domain-driven-design

Domain-Driven Design (Symfony)

Building blocks

Tactical pattern	In Symfony/PHP
Aggregate root	Entity class that guards invariants and is the only entry point to its cluster
Entity	Has identity (OrderId), mutable over its lifecycle
Value object	final readonly class, no identity, compared by value (Money, Email)
Domain event	final readonly class recorded on the aggregate, dispatched after persist
Repository	Interface in the domain, Doctrine adapter in infrastructure (see hexagonal-architecture)
Domain service	Stateless logic that doesn't belong to a single aggregate

Value objects

Immutable, self-validating, compared by value. Replace primitive obsession.

// ✅ GOOD — value object enforces its own invariants
final readonly class Email
{
    public function __construct(public string $value)
    {
        if (!filter_var($value, FILTER_VALIDATE_EMAIL)) {
            throw new \InvalidArgumentException("Invalid email: {$value}");
        }
    }

    public function equals(self $other): bool
    {
        return $this->value === $other->value;
    }
}

final readonly class Money
{
    public function __construct(public int $amountInCents, public Currency $currency) {}

    public function add(self $other): self
    {
        if ($this->currency !== $other->currency) {
            throw new \DomainException('Currency mismatch');
        }
        return new self($this->amountInCents + $other->amountInCents, $this->currency);
    }
}

// ❌ BAD — primitives everywhere, no invariants, validation scattered across services
function createOrder(string $email, int $totalCents): void { /* ... */ }

Map value objects with Doctrine embeddables (#[ORM\Embedded]) or a custom DBAL type, so the table stays flat while the model stays rich.

Aggregates protect invariants

• The aggregate root is the only object outside code may hold a reference to.
• Mutations go through behavior methods on the root, never setters.
• Invariants are checked inside those methods — an aggregate can never be in an invalid state.
• Keep aggregates small; reference other aggregates by ID, not object reference.

// ✅ GOOD — behavior methods guard invariants; children reached only through the root
#[ORM\Entity]
class Order
{
    #[ORM\Id, ORM\Column(type: 'uuid')]
    private Uuid $id;

    #[ORM\Column(enumType: OrderStatus::class)]
    private OrderStatus $status;

    /** @var Collection<int, OrderLine> */
    #[ORM\OneToMany(targetEntity: OrderLine::class, mappedBy: 'order', cascade: ['persist'], orphanRemoval: true)]
    private Collection $lines;

    /** @var DomainEvent[] */
    private array $recordedEvents = [];

    private function __construct(Uuid $id)
    {
        $this->id = $id;
        $this->status = OrderStatus::Pending;
        $this->lines = new ArrayCollection();
    }

    public static function place(): self
    {
        $order = new self(Uuid::v7());
        $order->record(new OrderPlaced($order->id));
        return $order;
    }

    public function addLine(Uuid $productId, int $quantity): void
    {
        if ($this->status !== OrderStatus::Pending) {
            throw new \DomainException('Cannot modify a confirmed order'); // invariant
        }
        $this->lines->add(new OrderLine($this, $productId, $quantity));
    }

    public function confirm(): void
    {
        if ($this->lines->isEmpty()) {
            throw new \DomainException('Cannot confirm an empty order'); // invariant
        }
        $this->status = OrderStatus::Confirmed;
        $this->record(new OrderConfirmed($this->id));
    }

    /** @return DomainEvent[] */
    public function releaseEvents(): array
    {
        $events = $this->recordedEvents;
        $this->recordedEvents = [];
        return $events;
    }

    private function record(DomainEvent $event): void
    {
        $this->recordedEvents[] = $event;
    }
}

// ❌ BAD — anemic entity: public setters let any caller break invariants
class Order
{
    public OrderStatus $status;
    public Collection $lines;
    public function setStatus(OrderStatus $s): void { $this->status = $s; }
}

Domain events

Record events inside the aggregate; dispatch them after the transaction commits so subscribers never see uncommitted state. A Doctrine postFlush listener or Messenger does the dispatch.

// ✅ Dispatch recorded events after flush via a Doctrine event subscriber
#[AsDoctrineListener(event: Events::postFlush)]
final class DispatchDomainEventsListener
{
    public function __construct(private readonly MessageBusInterface $eventBus) {}

    public function postFlush(PostFlushEventArgs $args): void
    {
        $uow = $args->getObjectManager()->getUnitOfWork();
        foreach ($uow->getIdentityMap() as $entities) {
            foreach ($entities as $entity) {
                if ($entity instanceof RecordsEvents) {
                    foreach ($entity->releaseEvents() as $event) {
                        $this->eventBus->dispatch($event);
                    }
                }
            }
        }
    }
}

Ubiquitous language

• Name classes and methods with the business vocabulary: Order::confirm(), not Order::setStatusToConfirmed().
• The same term means the same thing in code, tests, and conversation.
• Bounded contexts get their own namespace (App\Sales\…, App\Billing\…); the same word may differ between them.

Gotchas

• Agent generates anemic entities with public setters — model behavior methods that enforce invariants.
• Agent uses primitive string/int for Email, Money, OrderId — introduce value objects.
• Agent references other aggregates by object (#[ORM\ManyToOne] private Customer $customer) — reference by ID across aggregate boundaries.
• Agent dispatches domain events before flush() — record on the aggregate, dispatch after commit.
• Agent puts cross-aggregate logic on an entity — use a stateless domain service.
• Agent makes huge aggregates loading dozens of children — keep them small; consider a separate read model.
• Agent names methods after database operations (save, update) instead of intent (confirm, cancel).