Services

A service is an abstraction for pods, providing a stable, virtual IP (VIP) address. While pods may come and go, services allow clients to reliably connect to the containers running in the pods, using the VIP. The virtual in VIP means it’s not an actual IP address connected to a network interface but its purpose is purely to forward traffic to one or more pods. Keeping the mapping between the VIP and the pods up-to-date is the job of kube-proxy, a process that runs on every node, which queries the API server to learn about new services in the cluster.

Let’s create a pod supervised by an RC and a service along with it:

kubectl create -f https://raw.githubusercontent.com/mhausenblas/kbe/master/specs/services/rc.yaml

kubectl create -f https://raw.githubusercontent.com/mhausenblas/kbe/master/specs/services/svc.yaml

Now we have the supervised pod running:

kubectl get pods -l app=sise

kubectl describe pod rcsise-6nq3k

You can, from within the cluster, access the pod directly via its assigned IP XX.XX.X.X: 

curl 172.17.0.3:9876/info

This is however, as mentioned above, not advisable since the IPs assigned to pods may change. Hence, enter the simpleservice we’ve created:

kubectl get svc

kubectl describe svc simpleservice

The service keeps track of the pods it forwards traffic to through the label, in our case app=sise.

From within the cluster we can now access simpleservice like so:

curl 172.30.228.255:80/info

What makes the VIP 172.30.228.255 forward the traffic to the pod? The answer is: IPtables, which is essentially a long list of rules that tells the Linux kernel what to do with a certain IP package.

Looking at the rules that concern our service (executed on a cluster node) yields:

sudo iptables-save | grep simpleservice

Above you can see the four rules that kube-proxy has thankfully added to the routing table, essentially stating that TCP traffic to 172.30.228.255:80 should be forwarded to 172.17.0.3:9876, which is our pod.

Let’s now add a second pod by scaling up the RC supervising it:

kubectl scale --replicas=2 rc/rcsise

kubectl get pods -l app=sise

To delete apps.

 kubectl delete svc simpleservice

 kubectl delete rc rcsise

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