Allow setting consumer replicas though options

example_test.go

@@ -625,6 +625,11 @@ func ExampleSubOpt() {
 	js.Subscribe("foo", func(msg *nats.Msg) {
 		fmt.Printf("Received a message: %s\n", string(msg.Data))
 	}, nats.ManualAck(), nats.MaxDeliver(2), nats.BackOff([]time.Duration{50 * time.Millisecond, 250 * time.Millisecond}))
+
+	// Set consumer replicas count for a durable while subscribing.
+	js.Subscribe("foo", func(msg *nats.Msg) {
+		fmt.Printf("Received a message: %s\n", string(msg.Data))
+	}, nats.Durable("FOO"), nats.ConsumerReplicas(1))
 }
 
 func ExampleMaxWait() {

js.go

@@ -1332,6 +1332,9 @@ func checkConfig(s, u *ConsumerConfig) error {
 	if u.Heartbeat > 0 && u.Heartbeat != s.Heartbeat {
 		return makeErr("heartbeat", u.Heartbeat, s.Heartbeat)
 	}
+	if u.Replicas > 0 && u.Replicas != s.Replicas {
+		return makeErr("replicas", u.Replicas, s.Replicas)
+	}
 	return nil
 }
 
@@ -2449,6 +2452,17 @@ func InactiveThreshold(threshold time.Duration) SubOpt {
 	})
 }
 
+// ConsumerReplicas sets the number of replica count for a consumer.
+func ConsumerReplicas(replicas int) SubOpt {
+	return subOptFn(func(opts *subOpts) error {
+		if replicas < 1 {
+			return fmt.Errorf("invalid ConsumerReplicas value (%v), needs to be greater than 0", replicas)
+		}
+		opts.cfg.Replicas = replicas
+		return nil
+	})
+}
+
 func (sub *Subscription) ConsumerInfo() (*ConsumerInfo, error) {
 	sub.mu.Lock()
 	// TODO(dlc) - Better way to mark especially if we attach.

test/js_test.go

@@ -7272,3 +7272,36 @@ func TestJetStreamDirectGetMsg(t *testing.T) {
 		t.Fatalf("Unexpected error: %v", err)
 	}
 }
+
+func TestJetStreamConsumerReplicasOption(t *testing.T) {
+	withJSCluster(t, "CR", 3, func(t *testing.T, nodes ...*jsServer) {
+		nc, js := jsClient(t, nodes[0].Server)
+		defer nc.Close()
+
+		if _, err := js.AddStream(&nats.StreamConfig{
+				Name:     "ConsumerReplicasTest",
+				Subjects: []string{"foo"},
+				Replicas: 3,
+			}); err != nil {
+				t.Fatalf("Error adding stream: %v", err)
+			}
+
+			// Subscribe to the stream with a durable consumer "bar" and replica set to 1.
+			cb := func(msg *nats.Msg) {}
+			_, err := js.Subscribe("foo", cb, nats.Durable("bar"), nats.ConsumerReplicas(1))
+			if err != nil {
+				t.Fatalf("Error on subscribe: %v", err)
+			}
+
+			// Get consumer info
+			consInfo, err := js.ConsumerInfo("ConsumerReplicasTest", "bar")
+			if err != nil {
+				t.Fatalf("Error getting consumer info: %v", err)
+			}
+
+			// Check if the number of replicas is the same as we provided.
+			if consInfo.Config.Replicas != 1 {
+				t.Fatalf("Expected consumer replica to be %v, got %+v", 1, consInfo.Config.Replicas)
+			}
+	})
+}