Configuring target burst capacity¶

Target burst capacity is a global and per-revision numeric 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 target-burst-capacity 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/target-burst-capacity
Possible values: float (0 means the Activator is only in path when scaled to 0, -1 means the Activator is always in path)
Default: 211, as defined in v1.24 autoscaler configuration. The examples below explicitly choose 200 rather than using that default.

Example:

Per RevisionGlobal (ConfigMap)Global (Operator)

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  annotations:
  name: <service_name>
  namespace: default
spec:
  template:
    metadata:
      annotations:
        autoscaling.knative.dev/target-burst-capacity: "200"

apiVersion: v1
kind: ConfigMap
metadata:
  name: config-autoscaler
  namespace: knative-serving
data:
  target-burst-capacity: "200"

apiVersion: operator.knative.dev/v1beta1
kind: KnativeServing
metadata:
  name: knative-serving
spec:
  config:
    autoscaler:
      target-burst-capacity: "200"

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/target-burst-capacity 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.
If autoscaling.knative.dev/target-burst-capacity is set to -1, the Activator is always in the request path, regardless of the revision size.
If autoscaling.knative.dev/target-burst-capacity is set to a positive number, the Activator may be in the path, depending on the revision scale and load.