Kubernetes 1.33 version has just been released few days/weeks ago.
As this new release contains 64 enhancements (!), it can not be easy to know what are the interesting and useful features and how to use them.

In this blog post, let’s discover one of interesting and useful new feature: “Topology aware routing in multi-zones Kubernetes clusters”.

⚠️ Kubernetes 1.33 should be available on OVHcloud MKS clusters at the end of June/beginning of July but the demo is working also on MKS with Kubernetes 1.32 release 😉.

Topology aware routing

Since Kubernetes 1.33, the topology aware routing and traffic distribution feature is in General Availability (GA).

This feature allows to optimize service traffic in multi-zone clusters and reduce latency and cross-zone data transfer cost.

Topology Aware Routing provides a mechanism to help keep traffic within the zone it originated from.

In a context of multi-zone clusters, it helps reliability, performance, reduce costs or improve network performance.

As OVHcloud just released, in Beta, the launch of their Managed Kubernetes clusters (MKS) on 3 AZ (Availability Zones), it’s the perfect occasion for me to test this brand new Kubernetes feature 🙂.

Demo

Prerequisite: Have a Kubernetes cluster with at least 2 nodes running in 2 different zones.

If you already don’t have one, you can follow this blog post in order to create an OVHcloud MKS cluster with 3 nodes pools, one per AZ.

On my side I set-up a MKS cluster in 3AZ (one per node pool), with 3 nodes per node pool:

$ kubectx kubernetes-admin@multi-zone-mks
Switched to context "kubernetes-admin@multi-zone-mks".

$ kubectl get np
NAME             FLAVOR   AUTOSCALED   MONTHLYBILLED   ANTIAFFINITY   DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   MIN   MAX   AGE
my-pool-zone-a   b3-8     false        false           false          3         3         3            3           0     100   20d
my-pool-zone-b   b3-8     false        false           false          3         3         3            3           0     100   20d
my-pool-zone-c   b3-8     false        false           false          3         3         3            3           0     100   20d

$ kubectl get no
NAME                         STATUS   ROLES    AGE   VERSION
my-pool-zone-a-b9ztj-brgpq   Ready    <none>   20d   v1.32.3
my-pool-zone-a-b9ztj-gt5vd   Ready    <none>   20d   v1.32.3
my-pool-zone-a-b9ztj-mss8j   Ready    <none>   20d   v1.32.3
my-pool-zone-b-tr6wf-5wfgz   Ready    <none>   20d   v1.32.3
my-pool-zone-b-tr6wf-ct7fs   Ready    <none>   20d   v1.32.3
my-pool-zone-b-tr6wf-vlkwg   Ready    <none>   20d   v1.32.3
my-pool-zone-c-wgrl6-b2f9s   Ready    <none>   20d   v1.32.3
my-pool-zone-c-wgrl6-lp22l   Ready    <none>   20d   v1.32.3
my-pool-zone-c-wgrl6-slkq5   Ready    <none>   20d   v1.32.3

⚠️ As you saw, the Kubernetes version installed on my cluster is not equals to 1.33, but the ServiceTrafficDistribution feature gate is in Beta and it is activated:

$ kubectl get --raw /metrics | grep kubernetes_feature_enabled | grep Traffic

kubernetes_feature_enabled{name="ServiceTrafficDistribution",stage="BETA"} 1

A visual architecture of my MKS cluster:

⚠️ In MKS Standard clusters, don’t forget to enable the topology aware routing for 3AZ region.

In order to test this feature, in a new namespace, we will deploy:

• a deployment with two pods named receiver-xxx
• a ClusterIP service named svc-prefer-close with the feature enabled
• a Pod named sender

Let’s do that!

Create a deploy.yaml file with the following content:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app.kubernetes.io/name: service-traffic-example
  name: receiver
  namespace: prefer-close
spec:
  replicas: 2
  selector:
    matchLabels:
      app: service-traffic-example
  template:
    metadata:
      labels:
        app: service-traffic-example
    spec:
      containers:
      - image: scraly/hello-pod:1.0.1
        name: receiver
        ports:
        - containerPort: 8080
        env:
          - name: NODE_NAME
            valueFrom:
              fieldRef:
                fieldPath: spec.nodeName

Create a svc.yaml file with the following content:

apiVersion: v1
kind: Service
metadata:
  name: svc-prefer-close
  namespace: prefer-close
  annotations:
    service.kubernetes.io/topology-mode: auto
spec:
  ports:
    - name: http
      protocol: TCP
      port: 8080
      targetPort: 8080
  selector:
    app: service-traffic-example
  type: ClusterIP
  trafficDistribution: PreferClose

As you can see, this Service has two specific configurations.
First, we added the service.kubernetes.io/topology-mode: auto annotation to enable Topology Aware Routing for a Service.
Then, we configured the trafficDistribution to PreferClose in order to ask Kubernetes to send the traffic, preferably, to a pod that is “closed” to the sender.

Create a new namespace and apply the manifest files:

$ kubectl create ns prefer-close
$ kubectl apply -f deploy.yaml
$ kubectl apply -f svc.yaml

Result:
You should have two running Pods on 2 differents Nodes.

$ kubectl get po -o wide -n prefer-close

NAME                        READY   STATUS              RESTARTS   AGE   IP            NODE                         NOMINATED NODE   READINESS GATES
receiver-7cfd89d78d-dhv6z   1/1     Running             0          94s   10.240.4.91   my-pool-zone-c-wgrl6-slkq5   <none>           <none>
receiver-7cfd89d78d-hrxrt   1/1     Running             0          94s   10.240.5.63   my-pool-zone-a-b9ztj-mss8j   <none>           <none>

OK, receiver-xxxxxxxx-dhv6z is running on my-pool-zone-c-xxxx and the other pod is running on my-pool-zone-a-xxxx. There are running on differents Availability Zones.

Now, we can create a Pod sender. it will be scheduled on a Node:

Run it and execute a curl command to test the traffic redirection to the “svc-prefer-close” Service:

$ kubectl run sender -n prefer-close --image=curlimages/curl -it -- sh
If you don't see a command prompt, try pressing enter.
~ $ curl http://svc-prefer-close.prefer-close:8080
Version: 1.0.1
Hostname: receiver-7cfd89d78d-dhv6z
Node: my-pool-zone-c-wgrl6-slkq5

Let’s verify where are our Pods:

$ kubectl get po -n prefer-close -o wide
NAME                        READY   STATUS    RESTARTS     AGE   IP             NODE                         NOMINATED NODE   READINESS GATES
receiver-7cfd89d78d-dhv6z   1/1     Running   0            9d    10.240.4.91    my-pool-zone-c-wgrl6-slkq5   <none>           <none>
receiver-7cfd89d78d-hrxrt   1/1     Running   0            9d    10.240.5.63    my-pool-zone-a-b9ztj-mss8j   <none>           <none>
sender                      1/1     Running   1 (5s ago)   21s   10.240.3.134   my-pool-zone-c-wgrl6-b2f9s   <none>           <none>

Kube-proxy sent the traffic from sender to a receiver-xx Pod on the same Availability Zone 🎉

⚠️ Note that because preferClose means “topologically proximate”, it may vary across implementations and could encompass endpoints within the same node, rack, zone, or even region.

How is it working?

When calculating the endpoints for a Service, the EndpointSlice controller considers the topology (region and zone) of each endpoint and populates the hints field to allocate it to a zone.

Cluster components such as kube-proxy can then consume those hints, and use them to influence how the traffic is routed (favoring topologically closer endpoints).

So, with PreferClose value for trafficDistribution, we ask kube-proxy to redirect traffic to the nearest available endpoints based on the network topology.

That’s why the option is called PreferClose.

What’s next?

In the future you will be able to configure the trafficDistribution field with other values.

Indeed, two new values, more explicit, are currently in Alpha since the Kubernetes 1.33 release: PreferSameZone and PreferSameNode.

Personally I can’t wait to test them 😇.

Want to go further?

Want to learn more on this topic? In the coming days, we will publish a blog post about MKS Premium plan.

Visit our Managed Kubernetes Service (MKS) Premium plan in the OVHcloud Labs website to know more about Premium MKS.

Join the free Beta: https://labs.ovhcloud.com/en/managed-kubernetes-service-mks-premium-plan/

Read the documentation about the new Managed Kubernetes Service (MKS) Premium plan.

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