Affinity

Hard/Soft

• Hard affinity AKA "required node affinity", defined in the pod template under spec: affinity:nodeAffinity:requiredDuringSchedulingIgnoredDuringExecution. This specifies conditions that a node must meet for a pod to schedule to it.
• Soft affinity AKA "preferred known affinity", defined in the pod template under spec: affinity:nodeAffinity:preferredDuringSchedulingIgnoredDuringExecution. This specifies conditions that a node should preferably meet, but if they are not present, it is still okay to schedule the pod (as long as hard affinity criteria are met).

Node Selector vs. Node Affinity

• Both node selectors and node affinity can use Kubernetes labels—metadata assigned to a node.
• Labels allow you to specify that a pod should schedule to one of a set of nodes, which is more flexible than manually attaching a pod to a specific node.
• The difference between node selector and node affinity can be summarized as follows: ** A node selector is defined via the nodeSelector field in the pod template. It contains a set of key-value pairs that specify labels. The Kubernetes scheduler checks if a node has these labels to determine if it is suitable for running the pod. ** Node affinity is an expressive language that uses soft and hard scheduling rules, together with logical operators, to enable more granular control over pod placement.

Label nodes

kubectl label nodes <target-node-name> special-node=true

Use in deployment

apiVersion: apps/v1
kind: Deployment
metadata:
  name: php-apache
spec:
  selector:
    matchLabels:
      run: php-apache
  template:
    metadata:
      labels:
        run: php-apache
    spec:
      containers:
      - name: php-apache
        image: registry.k8s.io/hpa-example
        ports:
        - containerPort: 80
        resources:
          limits:
            cpu: 500m
          requests:
            cpu: 200m

      ## nodeAffinity
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: php-apache-node
                operator: In
                values:
                - "true"

      ## nodeSelector
      # nodeSelector:
      #  php-apache-node: "true"

---
apiVersion: v1
kind: Service
metadata:
  name: php-apache
  labels:
    run: php-apache
spec:
  # ports:
  # - port: 80
  ports:
    - port: 80
      targetPort: 80
      nodePort: 30009
  selector:
    run: php-apache
  type: NodePort

Assigning pods

apiVersion: v1
kind: Pod
metadata:
  name: your-pod
  namespace: your-namespace
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: special-node
            operator: In
            values:
            - "true"
  containers:
  - name: your-container
    image: your-image