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Basics of Traffic Splitting

The last super power 🚀 of Knative Serving we'll go over in this tutorial is traffic splitting.

What are some common traffic splitting use-cases?

Splitting traffic is useful for a number of very common modern infrastructure needs, such as blue/green deployments and canary deployments. Bringing these industry standards to bear on Kubernetes is as simple as a single CLI command on Knative or YAML tweak, let's see how!

Creating a new Revision

You may have noticed that when you created your Knative Service you assigned it a revision-name, "world". When your Service was created, Knative returned both a URL and a 'latest revision' name for your Knative Service. But what happens if you make a change to your Service?

What exactly is a Revision?"

You can think of a Revision as a stateless, autoscaling snapshot-in-time of application code and configuration.

A new Revision will get created each and every time you make changes to your Knative Service, whether you assign it a name or not. When splitting traffic, Knative splits traffic between different Revisions of your Knative Service.

Instead of TARGET="World," let's update the environment variable TARGET on our Knative Service hello to greet "Knative". Lets name this new revision hello-knative

kn service update hello \
--env TARGET=Knative \
--revision-name=knative


apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: hello
spec:
  template:
    metadata:
      name: knative
    spec:
      containers:
        - image: gcr.io/knative-samples/helloworld-go
          ports:
            - containerPort: 8080
          env:
            - name: TARGET
              value: "Knative"
Once you've edited your existing YAML file: 
kubectl apply -f hello.yaml

As before, Knative prints. out some helpful information to the CLI.

Expected output: 

Service hello created to latest revision 'hello-knative' is available at URL:
http://hello.default.127.0.0.1.nip.io

Note, since we are updating an existing Knative Service hello, the URL doesn't change, but our new Revision should have the new name "hello-knative"

Let's access our Knative Service again on the browser http://hello.default.127.0.0.1.nip.io to see the change, or use curl in your terminal: 

curl http://hello.default.127.0.0.1.nip.io

Expected output: 

Hello Knative!

Splitting Traffic

You may at this point be wondering, "where did 'Hello World!' go?" Revisions are a stateless snapshot-in-time of application code and configuration so your "hello-world" Revision is still available to you.

We can easily see a list of our existing revisions with the kn CLI:

kn revisions list

Though the following example doesn't cover it, you can peak under the hood to Kubernetes to see the revisions as Kubernetes sees them.

kubectl get revisions

Expected output: 

NAME            SERVICE   TRAFFIC   TAGS   GENERATION   AGE   CONDITIONS   READY   REASON
hello-knative   hello     100%             2            30s   3 OK / 4     True    
hello-world     hello                      1            5m    3 OK / 4     True

The column most relevant for our purposes is "TRAFFIC". It looks like 100% of traffic is going to our latest Revision ("hello-knative") and 0% of traffic is going to the Revision we configured earlier ("hello-world")

By default, when Knative creates a brand new Revision it directs 100% of traffic to the latest Revision of your service. We can change this default behavior by specifying how much traffic we want each of our Revisions to receive. Lets split traffic in half, using a percentage of 50%.

Info

@latest will always point to our "latest" Revision which, at the moment, is hello-knative.

kn service update hello \
--traffic hello-world=50 \
--traffic @latest=50


apiVersion: serving.knative.dev/v1
kind: Route
metadata:
  name: route-hello
spec:
  traffic:
  - revisionName: @latest
  - percent: 50
  - revisionName: hello-world
    percent: 50
Once you've edited your existing YAML file: 
kubectl apply -f hello.yaml

Verify traffic split configure correctly by listing the revisions again.

kn revisions list

Though the following example doesn't cover it, you can peak under the hood to Kubernetes to see the revisions as Kubernetes sees them.

kubectl get revisions

Expected output: 

NAME            SERVICE   TRAFFIC   TAGS   GENERATION   AGE   CONDITIONS   READY   REASON
hello-knative   hello     50%              2            10m   3 OK / 4     True
hello-world     hello     50%              1            36m   3 OK / 4     True

Access the Knative service on the browser again http://hello.default.127.0.0.1.nip.io, and refresh multiple times to see the different output being served by each revision. You can use curl to access multiple times. 

curl http://hello.default.127.0.0.1.nip.io

On the terminal you will see the output from the both revisions.

Expected output: 

curl http://hello.default.127.0.0.1.nip.io
Hello Knative!

curl http://hello.default.127.0.0.1.nip.io
Hello World!

Congratulations, 🎉 you've successfully split traffic between 2 different Revisions. Up next, Knative Eventing!