feat: backend service

backend/push/subscription.go

@@ -0,0 +1,43 @@
+package push
+
+import (
+	pushv1 "galaxy/backend/proto/push/v1"
+)
+
+// subscription is the per-gateway-instance delivery queue. Each
+// subscription owns a buffered channel; the publisher writes into it
+// without blocking by dropping the oldest queued event when the buffer
+// is full. The done channel is closed by the Service when the
+// subscription is replaced (a new connection arrived for the same
+// gateway_client_id) or when the Service is shutting down.
+type subscription struct {
+	clientID string
+	ch       chan *pushv1.PushEvent
+	done     chan struct{}
+	dropped  uint64
+}
+
+// deliver enqueues ev into the subscription's buffer. When the buffer
+// is full, the oldest queued event is dropped to make room and the
+// dropped counter increments. The bool reports whether a drop occurred,
+// so the publisher can update its drop metric.
+//
+// The Service holds its mutex while calling deliver, which means at
+// most one publisher writes to ch at a time. The reader goroutine runs
+// independently and only consumes from ch, so the second send below is
+// guaranteed not to block: after evicting the head, the channel has at
+// least one free slot which no other publisher can fill.
+func (s *subscription) deliver(ev *pushv1.PushEvent) bool {
+	select {
+	case s.ch <- ev:
+		return false
+	default:
+	}
+	select {
+	case <-s.ch:
+	default:
+	}
+	s.ch <- ev
+	s.dropped++
+	return true
+}