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Scale bounds

Scale bounds determine the minimum and maximum numbers of replicas that can serve an application at any given time. You can set scale bounds for an application to help prevent cold starts or control computing costs.

Minimum scale bounds

The minimum number of replicas that can serve an application is determined by the min-scale annotation. If scale to zero is not enabled, the min-scale value defaults to 1.

The min-scale value defaults to 0 replicas if the following conditions are met:

  • The min-scale annotation is not set

  • Scaling to zero is enabled

  • The class KPA is used

Example service spec with min-scale annotation
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: showcase
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/min-scale: "0"
...

Setting the min-scale annotation by using the Knative CLI

Using the Knative (kn) CLI to set the min-scale annotation provides a more streamlined and intuitive user interface over modifying YAML files directly. You can use the kn service command with the --scale-min flag to create or modify the min-scale value for a service.

Prerequisites

  • Knative Serving is installed on the cluster.

  • You have installed the Knative (kn) CLI.

Procedure

  • Set the minimum number of replicas for the service by using the --scale-min flag:

    $ kn service create <service_name> --image <image_uri> --scale-min <integer>
    Example command
    $ kn service create showcase --image quay.io/openshift-knative/showcase --scale-min 2

Maximum scale bounds

The maximum number of replicas that can serve an application is determined by the max-scale annotation. If the max-scale annotation is not set, there is no upper limit for the number of replicas created.

Example service spec with max-scale annotation
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: showcase
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/max-scale: "10"
...

Setting the max-scale annotation by using the Knative CLI

Using the Knative (kn) CLI to set the max-scale annotation provides a more streamlined and intuitive user interface over modifying YAML files directly. You can use the kn service command with the --scale-max flag to create or modify the max-scale value for a service.

Prerequisites

  • Knative Serving is installed on the cluster.

  • You have installed the Knative (kn) CLI.

Procedure

  • Set the maximum number of replicas for the service by using the --scale-max flag:

    $ kn service create <service_name> --image <image_uri> --scale-max <integer>
    Example command
    $ kn service create showcase --image quay.io/openshift-knative/showcase --scale-max 10