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.
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.
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.
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 stuffRails.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 stuffduration=finished-started# 1560979.429234 - 1560978.425334Rails.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 eventsend
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.transactiondo# 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.transactiondo|t1|User.count# Fires an event for t1.ActiveRecord::Base.transactiondo|t2|# The next line fires no event for t2, because the only# real database transaction in this example is t1.User.first.touchendend
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.transactiondo|t1|User.count# Fires an event for t1.ActiveRecord::Base.transaction(requires_new: true)do|t2|User.first.touch# Fires an event for t2.endend
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.
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: :datado# do your custom stuff hereend
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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