Multiple Buses

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A common architecture when building applications is to separate commands from queries. Commands are actions that do something and queries fetch data. This is called CQRS (Command Query Responsibility Segregation). See Martin Fowler's article about CQRS to learn more. This architecture could be used together with the Messenger component by defining multiple buses.

A command bus is a little different from a query bus. For example, command buses usually don't provide any results and query buses are rarely asynchronous. You can configure these buses and their rules by using middleware.

It might also be a good idea to separate actions from reactions by introducing an event bus. The event bus could have zero or more subscribers.

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framework:
    messenger:
        # The bus that is going to be injected when injecting MessageBusInterface
        default_bus: command.bus
        buses:
            command.bus:
                middleware:
                    - validation
                    - doctrine_transaction
            query.bus:
                middleware:
                    - validation
            event.bus:
                # the 'allow_no_handlers' middleware allows to have no handler
                # configured for this bus without throwing an exception
                default_middleware: allow_no_handlers
                middleware:
                    - validation

This will create three new services:

  • command.bus: autowireable with the MessageBusInterface type-hint (because this is the default_bus);
  • query.bus: autowireable with MessageBusInterface $queryBus;
  • event.bus: autowireable with MessageBusInterface $eventBus.

Restrict Handlers per Bus

By default, each handler will be available to handle messages on all of your buses. To prevent dispatching a message to the wrong bus without an error, you can restrict each handler to a specific bus using the messenger.message_handler tag:

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# config/services.yaml
services:
    App\MessageHandler\SomeCommandHandler:
        tags: [{ name: messenger.message_handler, bus: command.bus }]
        # prevent handlers from being registered twice (or you can remove
        # the MessageHandlerInterface that autoconfigure uses to find handlers)
        autoconfigure: false

This way, the App\MessageHandler\SomeCommandHandler handler will only be known by the command.bus bus.

You can also automatically add this tag to a number of classes by using the _instanceof service configuration. Using this, you can determine the message bus based on an implemented interface:

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# config/services.yaml
services:
    # ...

    _instanceof:
        # all services implementing the CommandHandlerInterface
        # will be registered on the command.bus bus
        App\MessageHandler\CommandHandlerInterface:
            tags:
                - { name: messenger.message_handler, bus: command.bus }

        # while those implementing QueryHandlerInterface will be
        # registered on the query.bus bus
        App\MessageHandler\QueryHandlerInterface:
            tags:
                - { name: messenger.message_handler, bus: query.bus }

Debugging the Buses

The debug:messenger command lists available messages & handlers per bus. You can also restrict the list to a specific bus by providing its name as an argument.

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$ php bin/console debug:messenger

  Messenger
  =========

  command.bus
  -----------

   The following messages can be dispatched:

   ---------------------------------------------------------------------------------------
    App\Message\DummyCommand
        handled by App\MessageHandler\DummyCommandHandler
    App\Message\MultipleBusesMessage
        handled by App\MessageHandler\MultipleBusesMessageHandler
   ---------------------------------------------------------------------------------------

  query.bus
  ---------

   The following messages can be dispatched:

   ---------------------------------------------------------------------------------------
    App\Message\DummyQuery
        handled by App\MessageHandler\DummyQueryHandler
    App\Message\MultipleBusesMessage
        handled by App\MessageHandler\MultipleBusesMessageHandler
   ---------------------------------------------------------------------------------------

Tip

Since Symfony 5.1, the command will also show the PHPDoc description of the message and handler classes.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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