I have a running GKE cluster with an HPA using a target CPU utilisation metric. This is OK but CPU utilisation is not the best scaling metric for us. Analysis suggests that active connection count is a good indicator of general platform load and thus, we'd like to look into this as our primary scaling metric.

To this end I have enabled custom metrics for the NGINX ingress that we use. From here we can see active connection counts, request rates, etc.

Here is the HPA specification using the NGINX custom metric:

apiVersion: autoscaling/v2beta1
kind: HorizontalPodAutoscaler
metadata:
  name: hpa-uat-active-connections
  namespace: default
spec:
  minReplicas: 3
  maxReplicas: 6
  metrics:
    - type: Pods
      pods:
        metricName: custom.googleapis.com|nginx-ingress-controller|nginx_ingress_controller_nginx_process_connections
        selector: 
          matchLabels:
            metric.labels.state: active
            resource.labels.cluster_name: "[redacted]"
        targetAverageValue: 5
  scaleTargetRef:
    apiVersion: apps/v1
    kind: Deployment
    name: "[redacted]"

However, while this specification does deploy OK, I always get this output from the HPA:

NAME                         REFERENCE                                 TARGETS       MINPODS   MAXPODS   REPLICAS   AGE
hpa-uat-active-connections   Deployment/[redacted]                     <unknown>/5   3         6         3          31s

In short, the target value is "unknown" and I have so far failed to understand / resolve why. The custom metric is indeed present:

kubectl get --raw "/apis/external.metrics.k8s.io/v1beta1/namespaces/default/custom.googleapis.com|nginx-ingress-controller|nginx_ingress_controller_nginx_process_connections?labelSelector=metric.labels.state%3Dactive,resource.labels.cluster_name%3D[redacted]" | jq

Which gives:

{
  "kind": "ExternalMetricValueList",
  "apiVersion": "external.metrics.k8s.io/v1beta1",
  "metadata": {
    "selfLink": "/apis/external.metrics.k8s.io/v1beta1/namespaces/default/custom.googleapis.com%7Cnginx-ingress-controller%7Cnginx_ingress_controller_nginx_process_connections"
  },
  "items": [
    {
      "metricName": "custom.googleapis.com|nginx-ingress-controller|nginx_ingress_controller_nginx_process_connections",
      "metricLabels": {
        "metric.labels.controller_class": "nginx",
        "metric.labels.controller_namespace": "ingress-nginx",
        "metric.labels.controller_pod": "nginx-ingress-controller-54f84b8dff-sml6l",
        "metric.labels.state": "active",
        "resource.labels.cluster_name": "[redacted]",
        "resource.labels.container_name": "",
        "resource.labels.instance_id": "[redacted]-eac4b327-stqn",
        "resource.labels.namespace_id": "ingress-nginx",
        "resource.labels.pod_id": "nginx-ingress-controller-54f84b8dff-sml6l",
        "resource.labels.project_id": "[redacted],
        "resource.labels.zone": "[redacted]",
        "resource.type": "gke_container"
      },
      "timestamp": "2019-12-30T14:11:01Z",
      "value": "1"
    }
  ]
}

So I have two questions, really:

1. (the main one): what am I doing wrong here to cause the HPA to not be able to read the metric?
2. Is this is right way to go about trying to scale to an average active connections load over a number of pods?

Many thanks in advance, Ben

Edit 1

kubectl get all

NAME                                                READY   STATUS    RESTARTS   AGE
pod/[redacted]-deployment-7f5fbc9ddf-l9tqk          1/1     Running   0          34h
pod/[redacted]-uat-deployment-7f5fbc9ddf-pbcns      1/1     Running   0          34h
pod/[redacted]-uat-deployment-7f5fbc9ddf-tjfrm      1/1     Running   0          34h

NAME                                TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)                      AGE
service/[redacted]-webapp-service   NodePort    [redacted]     <none>        [redacted]                   57d
service/kubernetes                  ClusterIP   [redacted]     <none>        [redacted]                   57d

NAME                                           READY   UP-TO-DATE   AVAILABLE   AGE
deployment.apps/[redacted]-uat-deployment      3/3     3            3           57d

NAME                                                      DESIRED   CURRENT   READY   AGE
replicaset.apps/[redacted]-uat-deployment-54b6bd5f9c      0         0         0       12d
replicaset.apps/[redacted]-uat-deployment-574c778cc9      0         0         0       35h
replicaset.apps/[redacted]-uat-deployment-66546bf76b      0         0         0       11d
replicaset.apps/[redacted]-uat-deployment-698dfbb6c4      0         0         0       4d
replicaset.apps/[redacted]-uat-deployment-69b5c79d54      0         0         0       6d17h
replicaset.apps/[redacted]-uat-deployment-6f67ff6599      0         0         0       10d
replicaset.apps/[redacted]-uat-deployment-777bfdbb9d      0         0         0       3d23h
replicaset.apps/[redacted]-uat-deployment-7f5fbc9ddf      3         3         3       34h
replicaset.apps/[redacted]-uat-deployment-9585454ff       0         0         0       6d21h
replicaset.apps/[redacted]-uat-deployment-97cbcfc6        0         0         0       17d
replicaset.apps/[redacted]-uat-deployment-c776f648d       0         0         0       10d

NAME                                                               REFERENCE                                 TARGETS   MINPODS   MAXPODS   REPLICAS   AGE
horizontalpodautoscaler.autoscaling/[redacted]-uat-deployment      Deployment/[redacted]-uat-deployment      4%/80%    3         6         3          9h