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本文主要研究一下ddd-by-examples的DomainEvent
DomainEvent
public interface DomainEvent {
UUID getEventId();
UUID getAggregateId();
Instant getWhen();
}
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DomainEvent接口定义了getEventId、getAggregateId、getWhen方法
DomainEvents
public interface DomainEvents {
void publish(DomainEvent event);
default void publish(List<DomainEvent> events) {
events.forEach(this::publish);
}
}
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DomainEvents接口定义了publish方法,同时提供一个批量的publish方法
JustForwardDomainEventPublisher
@AllArgsConstructor
public class JustForwardDomainEventPublisher implements DomainEvents {
private final ApplicationEventPublisher applicationEventPublisher;
@Override
public void publish(DomainEvent event) {
applicationEventPublisher.publishEvent(event);
}
}
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JustForwardDomainEventPublisher实现了DomainEvents接口,其publish方法通过spring的ApplicationEventPublisher来发布
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();
}
}
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MeteredDomainEventPublisher实现了DomainEvents接口,其publish先执行delegate.publish(event),然后递增metrics的counter计数
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);
}
}
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StoreAndForwardDomainEventPublisher实现了DomainEvents接口,其publish方法先通过eventsStorage.save(event)保存事件,之后有一个定时任务不断执行eventsStorage.toPublish()获取events,发布事件,最后执行eventsStorage.published(domainEvents)
EventsStorage
public interface EventsStorage {
void save(DomainEvent event);
List<DomainEvent> toPublish();
void published(List<DomainEvent> events);
}
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EventsStorage接口定义了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());
}
}
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InMemoryEventsStorage实现了EventsStorage接口,其save方法添加event到eventList,其toPublish方法返回当时的eventList,其published方法从eventList移除指定的events
小结
ddd-by-examples定义了DomainEvent、DomainEvents接口,其中JustForwardDomainEventPublisher、MeteredDomainEventPublisher、StoreAndForwardDomainEventPublisher实现了DomainEvents接口;StoreAndForwardDomainEventPublisher借助了EventsStorage,EventsStorage接口定义了save、toPublish、published方法。