Kubernetes/exercises/02-simple-rc

Resiliency of pods

1. Simulate a node failure

A pod is the smallest unit of compute in kubernetes. It is tied to a single node runtime and does not come with a lot of workload logic.

To ensure that controle plan no schedule pod run this command

kubectl taint node/k3d-upec-server-0 node-role.kubernetes.io/master:NoSchedule

This command prints the node running our simple-pod.

kubectl get pod/simple-pod-nginx -o jsonpath='{.spec.nodeName}{"\n"}'

We will observe the behaviour of this pod in the event of a node failure.

1. Stop the node where the pod is running with docker stop <node>

2. Check the state of the pod. Is it still running ?

Don't forget to run docker start <node> before pursuing.

2. Create replicas

A naive solution to this problem is to create replicas of this pod spread across multiple nodes.

You will create a ReplicationController resource that adds workload logic to our application by maintaining 3 replicas of our pod.

The spec.template object describes the pod that will be created in the case of insufficient replicas. It is the same as the pod declaration.

---
apiVersion: v1
kind: ReplicationController
metadata:
  name: simple-rc-nginx
spec: {}

Be careful in the usage of labels with selectors. Read the documentation.

As usual, you can monitor the state of the controller and the numbers of ready replicas.

kubectl get rc/simple-rc-nginx
NAME              DESIRED   CURRENT   READY   AGE
simple-rc-nginx   3         3         3       21s

If you delete one pod, the controller should detect it and schedule a new pod to replace it.

Additionally you can scale replicas up and down, either by modifying the resource/manifest or with kubectl.

kubectl scale --replicas=5 rc/simple-rc-nginx

3. Create a service

Port forwarding to a pod is not very convenient with multiple replicas. Ideally we need a way to address them in a load balanced manner. A Service resource is the standard way of exposing an application inside the cluster. It uses selectors to distribute traffic amongst selected pods.

Create a svc.spec.type.clusterIP service to expose the replicas inside the cluster.

You should consider using labelled selectors as you did for the RC.

apiVersion: v1
kind: Service
metadata:
  name: simple-rc-nginx
spec: {}

You can then access this service through a forwarded port.

kubectl port-forward svc/simple-rc-nginx 8080:80