MessageBus

A message dispatcher helps to separate presentation concerns from business logic by mapping inputs of various sources to simpler application messages. It also helps to decouple the domain from the implementation, for an application only has to know about the messages, not how they are handled. A design well known in Hexagonal architectures.

ICanBoogie/MessageBus provides an implementation of a message dispatcher, with support for permissions and voters. There's also a simple implementation of a message handler provider, and one more sophisticated that works with PSR-11 containers. Finally, there's special support for Symfony's Dependency Injection component.

Using a message dispatcher can be as simple as the following example:

<?php

namespace ICanBoogie\MessageBus;

/* @var Dispatcher $dispatcher */
/* @var object $message */

// The message is dispatched to its handler, the result is returned.
$result = $dispatcher->dispatch($message);

Installation

composer require icanboogie/message-bus

If you're upgrading to a newer version, please check the Migration guide.

Messages

Messages can be any type of object. What's important is that all input type considerations (e.g. HTTP details) are removed to keep what's essential for the domain. That is, a controller would create a message to dispatch, but controller concern would stay in the controller, they would not leak into the message.

The following example demonstrates how a DeleteMenu message could be defined. Note that there isn't a notion of input type or authorization. These are presentation concerns, and they should remain there.

<?php

// …

final class DeleteMenu
{
    public function __construct(
        public /*readonly*/ int $id
    ) {
    }
}

Messages that don't alter the state of an application—in other words, messages that lead to read-only operations—can be marked with the Safe interface. This is not a requirement, just a recommendation to help you identify messages.

<?php

use ICanBoogie\MessageBus\Safe;

// …

final class ShowMenu implements Safe
{
    public function __construct(
        public /*readonly*/ int $id
    ) {
    }
}

Message handlers

Message handlers handle messages. Usually the relation is 1:1, that is one handler for one message type. Message handlers are callables, typically a class implementing __invoke(T $message), where T is a type of message.

The following example demonstrates how a handler can be defined for a ShowMenu message:

<?php

final class ShowMenuHandler
{
    public function __invoke(ShowMenu $message): Menu
    {
        // …
    }
}

Providing message handlers

Message handlers are obtained through a provider, usually backed by a service container.

The following example demonstrates how to obtain a message handler for a given message, and how to invoke that handler to get a result.

<?php

/* @var ICanBoogie\MessageBus\HandlerProvider $provider */
/* @var object $message */

$handler = $provider->getHandlerForMessage($message);
$result = $handler($message);

Providing handlers with a PSR container

Handlers can be provided by an instance of PSR\HandlerProviderWithContainer, which is backed by a PSR container. You need to provide the mapping from "message class" to "handler service identifier".

<?php

use ICanBoogie\MessageBus\PSR\HandlerProviderWithContainer;
use Psr\Container\ContainerInterface;

/* @var $container ContainerInterface */

$handlerProvider = new HandlerProviderWithContainer($container, [

	Acme\MenuService\Application\MessageBus\CreateMenu::class => Acme\MenuService\Application\MessageBus\CreateMenuHandler::class,
	Acme\MenuService\Application\MessageBus\DeleteMenu::class => Acme\MenuService\Application\MessageBus\DeleteMenuHandler::class,

]);

Providing handlers with Symfony's Dependency Injection component

The easiest way to define messages, handlers, permissions, and voters, is with Symfony's Dependency Injection component. The handlers are defined as services, tags are used to identify them and the message type they support. It's also possible to define permissions, and voter for those permissions.

You can use the services.yaml file provided directly in your project, together with the compiler pass MessageBusPass.

The following example demonstrates how to define handlers, commands, permission, and voters:

services:
  Acme\MenuService\Application\MessageBus\CreateMenuHandler:
    tags:
    - { name: message_bus.handler, message: Acme\MenuService\Application\MessageBus\CreateMenu }
    - { name: message_bus.permission, permission: is_admin }
    - { name: message_bus.permission, permission: can_write_menu }

  Acme\MenuService\Application\MessageBus\DeleteMenuHandler:
    tags:
    - { name: message_bus.handler, message: Acme\MenuService\Application\MessageBus\DeleteMenu }
    - { name: message_bus.permission, permission: is_admin }
    - { name: message_bus.permission, permission: can_manage_menu }

  Acme\MenuService\Presentation\Security\Voters\IsAdmin:
    tags:
    - { name: message_bus.voter, permission: is_admin }

  Acme\MenuService\Presentation\Security\Voters\CanWriteMenu:
    tags:
    - { name: message_bus.voter, permission: can_write_menu }

  Acme\MenuService\Presentation\Security\Voters\CanManageMenu:
    tags:
    - { name: message_bus.voter, permission: can_manage_menu }

<?php

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
use Symfony\Component\Config\FileLocator;
use ICanBoogie\MessageBus\Symfony\MessageBusPass;

/* @var string $config */

$container = new ContainerBuilder();
$loader = new YamlFileLoader($container, new FileLocator(__DIR__));
$loader->load($config);
$container->addCompilerPass(new MessageBusPass());
$container->compile();

Using PHP 8 attributes instead of YAML

With the composer-attribute-collector Composer plugin, PHP 8 attributes can be used instead of YAML to define handlers, permissions, and voters.

For handlers and permissions, the Handler and Permission attributes can be used to replace YAML:

namespace Acme\MenuService\Application\MessageBus;

use ICanBoogie\MessageBus\Attribute\Handler;
use ICanBoogie\MessageBus\Attribute\Permission;

#[Permission('is_admin')]
#[Permission('can_write_menu')]
final class CreateMenu
{
    // ...
}

#[Handler]
final class CreateMenuHandler
{
    // ...
}

For voters, the Vote attribute can be used to replace YAML:

<?php

namespace Acme\MenuService\Presentation\Security\Voters;

use ICanBoogie\MessageBus\Attribute\Vote;

#[Vote('can_write_menu')]
final class CanWriteMenu
{
    // ...
}

  Acme\MenuService\Application\MessageBus\CreateMenuHandler:
    tags:
    - { name: message_bus.handler, message: Acme\MenuService\Application\MessageBus\CreateMenu }
    - { name: message_bus.permission, permission: is_admin }
    - { name: message_bus.permission, permission: can_write_menu }

You just need to add the compiler pass MessagePubPassWithAttributes before MessageBusPass:

<?php

// ...
$container->addCompilerPass(new MessageBusPassWithAttributes());
$container->addCompilerPass(new MessageBusPass());
// ...

Providing handlers using a chain of providers

With HandlerProviderWithChain you can chain multiple handler providers together. They will be used in sequence until a handler is found.

<?php

namespace ICanBoogie\MessageBus;

/* @var HandlerProviderWithHandlers $providerWithHandlers */
/* @var PSR\HandlerProviderWithContainer $providerWithContainer */

$provider = new HandlerProviderWithChain([ $providerWithHandlers, $providerWithContainer ]);

/* @var object $message */

$handler = $provider->getHandlerForMessage($message);

Permissions and voters

You probably want to restrict the dispatch of messages to certain conditions. For example, deleting records should only be possible for users having a certain scope in their JWT. For this, you want to make sure of a few things:

1. Define voters and the permission they vote for.

   services:
     Acme\MenuService\Presentation\Security\Voters\CanManageMenu:
       tags:
       - { name: message_bus.voter, permission: can_manage_menu }

2. Tag the permissions together with the handler and message definition.

   services:
     Acme\MenuService\Application\MessageBus\DeleteMenuHandler:
       tags:
       - { name: message_bus.handler, message: Acme\MenuService\Application\MessageBus\DeleteMenu }
       - { name: message_bus.permission, permission: can_manage_menu }

3. Require a RestrictedDispatcher instead of a Dispatcher.

   <?php
   
   // ...
   
   use ICanBoogie\MessageBus\RestrictedDispatcher;
   
   final class MenuController
   {
       public function __construct(
           private RestrictedDispatcher $dispatcher
       ) {}
   }

4. Put in the context whatever is required for the voters to make their decision. In the following example, that would be a token, for the voter to check for scopes.

   <?php
   
   // ...
   
   use ICanBoogie\MessageBus\Context;
   
   final class MenuController
   {
       // ...
   
       public function delete(Request $request): Response
       {
           // ...
   
           $this->dispatch(
               new MenuDelete($id),
               new Context([ $token ])
           );
       }
   }

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Continuous Integration

The project is continuously tested by GitHub actions.

Code of Conduct

This project adheres to a Contributor Code of Conduct. By participating in this project and its community, you are expected to uphold this code.

Contributing

Please see CONTRIBUTING for details.

License

icanboogie/message-bus is released under the BSD-3-Clause.