Notifications

ActiveSupport::Notifications provides an instrumentation API for Ruby.

Instrumenters

To instrument an event you just need to do:

ActiveSupport::Notifications.instrument('render', extra: :information) do
  render plain: 'Foo'
end

That first executes the block and then notifies all subscribers once done.

In the example above render is the name of the event, and the rest is called the payload. The payload is a mechanism that allows instrumenters to pass extra information to subscribers. Payloads consist of a hash whose contents are arbitrary and generally depend on the event.

Subscribers

You can consume those events and the information they provide by registering a subscriber.

ActiveSupport::Notifications.subscribe('render') do |name, start, finish, id, payload|
  name    # => String, name of the event (such as 'render' from above)
  start   # => Time, when the instrumented block started execution
  finish  # => Time, when the instrumented block ended execution
  id      # => String, unique ID for this notification
  payload # => Hash, the payload
end

For instance, let's store all “render” events in an array:

events = []

ActiveSupport::Notifications.subscribe('render') do |*args|
  events << ActiveSupport::Notifications::Event.new(*args)
end

That code returns right away, you are just subscribing to “render” events. The block is saved and will be called whenever someone instruments “render”:

ActiveSupport::Notifications.instrument('render', extra: :information) do
  render plain: 'Foo'
end

event = events.first
event.name      # => "render"
event.duration  # => 10 (in milliseconds)
event.payload   # => { extra: :information }

The block in the subscribe call gets the name of the event, start timestamp, end timestamp, a string with a unique identifier for that event (something like “535801666f04d0298cd6”), and a hash with the payload, in that order.

If an exception happens during that particular instrumentation the payload will have a key :exception with an array of two elements as value: a string with the name of the exception class, and the exception message. The :exception_object key of the payload will have the exception itself as the value.

As the previous example depicts, the class ActiveSupport::Notifications::Event is able to take the arguments as they come and provide an object-oriented interface to that data.

It is also possible to pass an object which responds to call method as the second parameter to the subscribe method instead of a block:

module ActionController
  class PageRequest
    def call(name, started, finished, unique_id, payload)
      Rails.logger.debug ['notification:', name, started, finished, unique_id, payload].join(' ')
    end
  end
end

ActiveSupport::Notifications.subscribe('process_action.action_controller', ActionController::PageRequest.new)

resulting in the following output within the logs including a hash with the payload:

notification: process_action.action_controller 2012-04-13 01:08:35 +0300 2012-04-13 01:08:35 +0300 af358ed7fab884532ec7 {
   controller: "Devise::SessionsController",
   action: "new",
   params: {"action"=>"new", "controller"=>"devise/sessions"},
   format: :html,
   method: "GET",
   path: "/login/sign_in",
   status: 200,
   view_runtime: 279.3080806732178,
   db_runtime: 40.053
 }

You can also subscribe to all events whose name matches a certain regexp:

ActiveSupport::Notifications.subscribe(/render/) do |*args|
  ...
end

and even pass no argument to subscribe, in which case you are subscribing to all events.

Temporary Subscriptions

Sometimes you do not want to subscribe to an event for the entire life of the application. There are two ways to unsubscribe.

WARNING: The instrumentation framework is designed for long-running subscribers, use this feature sparingly because it wipes some internal caches and that has a negative impact on performance.

Subscribe While a Block Runs

You can subscribe to some event temporarily while some block runs. For example, in

callback = lambda {|*args| ... }
ActiveSupport::Notifications.subscribed(callback, "sql.active_record") do
  ...
end

the callback will be called for all “sql.active_record” events instrumented during the execution of the block. The callback is unsubscribed automatically after that.

Manual Unsubscription

The subscribe method returns a subscriber object:

subscriber = ActiveSupport::Notifications.subscribe("render") do |*args|
  ...
end

To prevent that block from being called anymore, just unsubscribe passing that reference:

ActiveSupport::Notifications.unsubscribe(subscriber)

You can also unsubscribe by passing the name of the subscriber object. Note that this will unsubscribe all subscriptions with the given name:

ActiveSupport::Notifications.unsubscribe("render")

Default Queue

Notifications ships with a queue implementation that consumes and publishes events to all log subscribers. You can use any queue implementation you want.