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Knative Tutorials – Chapter – 10 – Knative Tutorials: servicing workflow between Components

December 1, 2023 Leave a Comment
  1. Request Initiation:
    • A user sends a request to an application hosted on Knative.
    • This request first hits the Istio Ingress Gateway, which is the entry point for external traffic into the Kubernetes cluster.
  2. VirtualService Routing:
    • The Istio Ingress Gateway uses Istio VirtualService to determine where to route the request. Knative sets up these VirtualServices when a service is created or updated.
    • Typically, the VirtualService will point to the Knative Activator when the application is scaled to zero or during scaling up from zero.
  3. Activator:
    • The Activator is responsible for starting up instances of the application (Pods) when there are none running. It plays a crucial role in scale-to-zero functionality.
    • If an application is scaled to zero (no active pods), the Activator receives the incoming request, triggers the Kubernetes deployment to start a new pod, and then buffers incoming requests until the pod is ready to handle them.
    • Once the pod is running and ready, the Activator forwards the buffered requests to the pod.
  4. Autoscaler:
    • The Autoscaler component in Knative observes traffic load and scales the application pods up or down based on the configuration (like concurrency-based metrics) and actual traffic.
    • It receives metrics about ongoing traffic and makes decisions on when to scale. The Activator also plays a part in this by sending metrics to the Autoscaler when it’s buffering requests for scaled-to-zero applications.
  5. Knative Service and Revisions:
    • In Knative, when you create a Service (not to be confused with Kubernetes Service), it represents a specific configuration of your application.
    • Each time you modify the Service, Knative creates a new Revision. Each Revision is a snapshot of the code and configurations for a particular version of the service.
    • The actual running code is encapsulated in a Kubernetes deployment associated with that Revision.
  6. Knative Route:
    • The Route is responsible for determining which Revision of a Service receives traffic. You can set up the Route to split traffic between multiple Revisions for blue-green deployments, canary releases, etc.
    • Combined with VirtualService, it ensures the right revisions of your application are being routed traffic.
  7. Request Completion:
    • After the application processes the request, the response travels back through the same path: from the pod to (possibly) the Activator, and then out through the Istio Ingress Gateway to the user.
Rajesh Kumar
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