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High availability on OpenShift Serverless

High availability (HA) is a standard feature of Kubernetes APIs that helps to ensure that APIs stay operational if a disruption occurs. In an HA deployment, if an active controller crashes or is deleted, another controller is available to take over processing of the APIs that were being serviced by the controller that is now unavailable.

HA in OpenShift Serverless is available through leader election, which is enabled by default after the Knative Serving control plane is installed.

When using a leader election HA pattern, instances of controllers are already scheduled and running inside the cluster before they are required. These controller instances compete to use a shared resource, known as the leader election lock. The instance of the controller that has access to the leader election lock resource at any given time is referred to as the leader.

Configuring high availability replicas on OpenShift Serverless

High availability (HA) functionality is available by default on OpenShift Serverless for the autoscaler-hpa, controller, activator, kourier-control, and kourier-gateway components. These components are configured with two replicas by default.

You modify the number of replicas that are created per controller by changing the configuration of the KnativeServing.spec.highAvailability spec in the KnativeServing custom resource definition (CRD).

Prerequisites

  • An OpenShift Container Platform account with cluster administrator access.

  • Installed the OpenShift Serverless Operator and Knative Serving.

Procedure

  1. In the OpenShift Container Platform web console Administrator perspective, navigate to OperatorHub → Installed Operators.

    Installed Operators page

  2. Select the knative-serving namespace.

  3. Click Knative Serving in the list of Provided APIs for the OpenShift Serverless Operator to go to the Knative Serving tab.

    Knative Serving tab

  4. Click knative-serving, then go to the YAML tab in the knative-serving page.

    Knative Serving YAML

  5. Edit the custom resource definition YAML:

    Example YAML
    apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
  namespace: knative-serving
spec:
  high-availability:
    replicas: 3

    Do not modify any YAML contained inside the config field. Some of the configuration values in this field are injected by the OpenShift Serverless Operator, and modifying them will cause your deployment to become unsupported.

    • The default replicas value is 2.

    • Changing the value to 1 will disable HA, or you can increase the number of replicas as required. The example configuration shown specifies a replica count of 3 for all HA controllers.