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116 lines
3.6 KiB
Markdown
116 lines
3.6 KiB
Markdown
# Kubernetes Concept: Affinity \ Anti-Affinity
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## Create a kubernetes cluster
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In this guide we we''ll need a Kubernetes cluster for testing. Let's create one using [kind](https://kind.sigs.k8s.io/) </br>
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```
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cd kubernetes/affinity
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kind create cluster --name demo --image kindest/node:v1.28.0 --config kind.yaml
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```
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Test the cluster:
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```
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kubectl get nodes
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NAME STATUS ROLES AGE VERSION
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demo-control-plane Ready control-plane 59s v1.28.0
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demo-worker Ready <none> 36s v1.28.0
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demo-worker2 Ready <none> 35s v1.28.0
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demo-worker3 Ready <none> 35s v1.28.0
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```
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## Node Affinity
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[Node Affinity](https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#node-affinity) is similar to `nodeSelector` however you can define more complex expressions. "Like my pods must run on SSD nodes or preffer SSD nodes"
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For example:
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* Node selector is a hard and fast rule meaning a pod will not be scheduled if the selection is not satisfied
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* 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`
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Node Affinity allows an expression.
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```
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kubectl apply -f node-affinity.yaml
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```
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We can see our pods are prefering SSD and are always going to `us-east`
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```
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kubectl get pods -owide
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#introduce more pods
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kubectl scale deploy app-disk --replicas 10
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#observe all pods on demo-worker
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```
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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` </br>
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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` </br>
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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`,
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notice that our new pods are now in `Pending` status because no nodes satisfy our node affinity rules </br>
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Fix our nodes.
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```
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kubectl taint nodes demo-worker type=ssd:NoSchedule-
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kubectl taint nodes demo-worker3 type=ssd:NoSchedule-
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```
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Scale back down to 0
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```
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kubectl scale deploy app-disk --replicas 0
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kubectl scale deploy app-disk --replicas 1
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# pod should go back to demo-worker , node 1
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kubectl get pods -owide
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```
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## Pod Affinity
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Now [Pod Affinity](https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#inter-pod-affinity-and-anti-affinity) is an expression to allow us to state that pods should gravitate towards other pods
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```
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kubectl apply -f pod-affinity.yaml
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# observe where pods get deployed
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kubectl get pods -owide
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kubectl scale deploy app-disk --replicas 3
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kubectl scale deploy web-disk --replicas 3
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```
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## Pod Anti-Affinity
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Let's say we observe our `app-disk` application disk usage is quite intense, and we would like to prevent `app-disk` pods from running together. </br>
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This is where anti-affinity comes in:
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```
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podAntiAffinity:
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requiredDuringSchedulingIgnoredDuringExecution:
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- labelSelector:
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matchExpressions:
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- key: app
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operator: In
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values:
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- app-disk
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topologyKey: "kubernetes.io/hostname"
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```
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After applying the above, we can roll it out and observe scheduling:
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```
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kubectl scale deploy app-disk --replicas 0
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kubectl scale deploy web-disk --replicas 0
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kubectl apply -f node-affinity.yaml
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kubectl get pods -owide
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kubectl scale deploy app-disk --replicas 2 #notice pending pods when scaling to 3
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kubectl get pods -owide
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kubectl scale deploy web-disk --replicas 2
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kubectl get pods -owide
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```
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