Horizontal auto scaling based on a custom metric

Setting up horizontal auto scaling based on a custom metric in Kubernetes requires several steps:

Install Metrics Server

Kubernetes Metrics Server is a scalable, efficient source of container resource metrics. If you haven’t installed the Metrics Server, you can do it by running the following command:
```
.. code-block:: bash

   kubectl apply -f https://github.com/kubernetes-sigs/metrics-server/releases/latest/download/components.yaml
```
Verify the Metrics Server installation:
```
.. code-block:: bash

   kubectl get deployment metrics-server -n kube-system
```
Install and Configure Custom Metrics API

The Custom Metrics API is often provided by “adapter” applications, which translate the results from monitoring systems into the format expected by the Custom Metrics API. One of these is the Prometheus adapter.

You can install the Prometheus adapter via Helm:
```
.. code-block:: bash

   helm repo add prometheus-community https://prometheus-community.github.io/helm-charts
   helm install [RELEASE_NAME] prometheus-community/prometheus-adapter
```
Replace [RELEASE_NAME] with the name you want to give your Prometheus Adapter release.
Set up Custom Metrics

With the adapter installed, you can set up your custom metrics. The Prometheus adapter can be configured to present the custom metrics you need. These are defined in a ConfigMap.

Define Horizontal Pod Autoscaler

Once you have your custom metric set up and exposed via the adapter, you can create an HPA resource to use it. Below is an example of how you might set up an HPA resource using a custom metric:

.. code-block:: yaml

   apiVersion: autoscaling/v2beta2
   kind: HorizontalPodAutoscaler
   metadata:
     name: my-app-hpa
   spec:
     scaleTargetRef:
       apiVersion: apps/v1
       kind: Deployment
       name: my-app
     minReplicas: 1
     maxReplicas: 10
     metrics:
     - type: Pods
       pods:
         metric:
           name: my_custom_metric
         target:
           type: AverageValue
           averageValue: 50

Replace “my_custom_metric” with the name of your custom metric. This HPA will maintain an average value of the custom metric of 50 per pod.

Please note that the setup process can be complex, and these are high-level steps. The specifics can vary depending on the infrastructure and the exact metric you’re scaling on. Make sure to refer to the specific documentation for each tool you are using.