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 the instrumenter that fired the event
payload # => Hash, the payload
end
Here, the start
and finish
values represent wall-clock time. If you are concerned about accuracy, you can register a monotonic subscriber.
ActiveSupport::Notifications.monotonic_subscribe('render') do |name, start, finish, id, payload|
name # => String, name of the event (such as 'render' from above)
start # => Monotonic time, when the instrumented block started execution
finish # => Monotonic time, when the instrumented block ended execution
id # => String, unique ID for the instrumenter that fired the event
payload # => Hash, the payload
end
The start
and finish
values above represent monotonic time.
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's instrumenter (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:
event.payload[:exception] # => ["ArgumentError", "Invalid value"]
event.payload[:exception_object] # => #<ArgumentError: Invalid value>
As the earlier 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.
To record started
and finished
values with monotonic time, specify the optional :monotonic
option to the subscribed
method. The :monotonic
option is set to false
by default.
callback = lambda {|name, started, finished, unique_id, payload| ... }
ActiveSupport::Notifications.subscribed(callback, "sql.active_record", monotonic: true) do
...
end
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")
Subscribers using a regexp or other pattern-matching object will remain subscribed to all events that match their original pattern, unless those events match a string passed to unsubscribe
:
subscriber = ActiveSupport::Notifications.subscribe(/render/) { }
ActiveSupport::Notifications.unsubscribe('render_template.action_view')
subscriber.matches?('render_template.action_view') # => false
subscriber.matches?('render_partial.action_view') # => true
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.
- CLASS ActiveSupport::Notifications::Event
- CLASS ActiveSupport::Notifications::Fanout
- CLASS ActiveSupport::Notifications::InstrumentationSubscriberError
- CLASS ActiveSupport::Notifications::Instrumenter
Attributes
[RW] | notifier |
Class Public methods
instrument(name, payload = {}) Link
instrumenter() Link
monotonic_subscribe(pattern = nil, callback = nil, &block) Link
Performs the same functionality as subscribe, but the start
and finish
block arguments are in monotonic time instead of wall-clock time. Monotonic time will not jump forward or backward (due to NTP or Daylights Savings). Use monotonic_subscribe
when accuracy of time duration is important. For example, computing elapsed time between two events.
publish(name, *args) Link
subscribe(pattern = nil, callback = nil, &block) Link
Subscribe to a given event name with the passed block
.
You can subscribe to events by passing a String
to match exact event names, or by passing a Regexp
to match all events that match a pattern.
ActiveSupport::Notifications.subscribe(/render/) do |*args|
@event = ActiveSupport::Notifications::Event.new(*args)
end
The block
will receive five parameters with information about the event:
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 the instrumenter that fired the event
payload # => Hash, the payload
end
If the block passed to the method only takes one parameter, it will yield an event object to the block:
ActiveSupport::Notifications.subscribe(/render/) do |event|
@event = event
end
Raises an error if invalid event name type is passed:
ActiveSupport::Notifications.subscribe(:render) {|*args| ...}
#=> ArgumentError (pattern must be specified as a String, Regexp or empty)