TappingDevice

Introduction

TappingDevice makes the objects tell you what they do, so you don't need to track them yourself.

Contact Tracing For Objects

The concept is very simple. It's basically like contact tracing for your Ruby objects. You can use

• print_calls(object) to see what the object does
• print_traces(object) to see how the object interacts with other objects (like used as an argument)
• print_mutations(object) to see what actions changed the object's state (instance variables)

Still sounds vague? Let's see some examples:

print_calls - Track Method Calls

In Discourse, it uses the Guardian class for authorization (like policy objects). It's barely visible in controller actions, but it does many checks under the hood. Now, let's say we want to know what the Guardian would do when a user creates a post; here's the controller action:

  def create
    @manager_params = create_params
    @manager_params[:first_post_checks] = !is_api?

    manager = NewPostManager.new(current_user, @manager_params)

    if is_api?
      memoized_payload = DistributedMemoizer.memoize(signature_for(@manager_params), 120) do
        result = manager.perform
        MultiJson.dump(serialize_data(result, NewPostResultSerializer, root: false))
      end

      parsed_payload = JSON.parse(memoized_payload)
      backwards_compatible_json(parsed_payload, parsed_payload['success'])
    else
      result = manager.perform
      json = serialize_data(result, NewPostResultSerializer, root: false)
      backwards_compatible_json(json, result.success?)
    end
  end

As you can see, it doesn't even exist in the controller action, which makes tracking it by reading code very hard to do.

But with TappingDevice. You can use print_calls to show what method calls the object performs

  def create
    # you can retrieve the current guardian object by calling guardian in the controller
    print_calls(guardian)
    @manager_params = create_params
   
    # .....

Now, if you execute the code, like via tests:

$ rspec spec/requests/posts_controller_spec.rb:603

You can get all the method calls it performs with basically everything you need to know

Let's take a closer look at each entry. Everyone of them contains the method call's

• method name
• method source class/module
• call site
• arguments
• return value

These are the information you'd have to look up one by one manually (probably with many debug code writing). Now you can get all of them in just one line of code.

print_traces - See The Object's Traces

If you're not interested in what an object does, but what it interacts with other parts of the program, e.g., used as arguments. You can use the print_traces helper. Let's see how Discourse uses the manager object when creating a post

  def create
    @manager_params = create_params
    @manager_params[:first_post_checks] = !is_api?
   
    manager = NewPostManager.new(current_user, @manager_params)

    print_traces(manager)
    # .....

And after running the test case

$ rspec spec/requests/posts_controller_spec.rb:603

You will see that it performs 2 calls: perform and perform_create_post. And it's also used as manager argument in various of calls of the NewPostManager class.

print_mutations - Display All State Changes At Once

Another thing that often bothers developers in debugging is to track an object's internal state changes. And tapping_device allows you to see all state changes with just one line of code. Let me keep using Discourse to demonstrate it.

When updating a post, it uses an object called PostRevisor to revise it:

# app/controllers/posts_controller.rb
class PostsController
  def update
    # ......
    revisor = PostRevisor.new(post, topic)
    revisor.revise!(current_user, changes, opts)
    # ......
  end
end

In the PostReviser#revise!, it uses many instance variables to track different information:

  # lib/post_revisor.rb
  def revise!(editor, fields, opts = {})
    @editor = editor
    @fields = fields.with_indifferent_access
    @opts = opts

    @topic_changes = TopicChanges.new(@topic, editor)
    
    # ......

    @revised_at = @opts[:revised_at] || Time.now
    @last_version_at = @post.last_version_at || Time.now

    @version_changed = false
    @post_successfully_saved = true

    @validate_post = true
    # ......
  end

Tracking the changes of that many instance variables can be a painful task, especially when we want to know the values before and after certain method call. This is why I created print_mutations to save us from this.

Like other helpers, you only need 1 line of code

# app/controllers/posts_controller.rb
class PostsController
  def update
    # ......
    revisor = PostRevisor.new(post, topic)
    print_mutations(revisor)
    revisor.revise!(current_user, changes, opts)
    # ......
  end
end

And then you'll see all the state changes:

Now you can see what method changes which states. And more importantly, you get to see all the sate changes at once!

You can try these examples on my fork of discourse

write_* helpers

tapping_device also provides helpers that write the events into files:

• write_calls(object)
• write_traces(object)
• write_mutations(object)

The default destination is /tmp/tapping_device.log. You can change it with the log_file option:

write_calls(object, log_file: "/tmp/another_file")

Installation

Add this line to your application's Gemfile:

gem 'tapping_device', group: :development

And then execute:

$ bundle

Or install it directly:

$ gem install tapping_device

Depending on the size of your application, TappingDevice could harm the performance significantly. So make sure you don't put it inside the production group

Advance Usages & Options

Add Conditions With .with

Sometimes we don't need to know all the calls or traces of an object; we just want some of them. In those cases, we can chain the helpers with .with to filter the calls/traces.

# only prints calls with name matches /foo/
print_calls(object).with do |payload|
  payload.method_name.to_s.match?(/foo/)
end

Options

There are many options you can pass when using a helper method. You can list all available options and their default value with

TappingDevice::Configurable::DEFAULTS #=> {
  :filter_by_paths=>[], 
  :exclude_by_paths=>[], 
  :with_trace_to=>50, 
  :event_type=>:return, 
  :hijack_attr_methods=>false, 
  :track_as_records=>false, 
  :inspect=>false, 
  :colorize=>true, 
  :log_file=>"/tmp/tapping_device.log"
}

Here are some commonly used options:

colorize: false

• default: true

By default print_calls and print_traces colorize their output. If you don't want the colors, you can use colorize: false to disable it.

print_calls(object, colorize: false)

inspect: true

• default: false

As you might have noticed, all the objects are converted into strings with #to_s instead of #inspect. This is because when used on some Rails objects, #inspect can generate a significantly larger string than #to_s. For example:

post.to_s #=> #<Post:0x00007f89a55201d0>
post.inspect #=> #<Post id: 649, user_id: 3, topic_id: 600, post_number: 1, raw: "Hello world", cooked: "<p>Hello world</p>", created_at: "2020-05-24 08:07:29", updated_at: "2020-05-24 08:07:29", reply_to_post_number: nil, reply_count: 0, quote_count: 0, deleted_at: nil, off_topic_count: 0, like_count: 0, incoming_link_count: 0, bookmark_count: 0, score: nil, reads: 0, post_type: 1, sort_order: 1, last_editor_id: 3, hidden: false, hidden_reason_id: nil, notify_moderators_count: 0, spam_count: 0, illegal_count: 0, inappropriate_count: 0, last_version_at: "2020-05-24 08:07:29", user_deleted: false, reply_to_user_id: nil, percent_rank: 1.0, notify_user_count: 0, like_score: 0, deleted_by_id: nil, edit_reason: nil, word_count: 2, version: 1, cook_method: 1, wiki: false, baked_at: "2020-05-24 08:07:29", baked_version: 2, hidden_at: nil, self_edits: 0, reply_quoted: false, via_email: false, raw_email: nil, public_version: 1, action_code: nil, image_url: nil, locked_by_id: nil, image_upload_id: nil>

hijack_attr_methods: true

• default: false
  • except for tap_mutation! and print_mutations

Because TracePoint doesn't track methods generated by attr_* helpers (see this issue for more info), we need to redefine those methods with the normal method definition.

For example, it generates

def name=(val)
  @name = val
end

for

attr_writer :name

This hack will only be applied to the target instance with instance_eval. So other instances of the class remain untouched.

The default is false because

1. Checking what methods are generated by attr_* helpers isn't free. It's an O(n) operation, where n is the number of methods the target object has.
2. It's still unclear if this hack safe enough for most applications.

Global Configuration

If you don't want to pass options every time you use a helper, you can use global configuration to change the default values:

TappingDevice.config[:colorize] = false
TappingDevice.config[:hijack_attr_methods] = true

And if you're using Rails, you can put the configs under config/initializers/tapping_device.rb like this:

if defined?(TappingDevice)
  TappingDevice.config[:colorize] = false
  TappingDevice.config[:hijack_attr_methods] = true
end

Lower-Level Helpers

print_calls and print_traces aren't the only helpers you can get from TappingDevice. They are actually built on top of other helpers, which you can use as well. To know more about them, please check this page

Related Blog Posts

• Optimize Your Debugging Process With Object-Oriented Tracing and tapping_device
• Debug Rails issues effectively with tapping_device
• Want to know more about your Rails app? Tap on your objects!

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/st0012/tapping_device. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open-source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the TappingDevice project's codebases, issue trackers, chat rooms, and mailing lists is expected to follow the code of conduct.