Omnes

Pub/sub for Ruby.

Omnes is a Ruby library implementing the publish-subscribe pattern. This pattern allows senders of messages to be decoupled from their receivers. An Event Bus acts as a middleman where events are published while interested parties can subscribe to them.

bundle add omnes

There're two ways to make use of the pub/sub features Omnes provides:

• Standalone, through an Omnes::Bus instance:

require "omnes"

bus = Omnes::Bus.new

• Mixing in the behavior in another class by including the Omnes module.

require "omnes"

class Notifier
  include Omnes
end

The following examples will use the direct Omnes::Bus instance. The only difference for the mixing use case is that the methods are directly called in the including instance.

Before being able to work with a given event, its name (which must be a Symbol) must be registered:

bus.register(:order_created)

An event can be anything responding to a method :omnes_event_name, which must match with a registered name.

Typically, there're two main ways to generate events.

1. Unstructured events

An event can be generated at publication time, where you provide its name and a payload to be consumed by its subscribers:

bus.publish(:order_created, number: order.number, user_email: user.email)

In that case, an instance of Omnes::UnstructuredEvent is generated under the hood.

Unstructured events are straightforward to create and use, but they're harder to debug as they're defined at publication time. On top of that, other features, such as event persistence, can't be reliably built on top of them.

2. Instance-backed events

You can also publish an instance of a class including Omnes::Event. The only fancy thing it provides is an OOTB event name generated based on the class name.

class OrderCreatedEvent
  include Omnes::Event

  attr_reader :number, :user_email
  
  def initialize(number:, user_email:)
    @number = number
    @user_email = user_email
  end
end

event = OrderCreatedEvent.new(number: order.number, user_email: user.email)
bus.publish(event)

By default, an event name instance equals the event class name downcased, underscored and with the Event suffix removed if present (:order_created in the previous example). However, you can configure your own name generator based on the event instance:

event_name_as_class = ->(event) { event.class.name.to_sym } # :OrderCreatedEvent in the example
Omnes.config.event.name_builder = event_name_as_class

Instance-backed events provide a well-defined structure, and other features, like event persistence, can be added on top of them.

You can subscribe to a specific event to run some code whenever it's published. The event is yielded to the subscription block:

bus.subscribe(:order_created) do |event|
  # ...
end

For unstructured events, the published data is made available through the payload method, although #[] can be used as a shortcut:

bus.subscribe(:order_created) do |event|
  OrderCreationEmail.new.send(number: event[:number], email: event[:user_email])
  # OrderCreationEmail.new.send(number: event.payload[:number], email: event.payload[:user_email])
end

Otherwise, use the event instance according to its structure:

bus.subscribe(:order_created) do |event|
  OrderCreationEmail.new.send(number: event.number, email: event.user_email)
end

The subscription code can also be given as anything responding to a method #call.

class OrderCreationEmailSubscription
  def call(event)
    OrderCreationEmail.new.send(number: event.number, email: event.user_email)
  end
end

bus.subscribe(:order_created, OrderCreationEmailSubscription.new)

However, see Event subscribers section bellow for a more powerful way to define standalone event handlers.

You can also create a subscription that will run for all events:

class LogEventsSubscription
  attr_reader :logger
  
  def initialize(logger: Logger.new(STDOUT))
    @logger = logger
  end
  
  def call(event)
    logger.info("Event #{event.omnes_event_name} published")
  end
end

bus.subscribe_to_all(LogEventsSubscription.new)

Custom event matchers can be defined. A matcher is something responding to #call and taking the event as an argument. It must return true or false to match or ignore the event.

ORDER_EVENTS_MATCHER = ->(event) { event.omnes_event_name.start_with?(:order) }

bus.subscribe_with_matcher(ORDER_EVENTS_MATCHER) do |event|
  # ...
end

For all subscription methods we've seen, an Omnes::Subscription instance is returned. Holding that reference can be useful for debugging and testing purposes.

Often though, you won't have the reference at hand when you need it. Thankfully, you can provide a subscription identifier on subscription time and use it later to fetch the subscription instance from the bus. A subscription identifier needs to be a Symbol:

bus.subscribe(:order_created, OrderCreationEmailSubscription.new, id: :order_created_email)
subscription = bus.subscription(:order_created_email)

Events subscribers offer a way to define event subscriptions from a custom class.

In its simplest form, you can match an event to a method in the class.

class OrderCreationEmailSubscriber
  include Omnes::Subscriber
  
  handle :order_created, with: :send_confirmation_email
  
  attr_reader :service
  
  def initialize(service: OrderCreationEmail.new)
    @service = service
  end

  def send_confirmation_email(event)
    service.send(number: event.number, email: event.user_email)
  end
end

You add the subscriptions by calling the #subscribe_to method on an instance:

OrderCreationEmailSubscriber.new.subscribe_to(bus)

Equivalent to the subscribe methods we've seen above, you can also subscribe to all events:

class LogEventsSubscriber
  include Omnes::Subscriber
  
  handle_all with: :log_event
  
  attr_reader :logger
  
  def initialize(logger: Logger.new(STDOUT))
    @logger = logger
  end

  def log_event(event)
    logger.info("Event #{event.omnes_event_name} published")
  end
end

You can also handle the event with your own custom matcher:

class OrderSubscriber
  include Omnes::Subscriber
  
  handle_with_matcher ORDER_EVENTS_MATCHER, with: :register_order_event
  
  def register_order_event(event)
    # ...
  end
end

Likewise, you can provide identifiers to reference subscriptions:

handle :order_created, with: :send_confirmation_email, id: :order_creation_email_subscriber

As you can subscribe multiple instances of a subscriber to the same bus, you might need to create a different identifier for each of them. For those cases, you can pass a lambda taking the subscriber instance:

handle :order_created, with: :send_confirmation_email, id: ->(subscriber) { :"#{subscriber.id}_order_creation_email_subscriber" }

You can let the event handlers to be automatically discovered.You need to enable the autodiscover feature and prefix the event name with on_ for your handler name.

class OrderCreationEmailSubscriber
  include Omnes::Subscriber[
    autodiscover: true
  ]
  
  # ...
  
  def on_order_created(event)
    # ...
  end
end

If you prefer, you can make autodiscover on by default:

Omnes.config.subscriber.autodiscover = true

You can also specify your own autodiscover strategy. It must be something callable, transforming the event name into the handler name.

AUTODISCOVER_STRATEGY = ->(event_name) { event_name }

class OrderCreationEmailSubscriber
  include Omnes::Subscriber[
    autodiscover: true,
    autodiscover_strategy: AUTODISCOVER_STRATEGY
  ]
  
  # ...

  def order_created(event)
    # ...
  end
end

The strategy can also be globally set:

Omnes.config.subscriber.autodiscover_strategy = AUTODISCOVER_STRATEGY

Subscribers are not limited to use a method as event handler. They can interact with the whole instance context and leverage it to build adapters.

Omnes ships with a few of them.

The Sidekiq adapter allows creating a subscription to be processed as a Sidekiq background job.

Sidekiq requires that the argument passed to #perform is serializable. By default, the result of calling #payload in the event is taken.

class OrderCreationEmailSubscriber
  include Omnes::Subscriber
  include Sidekiq::Job
  
  handle :order_created, with: Adapter::Sidekiq
  
  def perform(payload)
    OrderCreationEmail.send(number: payload["number"], email: payload["user_email"])
  end
end

bus = Omnes::Bus.new
bus.register(:order_created)
OrderCreationEmailSubscriber.new.subscribe_to(bus)
bus.publish(:order_created, "number" => order.number, "user_email" => user.email)

However, you can configure how the event is serialized thanks to the serializer: option. It needs to be something callable taking the event as argument:

handle :order_created, with: Adapter::Sidekiq[serializer: :serialized_payload.to_proc]

You can also globally configure the default serializer:

Omnes.config.subscriber.adapter.sidekiq.serializer = :serialized_payload.to_proc

You can delay the callback execution from the publication time with the .in method (analogous to Sidekiq::Job.perform_in):

handle :order_created, with: Adapter::Sidekiq.in(60)

The ActiveJob adapter allows creating a subscription to be processed as an ActiveJob background job.

ActiveJob requires that the argument passed to #perform is serializable. By default, the result of calling #payload in the event is taken.

class OrderCreationEmailSubscriber < ActiveJob
  include Omnes::Subscriber
  
  handle :order_created, with: Adapter::ActiveJob
  
  def perform(payload)
    OrderCreationEmail.send(number: payload["number"], email: payload["user_email"])
  end
end

bus = Omnes::Bus.new
bus.register(:order_created)
OrderCreationEmailSubscriber.new.subscribe_to(bus)
bus.publish(:order_created, "number" => order.number, "user_email" => user.email)

However, you can configure how the event is serialized thanks to the serializer: option. It needs to be something callable taking the event as argument:

handle :order_created, with: Adapter::ActiveJob[serializer: :serialized_payload.to_proc]

You can also globally configure the default serializer:

Omnes.config.subscriber.adapter.active_job.serializer = :serialized_payload.to_proc

Custom adapters can be built. They need to implement a method #call taking the instance of Omnes::Subscriber, the event and, optionally, the publication context (see debugging subscriptions).

Here's a custom adapter executing a subscriber method in a different thread (we add an extra argument for the method name, and we partially apply it at the definition time to obey the adapter requirements).

THREAD_ADAPTER = lambda do |method_name, instance, event|
  Thread.new { instance.method(method_name).call(event) }
end

class OrderCreationEmailSubscriber
  include Omnes::Subscriber
  
  handle :order_created, with: THREAD_ADAPTER.curry[:order_created]
  
  def order_created(event)
    # ...
  end
end

Alternatively, adapters can be curried and only take the instance as an argument, returning a callable taking the event. For instance, we could also have defined the thread adapter like this:

class ThreadAdapter
  attr_reader :method_name
  
  def initialize(method_name)
    @method_name = method_name
  end
  
  def call(instance)
    raise unless instance.respond_to?(method_name)
    
    ->(event) { instance.method(:call).(event) }
  end
end

# ...
handle :order_created, with: ThreadAdapter.new(:order_created)
# ...

You can unsubscribe a given subscription by passing its reference to Omnes::Bus#unsubscribe (see how to reference subscriptions):

subscription = bus.subscribe(:order_created, OrderCreationEmailSubscription.new)
bus.unsubscribe(subscription)

Sometimes you might need to leave your bus in a pristine state, with no events registered or active subscriptions. That can be useful for autoloading in development:

bus.clear
bus.registry.event_names # => []
bus.subscriptions # => []

Whenever you register an event, you get back an Omnes::Registry::Registration instance. It gives access to both the registered #event_name and the #caller_location of the registration.

An Omnes::Bus contains a reference to its registry, which can be used to retrieve a registration later on.

bus.registry.registration(:order_created)

You can also use the registry to retrieve all registered event names:

bus.registry.event_names

See Omnes::Registry for other available methods.

When you publish an event, you get back an Omnes::Publication instance. It contains some attributes that allow observing what happened:

• #event contains the event instance that has been published.
• #executions contains an array of Omnes::Execution(lib/omnes/execution.rb). Read more below.
• #context is an instance of Omnes::PublicationContext.

Omnes::Execution represents a subscription individual execution. It contains the following attributes:

• #subscription is an instance of Omnes::Subscription.
• #result contains the result of the execution.
• #benchmark of the operation.
• #time is the time where the execution started.

Omnes::PublicationContext represents the shared context for all triggered executions. See [Subscription][#subscription] for details.

If your subscription block or callable object takes a second argument, it'll contain an instance of an Omnes::PublicationContext. It allows you to inspect what triggered a given execution from within that execution code. It contains:

• #caller_location refers to the publication caller.
• #time is the time stamp for the publication.

class OrderCreationEmailSubscriber
  include Omnes::Subscriber
  
  handle :order_created, with: :send_confirmation_email

  def send_confirmation_email(event, publication_context)
    # debugging
    abort(publication_context.caller_location.inspect)

    OrderCreationEmail.send(number: event.number, email: event.user_email)
  end
end

In case you're developing your own async adapter, you can call #serialized on an instance of Omnes::PublicationContext to get a serialized version of it. It'll return a Hash with "caller_location" and "time" keys, and the respective String representations as values.

Ideally, you wouldn't need big setups to test your event-driven behavior. You could design your subscribers to use lightweight mocks for any external or operation at the integration level. Example:

if # test environment
  bus.subscribe(:order_created, OrderCreationEmailSubscriber.new(service: MockService.new)
else
  bus.subscribe(:order_created, OrderCreationEmailSubscriber.new)
end

Then, at the unit level, you can test your subscribers as any other class.

However, there's also a handy Omnes::Bus#performing_only method that allows running a code block with only a selection of subscriptions as potential callbacks for published events.

creation_subscription = bus.subscribe(:order_created, OrderCreationEmailSubscriber.new)
deletion_subscription = bus.subscribe(:order_deleted, OrderDeletionSubscriber.new)
bus.performing_only(creation_subscription) do
  bus.publish(:order_created, number: order.number, user_email: user.email) # `creation_subscription` will run
  bus.publish(:order_deleted, number: order.number) # `deletion_subscription` won't run
end
bus.publish(:order_deleted, number: order.number) # `deletion_subscription` will run

Remember that you can get previous subscription references thanks to subscription identifiers.

There's also a specialized Omnes::Bus#performing_nothing method that runs no subscriptions for the duration of the block.

We've seen the relevant configurable settings in the corresponding sections. You can also access the configuration in the habitual block syntax:

Omnes.configure do |config|
  config.subscriber.adapter.sidekiq.serializer = :serialized_payload.to_proc
end

Finally, nested settings can also be set directly from the affected class. E.g.:

Omnes::Subscriber::Adapter::Sidekiq.config.serializer = :serialized_payload.to_proc

Create an initializer in config/initializers/omnes.rb:

require "omnes"

Omnes.config.subscriber.autodiscover = true

Bus = Omnes::Bus.new

Rails.application.config.to_prepare do
  Bus.clear

  Bus.register(:order_created)

  OrderCreationEmailSubscriber.new.subscribe_to(Bus)
end

We can define OrderCreationEmailSubscriber in app/subscribers/order_creation_email_subscriber.rb:

# frozen_string_literal: true

class OrderCreationEmailSubscriber
  include Omnes::Subscriber

  def on_order_created(event)
    # ...
  end
end

Ideally, you'll publish your event in a custom service layer. If that's not possible, you can publish it in the controller.

We strongly discourage publishing events as part of an ActiveRecord callback. Subscribers should run code that is independent of the main business transaction. As such, they shouldn't run within the same database transaction, and they should be decoupled of persistence responsibilities altogether.

Why an Event Bus is called an Event Bus? It's a long story:

• The first leap leaves us with the hardware computer buses. They move data from one hardware component to another.
• The name leaked to the software to describe architectures that communicate parts by sending messages, like an Event Bus.
• That was given as an analogy of buses as vehicles, where not data but people are transported.
• Bus is a clipped version of the Latin omnibus. That's what buses used to be called (and they're still called like that in some places, like Argentina).
• Bus stands for the preposition for, while Omni means all. That's for all, but, for some reason, we decided to keep the part void of meaning.
• Why were they called omnibus? Let's move back to 1823 and talk about a man named Stanislas Baudry.
• Stanislas lived in a suburb of Nantes, France. There, he ran a corn mill.
• Hot water was a by-product of the mill, so Stanislas decided to build a spa business.
• As the mill was on the city's outskirts, he arranged some horse-drawn transportation to bring people to his spa.
• It turned out that people weren't interested in it, but they did use the carriage to go to and fro.
• The first stop of the service was in front of the shop of a hatter called Omnes.
• Omnes was a witty man. He'd named his shop with a pun on his Latin-sounding name: Omnes Omnibus. That means something like everything for everyone.
• Therefore, people in Nantes started to call Omnibus to the new service.

So, it turns out we call it the "Event Bus" because presumably, the parents of Omnes gave him that name. So, the name of this library, it's a tribute to Omnes, the hatter.

By the way, in case you're wondering, Stanislas, the guy of the mill, closed both it and the spa to run his service. Eventually, he moved to Paris to earn more money in a bigger city.

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.

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

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

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