aether/eventbus.go

package aether

import (
	"context"
	"sync"
)

// EventBroadcaster defines the interface for publishing and subscribing to events
type EventBroadcaster interface {
	Subscribe(namespaceID string) <-chan *Event
	Unsubscribe(namespaceID string, ch <-chan *Event)
	Publish(namespaceID string, event *Event)
	Stop()
	SubscriberCount(namespaceID string) int
}

// MetricsProvider is an optional interface that EventBroadcaster implementations
// can implement to expose metrics.
type MetricsProvider interface {
	// Metrics returns the metrics collector for this broadcaster.
	Metrics() BroadcasterMetrics
}

// EventBus broadcasts events to multiple subscribers within a namespace
type EventBus struct {
	subscribers map[string][]chan *Event // namespaceID -> channels
	mutex       sync.RWMutex
	ctx         context.Context
	cancel      context.CancelFunc
	metrics     *DefaultMetricsCollector
}

// NewEventBus creates a new event bus
func NewEventBus() *EventBus {
	ctx, cancel := context.WithCancel(context.Background())
	return &EventBus{
		subscribers: make(map[string][]chan *Event),
		ctx:         ctx,
		cancel:      cancel,
		metrics:     NewMetricsCollector(),
	}
}

// Metrics returns the metrics collector for this event bus.
func (eb *EventBus) Metrics() BroadcasterMetrics {
	return eb.metrics
}

// Subscribe creates a new subscription channel for a namespace
func (eb *EventBus) Subscribe(namespaceID string) <-chan *Event {
	eb.mutex.Lock()
	defer eb.mutex.Unlock()

	// Create buffered channel to prevent blocking publishers
	ch := make(chan *Event, 100)
	eb.subscribers[namespaceID] = append(eb.subscribers[namespaceID], ch)

	// Record subscription metric
	eb.metrics.RecordSubscribe(namespaceID)

	return ch
}

// Unsubscribe removes a subscription channel
func (eb *EventBus) Unsubscribe(namespaceID string, ch <-chan *Event) {
	eb.mutex.Lock()
	defer eb.mutex.Unlock()

	subs := eb.subscribers[namespaceID]
	for i, subscriber := range subs {
		if subscriber == ch {
			// Remove channel from slice
			eb.subscribers[namespaceID] = append(subs[:i], subs[i+1:]...)
			close(subscriber)

			// Record unsubscription metric
			eb.metrics.RecordUnsubscribe(namespaceID)
			break
		}
	}

	// Clean up empty namespace entries
	if len(eb.subscribers[namespaceID]) == 0 {
		delete(eb.subscribers, namespaceID)
	}
}

// Publish sends an event to all subscribers of a namespace
func (eb *EventBus) Publish(namespaceID string, event *Event) {
	eb.mutex.RLock()
	defer eb.mutex.RUnlock()

	// Record publish metric
	eb.metrics.RecordPublish(namespaceID)

	subscribers := eb.subscribers[namespaceID]
	for _, ch := range subscribers {
		select {
		case ch <- event:
			// Event delivered - record receive metric
			eb.metrics.RecordReceive(namespaceID)
		default:
			// Channel full, skip this subscriber (non-blocking)
			eb.metrics.RecordDroppedEvent(namespaceID)
		}
	}
}

// Stop closes the event bus
func (eb *EventBus) Stop() {
	eb.mutex.Lock()
	defer eb.mutex.Unlock()

	eb.cancel()

	// Close all subscriber channels and update metrics
	for namespaceID, subs := range eb.subscribers {
		for range subs {
			eb.metrics.RecordUnsubscribe(namespaceID)
		}
		for _, ch := range subs {
			close(ch)
		}
	}

	eb.subscribers = make(map[string][]chan *Event)
}

// SubscriberCount returns the number of subscribers for a namespace
func (eb *EventBus) SubscriberCount(namespaceID string) int {
	eb.mutex.RLock()
	defer eb.mutex.RUnlock()
	return len(eb.subscribers[namespaceID])
}