ABP Domain Events

前言

以下介紹一下 ABP 提供的 Publish Event 的方式(local event 及 distributed event)，

local event bus

在 Aggreate Root Entity 使用 AddLocalEvent(event物件);來加入要被 Publish 的 Event ，
透過 ILocalEventHandler<event物件> 來訂閱要 Handle 的 Event，例如，

public void SetTime(DateTime startTime, DateTime endTime)
{
	if (startTime > endTime)
	{
		throw new BusinessException(EventHubErrorCodes
		.EndTimeCantBeEarlierThanStartTime);
	}
	StartTime = startTime;
	EndTime = endTime;
	if (!IsDraft)
	{
		AddLocalEvent(new EventTimeChangedEventData(this));
	}
}

public class EventTimeChangedEventData
{
	public Event Event {get;}
	public EventTimeChangedEventData(Event @event)
	{
		Event = @event;
	}

}

public class UserEmailingHandler :
	ILocalEventHandler<EventTimeChangedEventData>,
	ITransientDependency
{
	public async Task HandleEventAsync(
		EventTimeChangedEventData eventData)
	{
		var @event = eventData.Event;
		// TODO: Send email to the registered users!
	}
}

• Handler 如果發生錯誤沒有 Handle 的話，會讓 DB 交易 Rollback

在 AppService 也可以使用 eventbus，注入 ILocalEventBus，再透過 PublishAsync

public class EventAppService : EventHubAppService,
	IEventAppService
{
	private readonly IRepository<Event, Guid>
		_eventRepository;
	private readonly ILocalEventBus _localEventBus;

	public EventAppService(
		IRepository<Event, Guid> eventRepository,
		ILocalEventBus localEventBus)
	{
		_eventRepository = eventRepository;
		_localEventBus = localEventBus;
	}
	public async Task SetTimeAsync(
		Guid eventId, DateTime startTime, DateTime endTime)
	{
		var @event = await _eventRepository.GetAsync(eventId);
		@event.SetTime(startTime, endTime);
		await _eventRepository.UpdateAsync(@event);
		await _localEventBus.PublishAsync(
			new EventTimeChangedEventData(@event));

	}
}

最好是在 Domain Services or Entity 中 Publish 會來的比較合適
因為在 AppService 可能會忘了 Publish

Local events 很適合用來實作一些 side-effects，
例如，某些狀態改變時，要做某些 Action
像上述範例是使用 Email 通知

• 不應該將 商業邏輯流程 交給 事件處理，以防整個過程難以掌控
• Domain Event(包含簡單資訊，Id, CustomId, ClientId)
• Intergation Event (包含詳細資訊，Id, CustomId, CustomName, ClientId, ClientName…)

distributed event bus

透過 message broker service, 像是 RabbitMQ or Kafka
在 Aggreate Root Entity 使用 AddDistributedEvent(ETO物件);來加入要被 Publish 的 Event ，

public void SetTime(DateTime startTime, DateTime endTime)
{
	if (startTime > endTime)
	{
		throw new BusinessException(EventHubErrorCodes
		.EndTimeCantBeEarlierThanStartTime);
	}
	StartTime = startTime;
	EndTime = endTime;
	if (!IsDraft)
	{
		AddDistributedEvent(new EventTimeChangedEto{
			EventId = Id, Title = Title,
			StartTime = StartTime, EndTime = EndTime
		});
	}
}

• 跟 local event bus 不同的是，這裡傳遞的是類似 DTO 的 ETO(Event Transfer Object)

其他地方使用則注入 IDistributeEventBus ，然後使用 PublishAsync

await _distributedEventBus.PublishAsync(ew EventTimeChangedEto{
		EventId = Id, Title = Title,
		StartTime = StartTime, EndTime = EndTime
	});

public class UserEmailingHandler :
	IDistributedEventHandler<EventTimeChangedEventEto>,
	ITransientDependency
{
	public async Task HandleEventAsync(
		EventTimeChangedEventData eventData)
	{
		var @event = eventData.Event;
		// TODO: Send email to the registered users!
	}
}

An aggregate should preserve its validity and consistency by implementing business rules.
The domain layer is used to implement the core, application-independent domain logic of the solution.

參考資源

ABP Event Bus
Should you publish Domain Events or Integration Events?