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 those 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 desired.

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

After reading this guide, you will know:

What instrumentation can provide.
How to add a subscriber to a hook.
The hooks inside the Rails framework for instrumentation.
How to build a custom instrumentation implementation.

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

Use ActiveSupport::Notifications.subscribe with a block to listen to any notification. Depending on the amount of arguments the block takes, you will receive different data.

The first way to subscribe to an event is to use a block with a single argument. The argument will be an instance of ActiveSupport::Notifications::Event.

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |event|
  event.name        # => "process_action.action_controller"
  event.duration    # => 10 (in milliseconds)
  event.allocations # => 1826
  event.payload     # => {:extra=>information}

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

If you don't need all the data recorded by an Event object, you can also specify a block that takes the following five arguments:

Name of the event
Time when it started
Time when it finished
A unique ID for the instrumenter that fired the event
The payload for the event

ActiveSupport::Notifications.subscribe "process_action.action_controller" do |name, started, finished, unique_id, payload|
  # 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, payload|
  # your own custom stuff
  duration = finished - started # 1560979.429234 - 1560978.425334
  Rails.logger.info "#{name} Received! (duration: #{duration})" # process_action.action_controller Received! (duration: 1.0039)
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 |event|
  # 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 events and their payloads are detailed below.

3.1. Action Controller

3.1.1. `start_processing.action_controller`

Key	Value
`:controller`	The controller name
`:action`	The action
`:request`	The `ActionDispatch::Request` object
`: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"
}

3.1.2. `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` object
`:response`	The `ActionDispatch::Response` object
`: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"},
  headers: #<ActionDispatch::Http::Headers:0x0055a67a519b88>,
  format: :html,
  method: "GET",
  path: "/posts",
  request: #<ActionDispatch::Request:0x00007ff1cb9bd7b8>,
  response: #<ActionDispatch::Response:0x00007f8521841ec8>,
  status: 200,
  view_runtime: 46.848,
  db_runtime: 0.157
}

3.1.3. `send_file.action_controller`

Key	Value
`:path`	Complete path to the file

Additional keys may be added by the caller.

3.1.4. `send_data.action_controller`

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

3.1.5. `redirect_to.action_controller`

Key	Value
`:status`	HTTP response code
`:location`	URL to redirect to
`:request`	The `ActionDispatch::Request` object

{
  status: 302,
  location: "http://localhost:3000/posts/new",
  request: <ActionDispatch::Request:0x00007ff1cb9bd7b8>
}

3.1.6. `halted_callback.action_controller`

Key	Value
`:filter`	Filter that halted the action

{
  filter: ":halting_filter"
}

3.1.7. `unpermitted_parameters.action_controller`

Key	Value
`:keys`	The unpermitted keys
`:context`	Hash with the following keys: `:controller`, `:action`, `:params`, `:request`

3.1.8. `send_stream.action_controller`

Key	Value
`:filename`	The filename
`:type`	HTTP content type
`:disposition`	HTTP content disposition

{
  filename: "subscribers.csv",
  type: "text/csv",
  disposition: "attachment"
}

3.2. Action Controller: Caching

3.2.1. `write_fragment.action_controller`

Key	Value
`:key`	The complete key

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

3.2.2. `read_fragment.action_controller`

Key	Value
`:key`	The complete key

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

3.2.3. `expire_fragment.action_controller`

Key	Value
`:key`	The complete key

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

3.2.4. `exist_fragment?.action_controller`

Key	Value
`:key`	The complete key

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

3.3. Action Dispatch

3.3.1. `process_middleware.action_dispatch`

Key	Value
`:middleware`	Name of the middleware

3.3.2. `redirect.action_dispatch`

Key	Value
`:status`	HTTP response code
`:location`	URL to redirect to
`:request`	The `ActionDispatch::Request` object

3.3.3. `request.action_dispatch`

Key	Value
`:request`	The `ActionDispatch::Request` object

3.4. Action View

3.4.1. `render_template.action_view`

Key	Value
`:identifier`	Full path to template
`:layout`	Applicable layout
`:locals`	Local variables passed to template

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

3.4.2. `render_partial.action_view`

Key	Value
`:identifier`	Full path to template
`:locals`	Local variables passed to template

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

3.4.3. `render_collection.action_view`

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

The :cache_hits key 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
}

3.4.4. `render_layout.action_view`

Key	Value
`:identifier`	Full path to template

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

3.5. Active Record

3.5.1. `sql.active_record`

Key	Value
`:sql`	SQL statement
`:name`	Name of the operation
`:connection`	Connection object
`:transaction`	Current transaction, if any
`:binds`	Bind parameters
`:type_casted_binds`	Typecasted bind parameters
`:statement_name`	SQL Statement name
`:async`	`true` if query is loaded asynchronously
`:cached`	`true` is added when cached queries used
`:row_count`	Number of rows returned by the query
`:affected_rows`	Number of rows affected by the query

Adapters may add their own data as well.

{
  sql: "SELECT \"posts\".* FROM \"posts\" ",
  name: "Post Load",
  connection: <ActiveRecord::ConnectionAdapters::SQLite3Adapter:0x00007f9f7a838850>,
  transaction: <ActiveRecord::ConnectionAdapters::RealTransaction:0x0000000121b5d3e0>
  binds: [<ActiveModel::Attribute::WithCastValue:0x00007fe19d15dc00>],
  type_casted_binds: [11],
  statement_name: nil,
  row_count: 5,
  affected_rows: 0
}

If the query is not executed in the context of a transaction, :transaction is nil.

3.5.2. `strict_loading_violation.active_record`

This event is only emitted when config.active_record.action_on_strict_loading_violation is set to :log.

Key	Value
`:owner`	Model with `strict_loading` enabled
`:reflection`	Reflection of the association that tried to load

3.5.3. `instantiation.active_record`

Key	Value
`:record_count`	Number of records that instantiated
`:class_name`	Record's class

{
  record_count: 1,
  class_name: "User"
}

3.5.4. `start_transaction.active_record`

This event is emitted when a transaction has been started.

Key	Value
`:transaction`	Transaction object
`:connection`	Connection object

Please, note that Active Record does not create the actual database transaction until needed:

ActiveRecord::Base.transaction do
  # We are inside the block, but no event has been triggered yet.

  # The following line makes Active Record start the transaction.
  User.count # Event fired here.
end

Remember that ordinary nested calls do not create new transactions:

ActiveRecord::Base.transaction do |t1|
  User.count # Fires an event for t1.
  ActiveRecord::Base.transaction do |t2|
    # The next line fires no event for t2, because the only
    # real database transaction in this example is t1.
    User.first.touch
  end
end

However, if requires_new: true is passed, you get an event for the nested transaction too. This might be a savepoint under the hood:

ActiveRecord::Base.transaction do |t1|
  User.count # Fires an event for t1.
  ActiveRecord::Base.transaction(requires_new: true) do |t2|
    User.first.touch # Fires an event for t2.
  end
end

3.5.5. `transaction.active_record`

This event is emitted when a database transaction finishes. The state of the transaction can be found in the :outcome key.

Key	Value
`:transaction`	Transaction object
`:outcome`	`:commit`, `:rollback`, `:restart`, or `:incomplete`
`:connection`	Connection object

In practice, you cannot do much with the transaction object, but it may still be helpful for tracing database activity. For example, by tracking transaction.uuid.

3.6. Action Mailer

3.6.1. `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_181b23fc3d4434d38138e5@mba.local.mail",
  subject: "Rails Guides",
  to: ["users@rails.com", "dhh@rails.com"],
  from: ["me@rails.com"],
  date: Sat, 10 Mar 2012 14:18:09 +0100,
  mail: "...", # omitted for brevity
  perform_deliveries: true
}

3.6.2. `process.action_mailer`

Key	Value
`:mailer`	Name of the mailer class
`:action`	The action
`:args`	The arguments

{
  mailer: "Notification",
  action: "welcome_email",
  args: []
}

3.7. Active Support: Caching

3.7.1. `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` if a read is done with `fetch`

3.7.2. `cache_read_multi.active_support`

Key	Value
`:key`	Keys used in the store
`:store`	Name of the store class
`:hits`	Keys of cache hits
`:super_operation`	`:fetch_multi` if a read is done with `fetch_multi`

3.7.3. `cache_generate.active_support`

This event is only emitted 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"
}

3.7.4. `cache_fetch_hit.active_support`

This event is only emitted 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"
}

3.7.5. `cache_write.active_support`

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

Cache stores may add their own data as well.

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

3.7.6. `cache_write_multi.active_support`

Key	Value
`:key`	Keys and values written to the store
`:store`	Name of the store class

3.7.7. `cache_increment.active_support`

Key	Value
`:key`	Key used in the store
`:store`	Name of the store class
`:amount`	Increment amount

{
  key: "bottles-of-beer",
  store: "ActiveSupport::Cache::RedisCacheStore",
  amount: 99
}

3.7.8. `cache_decrement.active_support`

Key	Value
`:key`	Key used in the store
`:store`	Name of the store class
`:amount`	Decrement amount

{
  key: "bottles-of-beer",
  store: "ActiveSupport::Cache::RedisCacheStore",
  amount: 1
}

3.7.9. `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"
}

3.7.10. `cache_delete_multi.active_support`

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

3.7.11. `cache_delete_matched.active_support`

This event is only emitted when using RedisCacheStore, FileStore, or MemoryStore.

Key	Value
`:key`	Key pattern used
`:store`	Name of the store class

{
  key: "posts/*",
  store: "ActiveSupport::Cache::RedisCacheStore"
}

3.7.12. `cache_cleanup.active_support`

This event is only emitted when using MemoryStore.

Key	Value
`:store`	Name of the store class
`:size`	Number of entries in the cache before cleanup

{
  store: "ActiveSupport::Cache::MemoryStore",
  size: 9001
}

3.7.13. `cache_prune.active_support`

This event is only emitted when using MemoryStore.

Key	Value
`:store`	Name of the store class
`:key`	Target size (in bytes) for the cache
`:from`	Size (in bytes) of the cache before prune

{
  store: "ActiveSupport::Cache::MemoryStore",
  key: 5000,
  from: 9001
}

3.7.14. `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"
}

3.8. Active Support: Messages

3.8.1. `message_serializer_fallback.active_support`

Key	Value
`:serializer`	Primary (intended) serializer
`:fallback`	Fallback (actual) serializer
`:serialized`	Serialized string
`:deserialized`	Deserialized value

{
  serializer: :json_allow_marshal,
  fallback: :marshal,
  serialized: "\x04\b{\x06I\"\nHello\x06:\x06ETI\"\nWorld\x06;\x00T",
  deserialized: { "Hello" => "World" },
}

3.9. Active Job

3.9.1. `enqueue_at.active_job`

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

3.9.2. `enqueue.active_job`

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

3.9.3. `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

3.9.4. `enqueue_all.active_job`

Key	Value
`:adapter`	QueueAdapter object processing the job
`:jobs`	An array of Job objects

3.9.5. `perform_start.active_job`

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

3.9.6. `perform.active_job`

Key	Value
`:adapter`	QueueAdapter object processing the job
`:job`	Job object
`:db_runtime`	Amount spent executing database queries in ms

3.9.7. `retry_stopped.active_job`

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

3.9.8. `discard.active_job`

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

3.10. Action Cable

3.10.1. `perform_action.action_cable`

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

3.10.2. `transmit.action_cable`

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

3.10.3. `transmit_subscription_confirmation.action_cable`

Key	Value
`:channel_class`	Name of the channel class

3.10.4. `transmit_subscription_rejection.action_cable`

Key	Value
`:channel_class`	Name of the channel class

3.10.5. `broadcast.action_cable`

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

3.11. Active Storage

3.11.1. `preview.active_storage`

Key	Value
`:key`	Secure token

3.11.2. `transform.active_storage`

3.11.3. `analyze.active_storage`

Key	Value
`:analyzer`	Name of analyzer e.g., ffprobe

3.12. Active Storage: Storage Service

3.12.1. `service_upload.active_storage`

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

3.12.2. `service_streaming_download.active_storage`

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

3.12.3. `service_download_chunk.active_storage`

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

3.12.4. `service_download.active_storage`

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

3.12.5. `service_delete.active_storage`

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

3.12.6. `service_delete_prefixed.active_storage`

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

3.12.7. `service_exist.active_storage`

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

3.12.8. `service_url.active_storage`

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

3.12.9. `service_update_metadata.active_storage`

This event is only emitted when using the Google Cloud Storage service.

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

3.13. Action Mailbox

3.13.1. `process.action_mailbox`

Key	Value
`:mailbox`	Instance of the Mailbox class inheriting from `ActionMailbox::Base`
`:inbound_email`	Hash with data about the inbound email being processed

{
  mailbox: #<RepliesMailbox:0x00007f9f7a8388>,
  inbound_email: {
    id: 1,
    message_id: "0CB459E0-0336-41DA-BC88-E6E28C697DDB@37signals.com",
    status: "processing"
  }
}

3.14. Railties

3.14.1. `load_config_initializer.railties`

Key	Value
`:initializer`	Path of loaded initializer in `config/initializers`

3.15. Rails

3.15.1. `deprecation.rails`

Key	Value
`:message`	The deprecation warning
`:callstack`	Where the deprecation came from
`:gem_name`	Name of the gem reporting the deprecation
`:deprecation_horizon`	Version where the deprecated behavior will be removed

4. 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

5. Creating Custom Events

Adding your own events is easy as well. Active Support will take care of all the heavy lifting for you. Simply call ActiveSupport::Notifications.instrument with a name, payload, and a block. The notification will be sent after the block returns. Active Support 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 may also 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.