new"
}

4.8 process_action.action_controller

Key Value
:controller The controller name
:action The action
:params Hash of request parameters without any filtered parameter
:headers Request headers
:format html/js/json/xml etc
:method HTTP request verb
:path Request path
:request The ActionDispatch::Request
:status HTTP status code
:location Location response header
: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"},
  headers: #<ActionDispatch::Http::Headers:0x0055a67a519b88>,
  format: :html,
  method: "GET",
  path: "/posts",
  request: #<ActionDispatch::Request:0x00007ff1cb9bd7b8>,
  status: 200,
  view_runtime: 46.848,
  db_runtime: 0.157
}

4.9 send_file.action_controller

Key Value
:path Complete path to the file

Additional keys may be added by the caller.

4.10 send_data.action_controller

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

4.11 redirect_to.action_controller

Key Value
:status HTTP response code
:location URL to redirect to
:request The ActionDispatch::Request
{
  status: 302,
  location: "http://localhost:3000/posts/new",
  request: #<ActionDispatch::Request:0x00007ff1cb9bd7b8>
}

4.12 halted_callback.action_controller

Key Value
:filter Filter that halted the action
{
  filter: ":halting_filter"
}

4.13 unpermitted_parameters.action_controller

Key Value
:keys Unpermitted keys

5 Action Dispatch

5.1 process_middleware.action_dispatch

Key Value
:middleware Name of the middleware

6 Action View

6.1 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"
}

6.2 render_partial.action_view

Key Value
:identifier Full path to template
{
  identifier: "/Users/adam/projects/notifications/app/views/posts/_form.html.erb"
}

6.3 render_collection.action_view

Key Value
:identifier Full path to template
:count Size of collection
:cache_hits Number of partials fetched from cache

:cache_hits is only included if the collection is rendered with cached: true.

{
  identifier: "/Users/adam/projects/notifications/app/views/posts/_post.html.erb",
  count: 3,
  cache_hits: 0
}

7 Active Record

7.1 sql.active_record

Key Value
:sql SQL statement
:name Name of the operation
:connection Connection object
:binds Bind parameters
:type_casted_binds Typecasted bind parameters
:statement_name SQL Statement name
:cached true is added when cached queries used

The adapters will add their own data as well.

{
  sql: "SELECT \"posts\".* FROM \"posts\" ",
  name: "Post Load",
  connection: #<ActiveRecord::ConnectionAdapters::SQLite3Adapter:0x00007f9f7a838850>,
  binds: [#<ActiveModel::Attribute::WithCastValue:0x00007fe19d15dc00>],
  type_casted_binds: [11],
  statement_name: nil
}

7.2 instantiation.active_record

Key Value
:record_count Number of records that instantiated
:class_name Record's class
{
  record_count: 1,
  class_name: "User"
}

8 Action Mailer

8.1 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
:perform_deliveries Whether delivery of this message is performed or not
{
  mailer: "Notification",
  message_id: "4f5b5491f1774_181b23fc3d443[email protected]",
  subject: "Rails Guides",
  to: ["[email protected]", "[email protected]"],
  from: ["[email protected]"],
  date: Sat, 10 Mar 2012 14:18:09 +0100,
  mail: "...", # omitted for brevity
  perform_deliveries: true
}

8.2 process.action_mailer

Key Value
:mailer Name of the mailer class
:action The action
:args The arguments
{
  mailer: "Notification",
  action: "welcome_email",
  args: []
}

9 Active Support

9.1 cache_read.active_support

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

9.2 cache_generate.active_support

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

Key Value
:key Key used in the store
:store Name of the store class

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

{
  key: "name-of-complicated-computation",
  store: "ActiveSupport::Cache::MemCacheStore"
}

9.3 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
:store Name of the store class

Options passed to fetch will be merged with the payload.

{
  key: "name-of-complicated-computation",
  store: "ActiveSupport::Cache::MemCacheStore"
}

9.4 cache_write.active_support

Key Value
:key Key used in the store
:store Name of the store class

Cache stores may add their own keys

{
  key: "name-of-complicated-computation",
  store: "ActiveSupport::Cache::MemCacheStore"
}

9.5 cache_delete.active_support

Key Value
:key Key used in the store
:store Name of the store class
{
  key: "name-of-complicated-computation",
  store: "ActiveSupport::Cache::MemCacheStore"
}

9.6 cache_exist?.active_support

Key Value
:key Key used in the store
:store Name of the store class
{
  key: "name-of-complicated-computation",
  store: "ActiveSupport::Cache::MemCacheStore"
}

10 Active Job

10.1 enqueue_at.active_job

Key Value
:adapter QueueAdapter object processing the job
:job Job object

10.2 enqueue.active_job

Key Value
:adapter QueueAdapter object processing the job
:job Job object

10.3 enqueue_retry.active_job

Key Value
:job Job object
:adapter QueueAdapter object processing the job
:error The error that caused the retry
:wait The delay of the retry

10.4 perform_start.active_job

Key Value
:adapter QueueAdapter object processing the job
:job Job object

10.5 perform.active_job

Key Value
:adapter QueueAdapter object processing the job
:job Job object

10.6 retry_stopped.active_job

Key Value
:adapter QueueAdapter object processing the job
:job Job object
:error The error that caused the retry

10.7 discard.active_job

Key Value
:adapter QueueAdapter object processing the job
:job Job object
:error The error that caused the discard

11 Action Cable

11.1 perform_action.action_cable

Key Value
:channel_class Name of the channel class
:action The action
:data A hash of data

11.2 transmit.action_cable

Key Value
:channel_class Name of the channel class
:data A hash of data
:via Via

11.3 transmit_subscription_confirmation.action_cable

Key Value
:channel_class Name of the channel class

11.4 transmit_subscription_rejection.action_cable

Key Value
:channel_class Name of the channel class

11.5 broadcast.action_cable

Key Value
:broadcasting A named broadcasting
:message A hash of message
:coder The coder

12 Active Storage

12.1 service_upload.active_storage

Key Value
:key Secure token
:service Name of the service
:checksum Checksum to ensure integrity

12.2 service_streaming_download.active_storage

Key Value
:key Secure token
:service Name of the service

12.3 service_download_chunk.active_storage

Key Value
:key Secure token
:service Name of the service
:range Byte range attempted to be read

12.4 service_download.active_storage

Key Value
:key Secure token
:service Name of the service

12.5 service_delete.active_storage

Key Value
:key Secure token
:service Name of the service

12.6 service_delete_prefixed.active_storage

Key Value
:prefix Key prefix
:service Name of the service

12.7 service_exist.active_storage

Key Value
:key Secure token
:service Name of the service
:exist File or blob exists or not

12.8 service_url.active_storage

Key Value
:key Secure token
:service Name of the service
:url Generated URL

12.9 service_update_metadata.active_storage

Key Value
:key Secure token
:service Name of the service
:content_type HTTP Content-Type field
:disposition HTTP Content-Disposition field

The only ActiveStorage service that provides this hook so far is GCS.

12.10 preview.active_storage

Key Value
:key Secure token

12.11 transform.active_storage

13 Railties

13.1 load_config_initializer.railties

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

14 Rails

14.1 deprecation.rails

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

15 Exceptions

If an exception happens during any instrumentation the payload will include information about it.

Key Value
:exception An array of two elements. Exception class name and the message
:exception_object The exception object

16 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 and add the instrumenter's 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 also have the option to call instrument without passing a block. This lets you leverage the instrumentation infrastructure for other messaging uses.

ActiveSupport::Notifications.instrument "my.custom.event", this: :data

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 your application is sending Tweets, you should create an event named tweet.twitter.

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