feat: expose websocket delivery pressure metrics

server/session.go

@@ -141,6 +141,10 @@ type Session struct {
 	// Outbound mesages, buffered.
 	// The content must be serialized in format suitable for the session.
 	send chan any
+	// WebSocket queue timestamps are kept separately to preserve the send channel's payload contract.
+	wsQueueMu    sync.Mutex
+	wsQueueTimes []time.Time
+	wsQueueHead  int
 
 	// Channel for shutting down the session, buffer 1.
 	// Content in the same format as for 'send'
@@ -182,6 +186,52 @@ type Subscription struct {
 	supd chan<- *sessionUpdate
 }
 
+func (s *Session) enqueueOutbound(payload any) bool {
+	if s.proto != WEBSOCK {
+		select {
+		case s.send <- payload:
+			return true
+		default:
+			return false
+		}
+	}
+
+	s.wsQueueMu.Lock()
+	defer s.wsQueueMu.Unlock()
+	select {
+	case s.send <- payload:
+		s.wsQueueTimes = append(s.wsQueueTimes, time.Now())
+		immetrics.ObserveWSSendQueueEnqueue(len(s.send), cap(s.send), true)
+		return true
+	default:
+		immetrics.ObserveWSSendQueueEnqueue(len(s.send), cap(s.send), false)
+		return false
+	}
+}
+
+func (s *Session) observeOutboundDequeue(purged bool) {
+	if s.proto != WEBSOCK {
+		return
+	}
+	s.wsQueueMu.Lock()
+	if s.wsQueueHead >= len(s.wsQueueTimes) {
+		s.wsQueueMu.Unlock()
+		return
+	}
+	queuedAt := s.wsQueueTimes[s.wsQueueHead]
+	s.wsQueueHead++
+	if s.wsQueueHead == len(s.wsQueueTimes) {
+		s.wsQueueTimes = s.wsQueueTimes[:0]
+		s.wsQueueHead = 0
+	} else if s.wsQueueHead >= 128 {
+		copy(s.wsQueueTimes, s.wsQueueTimes[s.wsQueueHead:])
+		s.wsQueueTimes = s.wsQueueTimes[:len(s.wsQueueTimes)-s.wsQueueHead]
+		s.wsQueueHead = 0
+	}
+	s.wsQueueMu.Unlock()
+	immetrics.ObserveWSSendQueueDequeue(queuedAt, purged)
+}
+
 func (s *Session) addSub(topic string, sub *Subscription) {
 	if s.multi != nil {
 		s.multi.addSub(topic, sub)
@@ -333,9 +383,7 @@ func (s *Session) queueOutBatch(msgs []*ServerComMessage) bool {
 	}
 
 	if s.supportsMessageBatching() {
-		select {
-		case s.send <- msgs:
-		default:
+		if !s.enqueueOutbound(msgs) {
 			// Never block here since it may also block the topic's run() goroutine.
 			logs.Err.Println("s.queueOut: session's send queue2 full", s.sid)
 			return false
@@ -394,9 +442,7 @@ func (s *Session) queueOut(msg *ServerComMessage) bool {
 		}
 	}
 
-	select {
-	case s.send <- msg:
-	default:
+	if !s.enqueueOutbound(msg) {
 		// Never block here since it may also block the topic's run() goroutine.
 		logs.Err.Println("s.queueOut: session's send queue full", s.sid)
 		return false
@@ -414,9 +460,7 @@ func (s *Session) queueOutBytes(data []byte) bool {
 		return true
 	}
 
-	select {
-	case s.send <- data:
-	default:
+	if !s.enqueueOutbound(data) {
 		logs.Err.Println("s.queueOutBytes: session's send queue full", s.sid)
 		return false
 	}
@@ -451,6 +495,7 @@ func (s *Session) stopSession(data any) {
 func (s *Session) purgeChannels() {
 	for len(s.send) > 0 {
 		<-s.send
+		s.observeOutboundDequeue(true)
 	}
 	for len(s.stop) > 0 {
 		<-s.stop