Merge pull request #220 from marcel-dempers/affinity

Kubernetes afffinity

kubernetes/affinity/README.md

@@ -0,0 +1,115 @@
+# 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](https://kind.sigs.k8s.io/) </br>
+
+```
+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](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"
+
+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` </br>
+
+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>
+
+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 </br>
+
+
+Fix our nodes.
+```
+kubectl taint nodes demo-worker type=ssd:NoSchedule-
+kubectl taint nodes demo-worker3 type=ssd:NoSchedule-
+```
+Scale back down to 0
+```
+kubectl scale deploy app-disk --replicas 0
+kubectl scale deploy app-disk --replicas 1
+
+# pod should go back to demo-worker , node 1
+kubectl get pods -owide
+```
+
+## Pod Affinity 
+
+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
+
+```
+kubectl apply -f pod-affinity.yaml
+
+# observe where pods get deployed
+kubectl get pods -owide
+
+kubectl scale deploy app-disk --replicas 3
+kubectl scale deploy web-disk --replicas 3
+```
+
+## Pod Anti-Affinity
+
+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>
+This is where anti-affinity comes in:
+
+```
+podAntiAffinity:
+  requiredDuringSchedulingIgnoredDuringExecution:
+  - labelSelector:
+      matchExpressions:
+      - key: app
+        operator: In
+        values:
+        - app-disk
+    topologyKey: "kubernetes.io/hostname"
+```
+
+After applying the above, we can roll it out and observe scheduling:
+
+```
+kubectl scale deploy app-disk --replicas 0
+kubectl scale deploy web-disk --replicas 0
+kubectl apply -f node-affinity.yaml
+kubectl get pods -owide
+
+kubectl scale deploy app-disk --replicas 2 #notice pending pods when scaling to 3
+kubectl get pods -owide
+kubectl scale deploy web-disk --replicas 2
+kubectl get pods -owide
+
+```
+

kubernetes/affinity/kind.yaml

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+kind: Cluster
+apiVersion: kind.x-k8s.io/v1alpha4
+nodes:
+- role: control-plane
+- role: worker  #demo-worker
+  labels:
+    zone: us-east
+    type: ssd
+- role: worker  #demo-worker2
+  labels:
+    zone: us-west
+    type: ssd
+- role: worker  #demo-worker3
+  labels:
+    zone: us-east
+- role: worker  #demo-worker4
+  labels:
+    zone: us-west

kubernetes/affinity/node-affinity.yaml

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+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: app-disk
+  labels:
+    app: app-disk
+spec:
+  selector:
+    matchLabels:
+      app: app-disk
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app: app-disk
+    spec:
+      containers:
+      - name: app-disk
+        image: nginx:latest
+      affinity:
+        podAntiAffinity:
+          requiredDuringSchedulingIgnoredDuringExecution:
+          - labelSelector:
+              matchExpressions:
+              - key: app
+                operator: In
+                values:
+                - app-disk
+            topologyKey: "kubernetes.io/hostname"
+        nodeAffinity:
+          requiredDuringSchedulingIgnoredDuringExecution:
+            nodeSelectorTerms:
+            - matchExpressions:
+              - key: zone
+                operator: In
+                values:
+                - us-east
+          preferredDuringSchedulingIgnoredDuringExecution:
+          - weight: 1
+            preference:
+              matchExpressions:
+              - key: type
+                operator: In
+                values:
+                - ssd

kubernetes/affinity/pod-affinity.yaml

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+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: web-disk
+  labels:
+    app: web-disk
+spec:
+  selector:
+    matchLabels:
+      app: web-disk
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app: web-disk
+    spec:
+      containers:
+      - name: web-disk
+        image: nginx:latest
+      affinity:
+        podAffinity:
+          requiredDuringSchedulingIgnoredDuringExecution:
+          - labelSelector:
+              matchExpressions:
+              - key: app
+                operator: In
+                values:
+                - app-disk
+            topologyKey: "kubernetes.io/hostname"