docker-development-youtube-series/kubernetes/affinity/README.md

Kubernetes Concept: Affinity \ Anti-Affinity

Create a kubernetes cluster

In this guide we we''ll need a Kubernetes cluster for testing. Let's create one using kind

cd kubernetes/affinity
kind create cluster --name demo --image kindest/node:v1.28.0 --config kind.yaml

Test the cluster:

kubectl get nodes
NAME                 STATUS   ROLES           AGE   VERSION
demo-control-plane   Ready    control-plane   59s   v1.28.0
demo-worker          Ready    <none>          36s   v1.28.0
demo-worker2         Ready    <none>          35s   v1.28.0
demo-worker3         Ready    <none>          35s   v1.28.0

Node Affinity

Node Affinity is similar to nodeSelector however you can define more complex expressions

For example:

• Node selector is a hard and fast rule meaning a pod will not be scheduled if the selection is not satisfied
• For example, when using os selector as linux , a pod can only be scheduled if there is a node available where os label is linux

Node Affinity allows an expression.

kubectl apply -f node-affinity.yaml

We can see our pods are prefering SSD and are always going to us-east

kubectl get pods -owide 

#introduce more pods
kubectl scale deploy app-disk --replicas 10

#observe all pods on demo-worker

If there is some trouble with our ssd disk, kubectl taint nodes demo-worker type=ssd:NoSchedule, we can see pods going to the non-ssd disk nodes in us-east

This is because our pods prefer SSD, however there is no SSD available, so would still go to non-SSD nodes as long as there are nodes available in us-east

If something goes wrong in our last us-east node: kubectl taint nodes demo-worker3 type=ssd:NoSchedule and we roll out more pods kubectl scale deploy app-disk --replicas 20, notice that our new pods are now in Pending status because no nodes satisfy our node affinity rules

Fix our nodes.

kubectl taint nodes demo-worker type=ssd:NoSchedule-
kubectl taint nodes demo-worker3 type=ssd:NoSchedule-