Knative Kubernetes Services

This guide describes the Kubernetes Services that are active when running Knative Serving.

Before You Begin

  1. This guide assumes that you have installed Knative Serving. If you have not, instructions on how to do this are located here.

  2. Verify that you have the proper components in your cluster. To view the services installed in your cluster, use the command:

    $ kubectl get services -n knative-serving

    This should return the following output:

    NAME                TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)                  AGE
activator-service   ClusterIP   10.96.61.11      <none>        80/TCP,81/TCP,9090/TCP   1h
autoscaler          ClusterIP   10.104.217.223   <none>        8080/TCP,9090/TCP        1h
controller          ClusterIP   10.101.39.220    <none>        9090/TCP                 1h
webhook             ClusterIP   10.107.144.50    <none>        443/TCP                  1h

  3. To view the deployments in your cluster, use the following command:

    $ kubectl get deployments -n knative-serving

    This should return the following output:

    NAME                     DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
activator                1         1         1            1           1h
autoscaler               1         1         1            1           1h
controller               1         1         1            1           1h
networking-certmanager   1         1         1            1           1h
networking-istio         1         1         1            1           1h
webhook                  1         1         1            1           1h

The networking deployment (in our case, networking-istio) may be called something different depending on what networking layer you chose to use when installing Knative.

These services and deployments are installed by the serving.yaml file during install. The next section describes their function.

Components

Service: activator

The activator is responsible for receiving & buffering requests for inactive revisions and reporting metrics to the autoscaler. It also retries requests to a revision after the autoscaler scales the revision based on the reported metrics.

Service: autoscaler

The autoscaler receives request metrics and adjusts the number of pods required to handle the load of traffic.

Service: controller

The controller service reconciles all the public Knative objects and autoscaling CRDs. When a user applies a Knative service to the Kubernetes API, this creates the configuration and route. It will convert the configuration into revisions and the revisions into deployments and Knative Pod Autoscalers (KPAs).

Service: webhook

The webhook intercepts all Kubernetes API calls as well as all CRD insertions and updates. It sets default values, rejects inconsitent and invalid objects, and validates and mutates Kubernetes API calls.

Deployment: networking-certmanager

The certmanager reconciles cluster ingresses into cert manager objects.

Deployment: networking

The networking layer deployment (eg. networking-istio, contour-ingress-controller, etc.) reconciles a cluster’s ingress into the appropriate networking layer primitives. In the example shown above, we’ve chosen to use Istio (which reconciles into a Istio Virtual Service.

What’s Next

  • For a deeper look at the services and deployments involved in Knative Serving, click here.
  • For a high-level analysis of Serving, look at the documentation here.
  • Check out the Knative Serving code samples here for more hands-on tutorials.