+ActiveSupport::Notifications+ provides an instrumentation API for Ruby.
To instrument an event you just need to do:
ActiveSupport::Notifications.instrument("render", :extra => :information) do render :text => "Foo" end
That executes the block first and 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.
You can consume those events and the information they provide by registering a subscriber. 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 will be called asynchronously whenever someone instruments “render”:
ActiveSupport::Notifications.instrument("render", :extra => :information) do render :text => "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.
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.
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.
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.
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.
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)
Notifications ships with a queue implementation that consumes and publish events to log subscribers in a thread. You can use any queue implementation you want.
# File lib/active_support/notifications.rb, line 121 def instrument(name, payload = {}) if @instrumenters[name] instrumenter.instrument(name, payload) { yield payload if block_given? } else yield payload if block_given? end end
# File lib/active_support/notifications.rb, line 147 def instrumenter Thread.current[:"instrumentation_#{notifier.object_id}"] ||= Instrumenter.new(notifier) end
# File lib/active_support/notifications.rb, line 117 def publish(name, *args) notifier.publish(name, *args) end
# File lib/active_support/notifications.rb, line 129 def subscribe(*args, &block) notifier.subscribe(*args, &block).tap do @instrumenters.clear end end
# File lib/active_support/notifications.rb, line 135 def subscribed(callback, *args, &block) subscriber = subscribe(*args, &callback) yield ensure unsubscribe(subscriber) end
# File lib/active_support/notifications.rb, line 142 def unsubscribe(args) notifier.unsubscribe(args) @instrumenters.clear end