Change how number of pending messages is calculated and add more error handling.

pkg/scalers/stan_scaler.go

@@ -108,47 +108,75 @@ func parseStanMetadata(metadata map[string]string) (stanMetadata, error) {
 
 // IsActive determines if we need to scale from zero
 func (s *stanScaler) IsActive(ctx context.Context) (bool, error) {
-	resp, err := http.Get(s.getMonitoringEndpoint())
+	monitoringEndpoint := s.getMonitoringEndpoint()
+
+	resp, err := http.Get(monitoringEndpoint)
 	if err != nil {
 		stanLog.Error(err, "Unable to access the nats streaming broker monitoring endpoint", "natsServerMonitoringEndpoint", s.metadata.natsServerMonitoringEndpoint)
 		return false, err
 	}
+
+	if resp.StatusCode == 404 {
+		baseResp, _ := http.Get(s.getSTANChannelsEndpoint())
+
+		if baseResp.StatusCode == 404 {
+			stanLog.Info("Streaming broker endpoint returned 404. Please ensure it has been created", "url", monitoringEndpoint, "channelName", s.metadata.subject)
+
+		} else {
+			stanLog.Info("Unable to connect to STAN. Please ensure you have configured the ScaledObject with the correct endpoint.", "baseResp.StatusCode", baseResp.StatusCode, "natsServerMonitoringEndpoint", s.metadata.natsServerMonitoringEndpoint)
+		}
+
+		return false, err
+	}
+
 	defer resp.Body.Close()
 	json.NewDecoder(resp.Body).Decode(&s.channelInfo)
 
 	return s.hasPendingMessage() || s.getMaxMsgLag() > 0, nil
 }
 
-func (s *stanScaler) getMonitoringEndpoint() string {
-	return "http://" + s.metadata.natsServerMonitoringEndpoint + "/streaming/channelsz?" + "channel=" + s.metadata.subject + "&subs=1"
+func (s *stanScaler) getSTANChannelsEndpoint() string {
+	return "http://" + s.metadata.natsServerMonitoringEndpoint + "/streaming/channelsz"
 }
 
-func (s *stanScaler) getTotalMessages() int64 {
-	return s.channelInfo.MsgCount
+func (s *stanScaler) getMonitoringEndpoint() string {
+	return s.getSTANChannelsEndpoint() + "?channel=" + s.metadata.subject + "&subs=1"
 }
 
 func (s *stanScaler) getMaxMsgLag() int64 {
-	var maxValue int64
-	maxValue = 0
+	maxValue := int64(0)
+	combinedQueueName := s.metadata.durableName + ":" + s.metadata.queueGroup
+
 	for _, subs := range s.channelInfo.Subscriber {
-		if subs.LastSent > maxValue && subs.QueueName == (s.metadata.durableName+":"+s.metadata.queueGroup) {
+		if subs.LastSent > maxValue && subs.QueueName == combinedQueueName {
 			maxValue = subs.LastSent
 		}
 	}
 
-	return s.channelInfo.MsgCount - maxValue
+	return s.channelInfo.LastSequence - maxValue
 }
 
-func (s *stanScaler) hasPendingMessage() bool {
-	var hasPending bool
-	hasPending = false
+func (s *stanScaler) hasPendingMessage() bool {	
+	subscriberFound := false
+	combinedQueueName := s.metadata.durableName + ":" + s.metadata.queueGroup
+
 	for _, subs := range s.channelInfo.Subscriber {
-		if subs.PendingCount > 0 && subs.QueueName == (s.metadata.durableName+":"+s.metadata.queueGroup) {
-			hasPending = true
+		if subs.QueueName == combinedQueueName {
+			subscriberFound = true
+
+			if subs.PendingCount > 0 {
+				return true
+			}
+
+			break
 		}
 	}
 
-	return hasPending
+	if !subscriberFound {
+		stanLog.Info("The STAN subscription was not found.", "combinedQueueName", combinedQueueName)
+	}
+
+	return false
 }
 
 func (s *stanScaler) GetMetricSpecForScaling() []v2beta1.MetricSpec {