ドメインイベントとドメインサービス

ビジネス上の事実を記録する

ドメインイベントとは

<?php
final class OrderPlaced {
    public function __construct(
        public readonly string $orderId,
        public readonly int $totalCents,
        public readonly DateTimeImmutable $occurredOn = new DateTimeImmutable()
    ) {}
}
$e = new OrderPlaced('o1', 1300);
echo $e->orderId, ' @ ', $e->occurredOn->format('H:i'), PHP_EOL;

集約でイベントを記録する

<?php
final class OrderPlaced {
    public function __construct(public readonly string $orderId) {}
}
abstract class AggregateRoot {
    private array $events = [];
    protected function record(object $e): void { $this->events[] = $e; }
    public function releaseEvents(): array {
        $out = $this->events; $this->events = []; return $out;
    }
}
final class Order extends AggregateRoot {
    public function __construct(public readonly string $id) {
        $this->record(new OrderPlaced($id));
    }
}
$o = new Order('o1');
foreach ($o->releaseEvents() as $e) { echo get_class($e), PHP_EOL; }

コミット後にディスパッチする

<?php
final class PlaceOrderService {
    public function __construct(
        private object $repo,
        private object $dispatcher
    ) {}
    public function handle(object $order): void {
        // 1. persist inside a transaction
        $this->repo->save($order);   // commit happens here
        // 2. only AFTER commit, release and dispatch
        foreach ($order->releaseEvents() as $event) {
            $this->dispatcher->dispatch($event);
        }
    }
}
echo "order persisted, then events dispatched", PHP_EOL;

ハンドラーが反応する

<?php
final class OrderPlaced { public function __construct(public readonly string $orderId) {} }

final class Dispatcher {
    private array $listeners = [];
    public function on(string $event, callable $l): void { $this->listeners[$event][] = $l; }
    public function dispatch(object $event): void {
        foreach ($this->listeners[$event::class] ?? [] as $l) { $l($event); }
    }
}
$d = new Dispatcher();
$d->on(OrderPlaced::class, fn($e) => print("email for {$e->orderId}\n"));
$d->on(OrderPlaced::class, fn($e) => print("analytics for {$e->orderId}\n"));
$d->dispatch(new OrderPlaced('o1'));

境界をまたぐイベント

ドメインサービスとは

コードで見るドメインサービス

<?php
final class Account {
    public function __construct(public readonly string $id, private int $cents) {}
    public function withdraw(int $c): void {
        if ($c > $this->cents) { throw new DomainException('insufficient funds'); }
        $this->cents -= $c;
    }
    public function deposit(int $c): void { $this->cents += $c; }
    public function balance(): int { return $this->cents; }
}
final class MoneyTransferService {
    public function transfer(Account $from, Account $to, int $cents): void {
        $from->withdraw($cents);
        $to->deposit($cents);
    }
}
$a = new Account('a', 1000); $b = new Account('b', 0);
(new MoneyTransferService())->transfer($a, $b, 400);
echo $a->balance(), '/', $b->balance(), PHP_EOL; // 600/400

ドメインサービスとアプリケーションサービス

サービスの乱用を避ける

アプリケーションサービスによる調整

<?php
final class Account {
    public function __construct(public readonly string $id, public int $cents) {}
}
final class MoneyTransferService { // domain service: the rule
    public function transfer(Account $from, Account $to, int $c): void {
        if ($c > $from->cents) { throw new DomainException('insufficient'); }
        $from->cents -= $c; $to->cents += $c;
    }
}
final class TransferApp { // application service: orchestration only
    public function __construct(private MoneyTransferService $domain) {}
    public function run(Account $a, Account $b, int $c): void {
        $this->domain->transfer($a, $b, $c); // then repo->save + dispatch
    }
}
$a = new Account('a', 1000); $b = new Account('b', 0);
(new TransferApp(new MoneyTransferService()))->run($a, $b, 250);
echo $a->cents, '/', $b->cents, PHP_EOL; // 750/250

クイックチェック

まとめ