4 - Knative Revisions

A Knative Revision is a specific version of a code deployment.

If you deploy a new version of an app in Kubernetes, you typically change the deployment.yaml file and apply the changed version using kubectl. Kubernetes will then perform a rolling update from the old to the new version.

If you have used Istio before, you know that with Istio you can deploy multiple versions of your application.

With Knative, every deployment of a new version will create a new revision. If you explicitly name the revisions, you will have to change the name with every new deployment, otherwise Knative will decline the deployment.

This is version 2 of the helloworld app, specified in service-v2.yaml:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld
spec:
  template:
    metadata:
      # This is the name of our new "Revision," it must follow the convention {service-name}-{revision-name}
      name: helloworld-v2
    spec:
      containers:
        - image: gcr.io/knative-samples/helloworld-go
          ports:
            - containerPort: 8080
          env:
            - name: TARGET
              value: "World V2 -- UPDATED"

Version 2 has a new value for the TARGET environment variable. It also has a new revision name (‘helloworld-v2’), if this does not change, Knative will deny the request.

1. Apply the new version:

   kubectl apply -f service-v2.yaml
   

2. Check the Knative Service:

   kn service describe helloworld
   

   Output:

   Name:       helloworld
   Namespace:  default
   Age:        53s
   URL:        http://helloworld....appdomain.cloud
   
   Revisions:  
     100%  @latest (helloworld-v2) [2] (44s)
           Image:  docker.io/ibmcom/kn-helloworld (at b17fc9)
     
   Conditions:  
     OK TYPE                   AGE REASON
     ++ Ready                  40s 
     ++ ConfigurationsReady    40s 
     ++ RoutesReady            40s 
   

   You can see that the ‘@latest’ revison is ‘helloworld-v2’.

3. Test it with curl, output:

   Hello World V2 -- UPDATED!
   

4. Display the Revisions:

   kn revision list
   

   Output:

   NAME            SERVICE      TRAFFIC   TAGS   GENERATION   AGE    CONDITIONS   READY   REASON
   helloworld-v2   helloworld   100%             2            104s   3 OK / 4     True    
   helloworld-v1   helloworld                    1            98m    3 OK / 4     True    
   

   As you can see, both revisions are there but 100% of all traffic goes to generation (revision) 2.

   So how is this different from a typical Kubernetes rolling update? The difference is that both revisions are available and we will see in the next section how to make use of that.

Continue with the next part 5 - Knative Traffic Management