聊聊ddd-by-examples的DomainEvent

序

本文主要研究一下ddd-by-examples的DomainEvent

DomainEvent

public interface DomainEvent {

    UUID getEventId();

    UUID getAggregateId();

    Instant getWhen();
}

DomainEvent接口定義了getEventId、getAggregateId、getWhen方法

DomainEvents

public interface DomainEvents {

    void publish(DomainEvent event);

    default void publish(List<DomainEvent> events) {
        events.forEach(this::publish);
    }
}

DomainEvents接口定義了publish方法，同時提供一個批量的publish方法

JustForwardDomainEventPublisher

@AllArgsConstructor
public class JustForwardDomainEventPublisher implements DomainEvents {

    private final ApplicationEventPublisher applicationEventPublisher;

    @Override
    public void publish(DomainEvent event) {
        applicationEventPublisher.publishEvent(event);
    }
}

JustForwardDomainEventPublisher實現了DomainEvents接口，其publish方法通過spring的ApplicationEventPublisher來發布

MeteredDomainEventPublisher

@AllArgsConstructor
public class MeteredDomainEventPublisher implements DomainEvents {

    private static final String DOMAIN_EVENTS = "domain_events";
    private static final String TAG_NAME = "name";

    private final DomainEvents delegate;
    private final MeterRegistry metricsRegistry;

    @Override
    public void publish(DomainEvent event) {
        delegate.publish(event);
        metricsRegistry.counter(DOMAIN_EVENTS, TAG_NAME, event.getClass().getSimpleName()).increment();
    }
}

MeteredDomainEventPublisher實現了DomainEvents接口，其publish先執行delegate.publish(event)，然後遞增metrics的counter計數

StoreAndForwardDomainEventPublisher

@AllArgsConstructor
public class StoreAndForwardDomainEventPublisher implements DomainEvents {

    private final DomainEvents eventsPublisher;
    private final EventsStorage eventsStorage;

    @Override
    public void publish(DomainEvent event) {
        eventsStorage.save(event);
    }

    @Scheduled(fixedRate = 3000L)
    @Transactional
    public void publishAllPeriodically() {
        List<DomainEvent> domainEvents = eventsStorage.toPublish();
        domainEvents.forEach(eventsPublisher::publish);
        eventsStorage.published(domainEvents);
    }
}

StoreAndForwardDomainEventPublisher實現了DomainEvents接口，其publish方法先通過eventsStorage.save(event)保存事件，之後有一個定時任務不斷執行eventsStorage.toPublish()獲取events，發佈事件，最後執行eventsStorage.published(domainEvents)

EventsStorage

public interface EventsStorage {

    void save(DomainEvent event);

    List<DomainEvent> toPublish();

    void published(List<DomainEvent> events);
}

EventsStorage接口定義了save、toPublish、published方法

InMemoryEventsStorage

public class InMemoryEventsStorage implements EventsStorage {

    //it's not thread safe, enough for testing
    private final java.util.List<DomainEvent> eventList = Collections.synchronizedList(new ArrayList<>());

    @Override
    synchronized public void save(DomainEvent event) {
        eventList.add(event);
    }

    @Override
    synchronized public List<DomainEvent> toPublish() {
        return List.ofAll(eventList);
    }

    @Override
    synchronized public void published(List<DomainEvent> events) {
        eventList.removeAll(events.asJava());
    }
}

InMemoryEventsStorage實現了EventsStorage接口，其save方法添加event到eventList，其toPublish方法返回當時的eventList，其published方法從eventList移除指定的events

小結

ddd-by-examples定義了DomainEvent、DomainEvents接口，其中JustForwardDomainEventPublisher、MeteredDomainEventPublisher、StoreAndForwardDomainEventPublisher實現了DomainEvents接口；StoreAndForwardDomainEventPublisher藉助了EventsStorage，EventsStorage接口定義了save、toPublish、published方法。

doc

• ddd-by-examples/library