聊聊eventhorizon的EventBus

序

本文主要研究一下eventhorizon的EventBus

EventBus

eventhorizon/eventbus.go

type EventBus interface {
	EventHandler

	// AddHandler adds a handler for an event. Returns an error if either the
	// matcher or handler is nil, the handler is already added or there was some
	// other problem adding the handler (for networked handlers for example).
	AddHandler(context.Context, EventMatcher, EventHandler) error

	// Errors returns an error channel where async handling errors are sent.
	Errors() <-chan EventBusError

	// Wait wait for all handlers to be cancelled by their context.
	Wait()
}

type EventHandler interface {
	// HandlerType is the type of the handler.
	HandlerType() EventHandlerType

	// HandleEvent handles an event.
	HandleEvent(context.Context, Event) error
}

type EventMatcher interface {
	// Match returns true if the matcher matches an event.
	Match(Event) bool
}

EventBus接口內嵌了EventHandler接口，定義了AddHandler、Errors、Wait方法

EventBus

eventhorizon/eventbus/local/eventbus.go

type EventBus struct {
	group        *Group
	registered   map[eh.EventHandlerType]struct{}
	registeredMu sync.RWMutex
	errCh        chan eh.EventBusError
	wg           sync.WaitGroup
	codec        eh.EventCodec
}

// HandleEvent implements the HandleEvent method of the eventhorizon.EventHandler interface.
func (b *EventBus) HandleEvent(ctx context.Context, event eh.Event) error {
	data, err := b.codec.MarshalEvent(ctx, event)
	if err != nil {
		return fmt.Errorf("could not marshal event: %w", err)
	}

	return b.group.publish(ctx, data)
}

EventBus定義了group、registered、registeredMu、errCh、wg、codec屬性；HandleEvent方法先序列化event，然後通過group.publish發佈event

Group

eventhorizon/eventbus/local/eventbus.go

type Group struct {
	bus   map[string]chan []byte
	busMu sync.RWMutex
}

// NewGroup creates a Group.
func NewGroup() *Group {
	return &Group{
		bus: map[string]chan []byte{},
	}
}

func (g *Group) publish(ctx context.Context, b []byte) error {
	g.busMu.RLock()
	defer g.busMu.RUnlock()

	for _, ch := range g.bus {
		// Marshal and unmarshal the context to both simulate only sending data
		// that would be sent over a network bus and also break any relationship
		// with the old context.
		select {
		case ch <- b:
		default:
			log.Printf("eventhorizon: publish queue full in local event bus")
		}
	}

	return nil
}

// Handles all events coming in on the channel.
func (b *EventBus) handle(ctx context.Context, m eh.EventMatcher, h eh.EventHandler, ch <-chan []byte) {
	defer b.wg.Done()

	for {
		select {
		case data := <-ch:
			// Artificial delay to simulate network.
			time.Sleep(10 * time.Millisecond)

			event, ctx, err := b.codec.UnmarshalEvent(ctx, data)
			if err != nil {
				err = fmt.Errorf("could not unmarshal event: %w", err)
				select {
				case b.errCh <- eh.EventBusError{Err: err, Ctx: ctx}:
				default:
					log.Printf("eventhorizon: missed error in local event bus: %s", err)
				}
				return
			}

			// Ignore non-matching events.
			if !m.Match(event) {
				continue
			}

			// Handle the event if it did match.
			if err := h.HandleEvent(ctx, event); err != nil {
				err = fmt.Errorf("could not handle event (%s): %s", h.HandlerType(), err.Error())
				select {
				case b.errCh <- eh.EventBusError{Err: err, Ctx: ctx, Event: event}:
				default:
					log.Printf("eventhorizon: missed error in local event bus: %s", err)
				}
			}
		case <-ctx.Done():
			return
		}
	}
}

Group的publish方法遍歷bus的channel，通過select寫入event；handle方法循環select讀取event，然後通過m.Match(event)判斷是符合，是的話執行h.HandleEvent

小結

eventhorizon的EventBus接口內嵌了EventHandler接口，定義了AddHandler、Errors、Wait方法。

doc

• eventhorizon