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Small convention based CQRS library for PHP

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README.md

LiteCQRS for PHP

Small naming-convention based CQRS library for PHP (loosely based on LiteCQRS for C#) that relies on the MessageBus, Command, EventSourcing and Domain Event patterns.

Build Status (Master)

NOTE Use the 1.1 branch, as the dev-master is currently in heavy refactoring.

Terminology

CQS is Command-Query-Separation: A paradigm where read methods never change state and write methods never return data. Build on top, CQRS suggests the separation of read- from write-model and uses the DomainEvent pattern to notify the read model about changes in the write model.

LiteCQRS uses the command pattern and a central message bus service that finds the corresponding handler to execute a command. A command is just a class with some properties describing it, it can optionally implement LiteCQRS\Command.

During the execution of a command, domain events can be triggered. These are again just simple classes with some properties and they can optionally implement LiteCQRS\DomainEvent.

An event queue knows what domain events have been triggered during a command and then publishes them to an event message bus, where many listeners can listen to them.

Changes

From 1.0 to 1.1

  • Extending LiteCQRS\Command and LiteCQRS\DomainEvent is NOT required anymore. In fact you can use any class as command or event. The naming conventions alone make sure command handlers and event listeners are detected.

  • JMS Serializer Plugin cannot "detach" aggregate root properties that are part of an event that is serialized anymore. Putting related aggregate roots into an Event is therefore not supported anymore (and not a good idea even with JMS Serializer 0.9 anyways).

Conventions

  • All public methods of a command handler class are mapped to Commands "Command Class Shortname" => "MethodName" when the method and command class shortname match. Implementing an interface for the commands is NOT required (since 1.1)
  • Domain Events are applied to Event Handlers "Event Class Shortname" => "onEventClassShortname". Only if this matches is an event listener registered.
  • Domain Events are applied on Entities/Aggregate Roots "Event Class Shortname" => "applyEventClassShortname"
  • You can optionally extend the DefaultDomainEvent which has a constructor that maps its array input to properties and throws an exception if an unknown property is passed.
  • There is also a DefaultCommand with the same semantics as DefaultDomainEvent. Extending this is not required.

Examples:

  • HelloWorld\GreetingCommand maps to the greeting(GreetingCommand $command) method on the registered handler.
  • HelloWorld\Commands\Greeting maps to the greeting(Greeting $command) method on the registered handler.
  • HelloWorld\GreetedEvent is passed to all event handlers that have a method onGreeted(GreetedEvent $event).
  • HelloWorld\Events\Greeted is passed to all event handlers that have a method onGreeted(Greeted $event).
  • HelloWorld\GreetedEvent is delegated to applyGreeted($event) when created on the aggregate root

Installation & Requirements

Use the 1.1 branch, as the dev-master is currently in heavy refactoring.

The core library has no dependencies on other libraries. Plugins have dependencies on their specific libraries.

Install with Composer:

{
    "require": {
        "beberlei/lite-cqrs": "1.1"
    }
}

Workflow

These are the steps that a command regularly takes through the LiteCQRS stack during execution:

  1. You push commands into a CommandBus. Commands are simple objects extending Command created by you.
  2. The CommandBus checks for a handler that can execute your command. Every command has exactly one handler.
  3. The command handler changes state of the domain model. It does that by creating events (that represent state change) and passing them to the AggregateRoot::apply() or DomainEventProvider::raise() method of your domain objects.
  4. When the command is completed, the command bus will check all objects in the identity map for events.
  5. All found events will be passed to the EventMessageBus#publish() method.
  6. The EventMessageBus dispatches all events to observing event handlers.
  7. Event Handlers can create new commands again using the CommandBus.

Command and Event handler execution can be wrapped in handlers that manage transactions. Event handling is always triggered outside of any command transaction. If the command fails with any exception all events created by the command are forgotten/ignored. No event handlers will be triggered in this case.

In the case of InMemory CommandBus and EventMessageBus LiteCQRS makes sure that the execution of command and event handlers is never nested, but in sequential linearized order. This prevents independent transactions for each command from affecting each other.

Examples

See examples/ for some examples:

  1. example1.php shows usage of the Command- and EventMessageBus with one domain object
  2. example2_event.php shows direct usage of the EventMessageBus inside a command
  3. example3_sequential_commands.php demonstrates how commands are processed sequentially.
  4. tictactoe.php implements a tic tac toe game with CQRS.
  5. SymfonyExample.md shows example1.php implemented within the scope of a Symfony2 project.

Setup

  1. In Memory Command Handlers, no event publishing/observing
<?php
$userService = new UserService();

$commandBus = new DirectCommandBus()
$commandBus->register('MyApp\ChangeEmailCommand', $userService);
  1. In Memory Commands and Events Handlers

This uses LiteCQRS\EventProviderInterface instances to trigger domain events.

<?php
// 1. Setup the Library with InMemory Handlers
$messageBus = new InMemoryEventMessageBus();
$identityMap = new SimpleIdentityMap();
$queue = new EventProviderQueue($identityMap);
$commandBus = new DirectCommandBus(array(
    new EventMessageHandlerFactory($messageBus, $queue)
));

// 2. Register a command service and an event handler
$userService = new UserService($identityMap);
$commandBus->register('MyApp\ChangeEmailCommand', $userService);

$someEventHandler = new MyEventHandler();
$messageBus->register($someEventHandler);
  1. In Memory Commands + Custom Event Queue

LiteCQRS knows about triggered events by asking LiteCQRS\Bus\EventQueue. Provide your own implementation to be independent of your domain objects having to implement EventProviderInterface.

<?php
$messageBus = new InMemoryEventMessageBus();
$queue = new MyCustomEventQueue();

$commandBus = new DirectCommandBus(array(
    new EventMessageHandlerFactory($messageBus, $queue)
));

Usage

To implement a Use Case of your application

  1. Create a command object that receives all the necessary input values. Use public properties and extend LiteCQRS\DefaultCommand to simplify.
  2. Add a new method with the name of the command to any of your services (command handler)
  3. Register the command handler to handle the given command on the CommandBus.
  4. Have your entities implement LiteCQRS\AggregateRoot or LiteCQRS\DomainEventProvider
  5. Use protected method raise(DomainEvent $event) or apply(DomainEvent $event)`` to attach events to your aggregate root objects.

That is all there is for simple use-cases.

If your command triggers events that listeners check for, you should:

  1. Create a domain specific event class. Use public properties to simplify.
  2. Create a event handler(s) or add method(s) to existing event handler(s).

While it seems "complicated" to create commands and events for every use-case. These objects are really dumb and only contain public properties. Using your IDE or editor functionality you can easily generate them in no time. In turn, they will make your code very explicit.

Difference between apply() and raise()

There are two ways to publish events to the outside world.

  • DomainEventProvider#raise(DomainEvent $event) is the simple one, it emits an event and does nothing more.
  • AggregateRoot#apply(DomainEvent $event) requires you to add a method apply$eventName($event) that can be used to replay events on objects. This is used to replay an object from events.

If you don't use event sourcing then you are fine just using raise() and ignoring apply() altogether.

Failing Events

The EventMessageBus prevents exceptions from bubbling up. To allow some debugging of failed event handler execution there is a special event "EventExecutionFailed" that you can listen to. You will get passed an instance of LiteCQRS\Bus\EventExecutionFailed with properties $exception, $service and $event to allow analysing failures in your application.

Extension Points

You should implement your own CommandBus or extend the existing to wire the whole process together exactly as you need it to work.

Plugins

Symfony

Inside symfony you can use LiteCQRS by registering services with lite_cqrs.command_handler or the lite_cqrs.event_handler tag. These services are then autodiscovered for commands and events.

Command- and Event-Handlers are lazily loaded from the Symfony Dependency Injection Container.

To enable the bundle put the following in your Kernel:

new \LiteCQRS\Plugin\SymfonyBundle\LiteCQRSBundle(),

You can enable/disable the bundle by adding the following to your config.yml:

lite_cqrs: ~

Please refer to the SymfonyExample.md document for a full demonstration of using LiteCQRS from within a Symfony2 project.

Monolog

A plugin that logs the execution of every command and handler using Monolog. It includes the type and name of the message, its parameters as json and if its execution succeeded or failed.

The Monolog integration into Symfony registers a specific channel lite_cqrs which you can configure differently from the default channels in Symfony. See the Symfony cookbook for more information.

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