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Kubernetes: 11. Node Affinity

Scheduler

  • By default Pods gets scheduled based on node availability for the scheduler
  • There may be cases where in one of the node has more resources and the pod required to be scheduled on this node
  • There are two ways to achieve this
    1. Node Selector
    2. Node Affinity
Node Affinity
  • The primary purpose of node affinity is to make sure that pods are hosted correctly on the nodes
  • Assume that during pod creation the affinity rules match and the pod is created, what if the node labels are changed after the pod creation
  • What happens to pod depends on the nodeAffinity values set. These are
    1. requiredDuringSchedulingIgnoredDuringExecution
    2. preferredDuringSchedulingIgnoredDuringExecution
    3. requiredDuringSchedulingRequiredDuringExecution
  • 3rd option still does not exist in Kubernetes, it will be/or is already released in the future releases
  • Operators can be In, NotIn, Exists
  • For Exists, we don't need to specify any value in the pod-definition. This is because affinity rules only check if the key exists, it does not look for any values
pod-definition.yaml
apiVersion: v1
kind: Pod
metadata:
    name: myapp-pod
    labels:
        app: myapp


spec:
    containers:
    - name: nginx-container
      image: nginx

    affinity:
        nodeAffinity:
            requiredDuringSchedulingIgnoredDuringExecution:
                nodeSelectorTerms:
                - matchExpressions:
                  - key: size
                    operator: In
                    values:
                    - Large
                    - Medium
  
How to force a pod to schedule on a node?
  • With taints & toleration, only the pod that can tolerate a taint gets scheduled on that node.
  • But the pod can be scheduled on a node that has no taint defined
  • With node selectors, only the pod that matches the with the node label gets scheduled on that node.
  • But a pod with no selector rules can be scheduled on a node with label
  • So a combination of taints and toleration along with node selector has to be used
  • With taint & toleration only pods that can tolerate the taint will be scheduled and
  • with node selector pod will be scheduled only on the node it is supposed to
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