Lesson 2: Tactical DDD

DDD PHP

Not having code for Lesson 2?

git checkout lesson-2

Aggregate

An Aggregate is an entity or group of entities that is always kept in a consistent state. Aggregates are very explicitly present in the Command Model, as that is where change is initiated and business behaviour is placed.

Let's create our first Aggregate Product.

namespace App\Domain\Product;

use Ecotone\Modelling\Attribute\Aggregate;
use Ecotone\Modelling\Attribute\Identifier;
use Ecotone\Modelling\Attribute\CommandHandler;
use Ecotone\Modelling\Attribute\QueryHandler;

#[Aggregate]
class Product
{
    #[Identifier]
    private int $productId;

    private int $cost;

    private function __construct(int $productId, int $cost)
    {
        $this->productId = $productId;
        $this->cost = $cost;
    }

    #[CommandHandler]
    public static function register(RegisterProductCommand $command) : self
    {
        return new self($command->getProductId(), $command->getCost());
    }

    #[QueryHandler]
    public function getCost(GetProductPriceQuery $query) : int
    {
        return $this->cost;
    }
}

1. Aggregate attribute marks class to be known as Aggregate

2. Identifier marks properties as identifiers of specific Aggregate instance. Each Aggregate must contains at least one identifier.

3. CommandHandler enables command handling on specific method just as we did in Lesson 1. If method is static, it's treated as a factory method and must return a new aggregate instance. Rule applies as long as we use State-Stored Aggregate instead of Event Sourcing Aggregate.

4. QueryHandler enables query handling on specific method just as we did in Lesson 1.

If you want to known more details about Aggregate start with chapter State-Stored Aggregate

Now remove App\Domain\Product\ProductService as it contains handlers for the same command and query classes. Before we will run our test scenario, we need to register Repository.

Usually you will mark services as Query Handlers not aggregates .However Ecotone does not block possibility to place Query Handler on Aggregate. It's up to you to decide.

Repository

Repositories are used for retrieving and saving the aggregate to persistent storage. We will build an in-memory implementation for now.

namespace App\Domain\Product;

use Ecotone\Modelling\Attribute\Repository;
use Ecotone\Modelling\StandardRepository;

 #[Repository] // 1
class InMemoryProductRepository implements StandardRepository // 2
{
    /**
     * @var Product[]
     */
    private $products = [];

    // 3
    public function canHandle(string $aggregateClassName): bool
    {
        return $aggregateClassName === Product::class;
    }

    // 4
    public function findBy(string $aggregateClassName, array $identifiers): ?object
    {
        if (!array_key_exists($identifiers["productId"], $this->products)) {
            return null;
        }

        return $this->products[$identifiers["productId"]];
    }

    // 5
    public function save(array $identifiers, object $aggregate, array $metadata, ?int $expectedVersion): void
    {
        $this->products[$identifiers["productId"]] = $aggregate;
    }
}

1. Repository attribute marks class to be known to Ecotone as Repository.

2. We need to implement some methods in order to allow Ecotone to retrieve and save Aggregate. Based on implemented interface, Ecotone knowns, if Aggregate is state-stored or event sourced.

3. canHandle tells which classes can be handled by this specific repository.

4. findBy return found aggregate instance or null. As there may be more, than single indentifier per aggregate, identifiers are array.

5. save saves an aggregate instance. You do not need to bother right what is $metadata and $expectedVersion.

If you want to known more details about Repository start with chapter Repository

Laravel

# As default auto wire of Laravel creates new service instance each time 
# service is requested from Depedency Container, we need to register 
# ProductService as singleton.

# Go to bootstrap/QuickStartProvider.php and register our ProductService

namespace Bootstrap;

use App\Domain\Product\InMemoryProductRepository;
use Illuminate\Support\ServiceProvider;

class QuickStartProvider extends ServiceProvider
{
    public function register()
    {
        $this->app->singleton(InMemoryProductRepository::class, function(){
            return new InMemoryProductRepository();
        });
    }
(...)

Symfony

Lite

Let's run our testing command:

bin/console ecotone:quickstart
Running example...
100
Good job, scenario ran with success!

Have you noticed what we are missing here? Our Event Handler was not called, as we do not publish the ProductWasRegistered event anymore.

Event Publishing

In order to automatically publish events recorded within Aggregate, we need to add method annotated with AggregateEvents. This will tell Ecotone where to get the events from. Ecotone comes with default implementation, that can be used as trait WithEvents.

use Ecotone\Modelling\WithEvents;

#[Aggregate]
class Product
{
    use WithEvents;

    #[Identifier]
    private int $productId;

    private int $cost;

    private function __construct(int $productId, int $cost)
    {
        $this->productId = $productId;
        $this->cost = $cost;

        $this->recordThat(new ProductWasRegisteredEvent($productId));
    }
(...)

You may implement your own method for returning events, if you do not want to be coupled with the framework.

Let's run our testing command:

bin/console ecotone:quickstart
Running example...
Product with id 1 was registered!
100
Good job, scenario ran with success!

Congratulations, we have just finished Lesson 2. In this lesson we have learnt how to make use of Aggregates and Repositories. Now we will learn about Converters and Metadata