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Traffic Splitting

So far, whenever we have a configuration change (code or env variable), we apply that change which in turn creates a new revision and the route is updated to direct 100% of the traffic to the new revision.

What if you want to control how the traffic is routed between current and latest revision? You can do that in Knative.

Named revisions and pinning to a revision

We deployed three versions of our Knative that created 3 revisions with random names that you can verify:

kubectl get revision

NAME               SERVICE NAME
helloworld-z9clz   helloworld-z9clz
helloworld-f4xvr   helloworld-f4xvr
helloworld-ln8rv   helloworld-ln8rv

For traffic splitting, it's useful to have meaningful revision names. It's also useful to pin the traffic to a certain revision. Let's do both.

Create a service-v1-pinned.yaml file as follows:

apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: helloworld
  namespace: default
spec:
  template:
    metadata:
      name: helloworld-v1
    spec:
      containers:
        # Replace {username} with your actual DockerHub
        - image: docker.io/{username}/helloworld:v1
          env:
            - name: TARGET
              value: "v1"
  traffic:
  - tag: current
    revisionName: helloworld-v1
    percent: 100
  - tag: latest
    latestRevision: true
    percent: 0

Notice a couple of things:

  1. The revision of the Service has now a specific name: helloworld-v1
  2. There's a traffic section where we pin 100% of the traffic to the named revision.

In this setup, we can still deploy a new revision but that revision (latest) is not going to get the traffic.

Apply the change:

kubectl apply -f service-v1-pinned.yaml

You should see the new named revision created:

kubectl get revision

NAME               SERVICE NAME       GENERATION
helloworld-z9clz   helloworld-z9clz   1
helloworld-f4xvr   helloworld-f4xvr   2
helloworld-ln8rv   helloworld-ln8rv   3
helloworld-v1      helloworld-v1      4

And helloworld-v1 is the one getting the traffic:

curl http://helloworld.default.$ISTIO_INGRESS.xip.io

Hello v1

Deploy a new version

Let's create a new revision. Create a service-v4.yaml file that has TARGET value of v4:

apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: helloworld
  namespace: default
spec:
  template:
    metadata:
      name: helloworld-v4
    spec:
      containers:
        # Replace {username} with your actual DockerHub
        - image: docker.io/{username}/helloworld:v1
          env:
            - name: TARGET
              value: "v4"
  traffic:
  - tag: current
    revisionName: helloworld-v1
    percent: 100
  - tag: latest
    latestRevision: true
    percent: 0

Notice that even though a new revision is being created helloworld-v4, the old revision helloworld-v1 is the one getting the traffic.

Apply the change:

kubectl apply -f service-v4.yaml

You should see the new named revision created:

kubectl get revision

NAME               SERVICE NAME       GENERATION
helloworld-z9clz   helloworld-z9clz   1
helloworld-f4xvr   helloworld-f4xvr   2
helloworld-ln8rv   helloworld-ln8rv   3
helloworld-v1      helloworld-v1      4
helloworld-v4      helloworld-v4      5

But helloworld-v1 is still one getting the traffic:

curl http://helloworld.default.$ISTIO_INGRESS.xip.io

Hello v1

You can verify that the new version is deployed by accessing the latest endpoint:

curl http://latest-helloworld.default.$ISTIO_INGRESS.xip.io

Hello v4

But the current one is helloworld-v1:

curl http://current-helloworld.default.$ISTIO_INGRESS.xip.io

Hello v1

Split the traffic 50-50

In this last section, let's split the traffic 50-50 between helloworld-v1 and helloworld-v4. Create a service-v1v4-split.yaml file as follows:

apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: helloworld
  namespace: default
spec:
  template:
    metadata:
      name: helloworld-v4
    spec:
      containers:
        # Replace {username} with your actual DockerHub
        - image: docker.io/{username}/helloworld:v4
          env:
            - name: TARGET
              value: "v4"
  traffic:
  - tag: current
    revisionName: helloworld-v1
    percent: 50
  - tag: candidate
    revisionName: helloworld-v4
    percent: 50
  - tag: latest
    latestRevision: true
    percent: 0

Apply the change:

kubectl apply -f service-v1v4-split.yaml

You should see roughly 50% of the requests split between revisions:

for i in {1..10}; do curl http://helloworld.default.$ISTIO_INGRESS.xip.io; sleep 1; done
Hello v1
Hello v4
Hello v1
Hello v4