Node Affinity

For multiple reasons, there can be a need to run a pod on a specific worker node.

There can be multiple reasons, node hardware being the common one.

Node affinity is a set of rules used by the scheduler to determine where a pod can be placed.

In Kubernetes terms, it is referred to as nodeSelector, and nodeAffinity/podAffinity fields under PodSpec.

In Kubernetes, we can achieve nodeAffinity with the help of:

nodeSelector

nodeAffinity (more flexibility)

Node affinity is conceptually similar to nodeSelector – it allows you to constrain which nodes your pod is eligible to be scheduled on, based on labels on the node.

nodeAffinity-required.yaml


apiVersion: v1
kind: Pod
metadata:
  name: kplabs-node-affinity
spec:
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: color
            operator: In
            values:
            - blue
  containers:
  - name: with-node-affinity
    image: nginx

nodeAffinity-preferred.yaml


apiVersion: v1
kind: Pod
metadata:
  name: kplabs-node-affinity-preferred
spec:
  affinity:
    nodeAffinity:
      preferredDuringSchedulingIgnoredDuringExecution:
      - weight: 1
        preference:
          matchExpressions:
          - key: memory
            operator: In
            values:
            - high
            - medium
  containers:
  - name: kplabs-affinity-preferred
    image: nginx