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 that 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 it is so desired.
In this guide, you will learn how to use the instrumentation API inside of Active Support to measure events inside of Rails and other Ruby code.
After reading this guide, you will know:
- What instrumentation can provide.
- The hooks inside the Rails framework for instrumentation.
- Adding a subscriber to a hook.
- Building a custom instrumentation implementation.
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, as described below in <TODO: link to section detailing each hook point>. 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.
Rails framework hooks
Within the Ruby on Rails framework, there are a number of hooks provided for common events. These are detailed below.
Action Controller
write_fragment.action_controller
Key |
Value |
:key |
The complete key |
{
key: 'posts/1-dashboard-view'
}
read_fragment.action_controller
Key |
Value |
:key |
The complete key |
{
key: 'posts/1-dashboard-view'
}
expire_fragment.action_controller
Key |
Value |
:key |
The complete key |
{
key: 'posts/1-dashboard-view'
}
exist_fragment?.action_controller
Key |
Value |
:key |
The complete key |
{
key: 'posts/1-dashboard-view'
}
write_page.action_controller
Key |
Value |
:path |
The complete path |
{
path: '/users/1'
}
expire_page.action_controller
Key |
Value |
:path |
The complete path |
{
path: '/users/1'
}
start_processing.action_controller
Key |
Value |
:controller |
The controller name |
:action |
The action |
:params |
Hash of request parameters without any filtered parameter |
:format |
html/js/json/xml etc |
:method |
HTTP request verb |
:path |
Request path |
{
controller: "PostsController",
action: "new",
params: { "action" => "new", "controller" => "posts" },
format: :html,
method: "GET",
path: "/posts/new"
}
process_action.action_controller
Key |
Value |
:controller |
The controller name |
:action |
The action |
:params |
Hash of request parameters without any filtered parameter |
:format |
html/js/json/xml etc |
:method |
HTTP request verb |
:path |
Request path |
:view_runtime |
Amount spent in view in ms |
{
controller: "PostsController",
action: "index",
params: {"action" => "index", "controller" => "posts"},
format: :html,
method: "GET",
path: "/posts",
status: 200,
view_runtime: 46.848,
db_runtime: 0.157
}
send_file.action_controller
Key |
Value |
:path |
Complete path to the file |
INFO. Additional keys may be added by the caller.
send_data.action_controller
ActionController
does not had any specific information to the payload. All options are passed through to the payload.
redirect_to.action_controller
Key |
Value |
:status |
HTTP response code |
:location |
URL to redirect to |
{
status: 302,
location: "http://localhost:3000/posts/new"
}
halted_callback.action_controller
Key |
Value |
:filter |
Filter that halted the action |
{
filter: ":halting_filter"
}
Action View
render_template.action_view
Key |
Value |
:identifier |
Full path to template |
:layout |
Applicable layout |
{
identifier: "/Users/adam/projects/notifications/app/views/posts/index.html.erb",
layout: "layouts/application"
}
render_partial.action_view
Key |
Value |
:identifier |
Full path to template |
{
identifier: "/Users/adam/projects/notifications/app/views/posts/_form.html.erb",
}
Active Record
sql.active_record
Key |
Value |
:sql |
SQL statement |
:name |
Name of the operation |
:object_id |
self.object_id |
INFO. The adapters will add their own data as well.
{
sql: "SELECT \"posts\".* FROM \"posts\" ",
name: "Post Load",
connection_id: 70307250813140,
binds: []
}
identity.active_record
Key |
Value |
:line |
Primary Key of object in the identity map |
:name |
Record's class |
:connection_id |
self.object_id |
Action Mailer
receive.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 |
{
mailer: "Notification",
message_id: "4f5b5491f1774_181b23fc3d4434d38138e5@mba.local.mail",
subject: "Rails Guides",
to: ["users@rails.com", "ddh@rails.com"],
from: ["me@rails.com"],
date: Sat, 10 Mar 2012 14:18:09 +0100,
mail: "..." # omitted for brevity
}
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 |
{
mailer: "Notification",
message_id: "4f5b5491f1774_181b23fc3d4434d38138e5@mba.local.mail",
subject: "Rails Guides",
to: ["users@rails.com", "ddh@rails.com"],
from: ["me@rails.com"],
date: Sat, 10 Mar 2012 14:18:09 +0100,
mail: "..." # omitted for brevity
}
ActiveResource
request.active_resource
Key |
Value |
:method |
HTTP method |
:request_uri |
Complete URI |
:result |
HTTP response object |
Active Support
cache_read.active_support
Key |
Value |
:key |
Key used in the store |
:hit |
If this read is a hit |
:super_operation |
:fetch is added when a read is used with #fetch |
cache_generate.active_support
This event is only used when #fetch
is called with a block.
Key |
Value |
:key |
Key used in the store |
INFO. Options passed to fetch will be merged with the payload when writing to the store
{
key: 'name-of-complicated-computation'
}
cache_fetch_hit.active_support
This event is only used when #fetch
is called with a block.
Key |
Value |
:key |
Key used in the store |
INFO. Options passed to fetch will be merged with the payload.
{
key: 'name-of-complicated-computation'
}
cache_write.active_support
Key |
Value |
:key |
Key used in the store |
INFO. Cache stores my add their own keys
{
key: 'name-of-complicated-computation'
}
cache_delete.active_support
Key |
Value |
:key |
Key used in the store |
{
key: 'name-of-complicated-computation'
}
cache_exist?.active_support
Key |
Value |
:key |
Key used in the store |
{
key: 'name-of-complicated-computation'
}
Rails
deprecation.rails
Key |
Value |
:message |
The deprecation warning |
:callstack |
Where the deprecation came from |
Subscribing to an event
Subscribing to an event is easy. Use ActiveSupport::Notifications.subscribe
with a block to
listen to any notification.
The block receives the following arguments:
- The name of the event
- Time when it started
- Time when it finished
- An unique ID for this event
- The payload (described in previous sections)
ActiveSupport::Notifications.subscribe "process_action.action_controller" do |name, started, finished, unique_id, data|
# your own custom stuff
Rails.logger.info "#{name} Received!"
end
Defining all those block arguments each time can be tedious. You can easily create an ActiveSupport::Notifications::Event
from block arguments like this:
ActiveSupport::Notifications.subscribe "process_action.action_controller" do |*args|
event = ActiveSupport::Notifications::Event.new *args
event.name # => "process_action.action_controller"
event.duration # => 10 (in milliseconds)
event.payload # => {:extra=>information}
Rails.logger.info "#{event} Received!"
end
Most times you only care about the data itself. Here is a shortcut to just get the data.
ActiveSupport::Notifications.subscribe "process_action.action_controller" do |*args|
data = args.extract_options!
data # { extra: :information }
You may also subscribe to events matching a regular expression. This enables you to subscribe to
multiple events at once. Here's you could subscribe to everything from ActionController
.
ActiveSupport::Notifications.subscribe /action_controller/ do |*args|
# inspect all ActionController events
end
Creating custom events
Adding your own events is easy as well. ActiveSupport::Notifications
will take care of
all the heavy lifting for you. Simply call instrument
with a name
, payload
and a block.
The notification will be sent after the block returns. ActiveSupport
will generate the start and end times
as well as the 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 should follow Rails conventions when defining your own events. The format is: event.library
.
If you application is sending Tweets, you should create an event named tweet.twitter
.