# Kubernetes Autoscaling (HPA) with Custom Metrics

You must have the following to scale components based on custom metrics:

- To collect metrics from applications and store them in the Kloudfuse database.
- To extend the Kubernetes Custom Metrics API with metrics supplied by a collector, the `k8s-prometheus-adapter`. The adapter is an implementation of the custom metrics API that supports arbitrary metrics.

Consider a case where Kloudfuse ingests an application-specific metric, `some_metric`. You plan to modify it, and use its value for autoscaling the Kubernetes pods. Follow these steps:

1. Install Prometheus adapter with proper configuration to emit a `custom_metric` based on a query for `some_metric`.

2. Create `prometheus-values.yaml` file for the Prometheus Adapter, configuring the proper endpoint, authentication, and custom metric rules.

```yaml
   metricsRelistInterval: 5m

prometheus:
        # Replace with your endpoint for Kloudfuse
        url: <<https://your.hostname.kloudfuse>>
        port: 443
        path: ""

# Rules to generate `some_custom_metric` as an external metric
   rules:
        default: false
        external:
     - seriesQuery: '{__name__=~"some_metric"}'
       resources:
         template: <<.Resource>>
       name:
         as: "some_custom_metric"
       metricsQuery: avg (some_metric)

# Base64 Encoded Auth string in the format <<username:password>>
   extraArguments:
     - --prometheus-header=Authorization=Basic YWRtaW46cGFzc3dvcmQ=
   ```

3. Deploy Prometheus adapter on your cluster.

```console
   helm install --name prometheus-adapter >prometheus-community/prometheus-adapter -f prometheus-values.yaml
   ```

4. Verify that the adapter emits `custom-metric` correctly, using the Kubernetes External Metrics API.

See [Prometheus Adapter](https://github.com/prometheus-community/helm-charts/tree/main/charts/prometheus-adapter) code and documentation for additional configuration information.

```yaml
   kubectl get --raw /apis/external.metrics.k8s.io/v1beta1 | jq .

#Should show some output like this
   {
        "kind": "APIResourceList",
        "apiVersion": "v1",
        "groupVersion": "external.metrics.k8s.io/v1beta1",
        "resources": [\
          {\
            "name": "some_custom_metric",\
            "singularName": "",\
            "namespaced": true,\
            "kind": "ExternalMetricValueList",\
            "verbs": [\
              "get"\
            ]\
          }\
        ]
   }
   ```

5. After `custom-metric` becomes available to use within the Kubernetes Metrics API, configure HPA to use the metric.

```yaml
   apiVersion: autoscaling/v2
   kind: HorizontalPodAutoscaler
   metadata:
        name: <<some_name>>
   spec:
        scaleTargetRef:
          apiVersion: apps/v1
          kind: Deployment
          name: <<some_name>>
        minReplicas: 1
        maxReplicas: 10
        metrics:
     - type: External
       external:
         metric:
           name: some_custom_metric
         target:
           type: AverageValue
           averageValue: 100
   ```

6. Explore how to configure HPA with Custom metrics in Kubernetes documentation for [Autoscaling on multiple metrics and custom metrics](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale-walkthrough/#autoscaling-on-multiple-metrics-and-custom-metrics).