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Configuring target burst capacity

Target burst capacity is a global and per-revision integer setting that determines the size of traffic burst a Knative application can handle without buffering. If a traffic burst is too large for the application to handle, the Activator service will be placed in the request path to protect the revision and optimize request load balancing.

The Activator service is responsible for receiving and buffering requests for inactive revisions, or for revisions where a traffic burst is larger than the limits of what can be handled without buffering for that revision. It can also quickly spin up additional pods for capacity, and throttle how quickly requests are sent to pods.

Target burst capacity can be configured using a combination of the following parameters:

  • Setting the targeted concurrency limits for the revision. See concurrency.
  • Setting the target utilization parameters. See target utilization.
  • Setting the target burst capacity. You can configure target burst capacity using the autoscaling.knative.dev/targetBurstCapacity annotation key in the config-autoscaler ConfigMap. See Setting the target burst capacity.

Setting the target burst capacity

  • Global key: target-burst-capacity
  • Per-revision annotation key: autoscaling.knative.dev/targetBurstCapacity
  • Possible values: float (0 means the Activator is only in path when scaled to 0, -1 means the Activator is always in path)
  • Default: 200

Example:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  annotations:
  name: <service_name>
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/targetBurstCapacity: "200"
apiVersion: v1
kind: ConfigMap
metadata:
  name: config-autoscaler
  namespace: knative-serving
data:
  target-burst-capacity: "200"
apiVersion: operator.knative.dev/v1alpha1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      target-burst-capacity: "200"

  • If autoscaling.knative.dev/targetBurstCapacity is set to 0, the Activator is only added to the request path during scale from zero scenarios, and ingress load balancing will be applied.

    NOTE: Ingress gateway load balancing requires additional configuration. For more information about load balancing using an ingress gateway, see the Serving API documentation.

  • If autoscaling.knative.dev/targetBurstCapacity is set to -1, the Activator is always in the request path, regardless of the revision size.

  • If autoscaling.knative.dev/targetBurstCapacity is set to another integer, the Activator may be in the path, depending on the revision scale and load.