The 10-minute Rails Pub/Sub

This time we’ll experiment with a quick way to architecture a Rails application to use Pub/Sub instead of model callbacks.

What’s wrong with callbacks

Rails active record models easily become bloated, that’s where most of the business logic tends to live after all. One of the most common sources of technical debt in Rails apps is callbacks. Models become god-objects with dependencies to other models, mailers and even 3rd party services.

When it comes to refactoring this coupling, I usually recommend extracting all callbacks to stateless functions which can be composed to form pipelines. One can use dry-transaction for that. My love for such composable architectures led me to create Opus for Elixir.

I’m also quite proud that callbacks got deprecated in Ecto 🎉.

About Pub/Sub

The solution which is the focus of this post is Pub/Sub. The models will publish events concerning database updates. A database record gets created / updated / destroyed and then a subscriber does something, or ignores the event.

Enter ActiveSupport::Notifications

We’ll lay the foundations for this ten minute implementation on top of ActiveSupport::Notifications. Originally introduced as an instrumentation API for Rails, but there’s nothing preventing us from using it for custom events.

Some facts about ActiveSupport::Notifications.

It’s basically a thread-safe queue
Events are synchronous
Events are process-local
It’s simple to use 😎

The Code

In this experiment, we’ll cover the following scenario:

When a User "zorbash" is created
And "zorbash" had been invited by "gandalf"
Then the field signups_count for "gandalf" should increase by 1

First we’ll create a model concern which we can include to our User model to publish events each time a record is created.

# frozen_string_literal: true

module Publishable
  extend ActiveSupport::Concern

  included do
    after_create_commit :publish_create
    after_update_commit :publish_update
    after_destroy_commit :publish_destroy
  end

  class_methods do
    def subscribe(event = :any)
      event_name = event == :any ? /#{table_name}/ : "#{table_name}.#{event}"

      ActiveSupport::Notifications.subscribe(event_name) do |_event_name, **payload|
        yield payload
      end

      self
    end
  end

  private

  def publish_create
    publish(:create)
  end

  def publish_update
    publish(:update)
  end

  def publish_destroy
    publish(:destroy)
  end

  def publish(event)
    event_name = "#{self.class.table_name}.#{event}"

    ActiveSupport::Notifications.publish(event_name, event: event, model: self)
  end
end

Then we must include it in our model.

# frozen_string_literal: true

class User < ApplicationRecord
  include Publishable # 👈 Added here

  devise :invitable

  # other omitted code
end

Let’s implement a subscriber.

module UserSubscriber
  extend self

  def subscribe
    User.subscribe(:create) do |event|
      event[:model].increment!(:signups_count)
    end
  end
end

Finally, we have to initialize the subscription.

# File: config/initializers/subscriptions.rb
Rails.application.config.after_initialize do
  UserSubscriber.subscribe
end

Caveats

The more listeners you add, the slower it becomes for an event to be handled in sequence across all listeners. This is similar to how an object would call all callback handler methods one after the other.

See: active_support/notifications/fanout.rb

def publish(name, *args)
  listeners_for(name).each { |s| s.publish(name, *args) }
end

They’re also not suitable for callbacks used to mutate a record like before_validation or after_initialize.

Furthermore there are no guarantees that an event will be processed successfully. Where things can go wrong, will go wrong. Prefer a solution with robust recovery semantics.

Next Steps

For enhanced flexibility, we can push events to Redis or RabbitMQ or Kafka. How to pick one according to your needs is beyond the scope of this post. However you can consider yourself lucky, since there are tons of resources out there and mature libraries to build your event-driven system on top of.

Alternatives

Notable Pub/Sub gems:

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Dimitris Zorbas

Code spells, smells and sourcery