Introduction to Node Affinity in Kubernetes

In Kubernetes, we can specify rules to control scheduling our pods on nodes or machines using Node Affinity. It is one of the suitable ways to place specific pods on different nodes by using the pod affinity/anti-affinity rules and node labels.

Node Affinity contains two major components:

  • Node selectors
  • Node affinity rules

Node selectors

We can use it to define labels on nodes and then compare these labels with labels on pods. We can identify the characteristics of our nodes by assigning labels to them, e.g., network configuration, hardware capabilities, or location. After that, we can add node selectors in our pod specification to indicate how we can schedule any pod on nodes with the help of the matching labels.

A coding example related to the usage of a node selector in a pod specification is given below:

apiVersion: v1
kind: Pod
metadata:
  name: abc-pod
spec:
  containers:
  - name: abc-container
    image: nginx
  nodeSelector:
    disktype: ssd

In the coding example above, the nodeSelector field indicates that we can schedule the pod only on nodes labeled with disktype: ssd.

Node affinity rules

Based on node labels and pod affinity/anti-affinity requirements, node affinity rules enable more advanced control over pod scheduling. Node affinity rules define preferences and constraints for pod placement on nodes, whereas pod affinity/anti-affinity rules define preferences and restrictions for other pods.

A coding example related to the usage of node affinity rules in a pod specification is given below:

apiVersion: v1
kind: Pod
metadata:
  name: abc-pod
spec:
  containers:
  - name: abc-container
    image: nginx
  affinity:
    nodeAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
        nodeSelectorTerms:
        - matchExpressions:
          - key: disktype
            operator: In
            values:
            - ssd

In this example, the nodeAffinity field indicates that we can schedule the pod only on nodes labeled with disktype: ssd. The requiredDuringSchedulingIgnoredDuringExecution attribute ensures we can schedule the pod only on nodes meeting the specified node affinity rules.

Conclusion

In short, if we have nodes having different characteristics and we want to control the scheduling of pods by using these characteristics, then the Node Affinity technique would be beneficial. We can easily schedule pods on nodes that meet specific requirements, for example, geographic location or hardware capabilities, with the help of this technique.

Unlock your potential: Kubernetes Essentials series, all in one place!

If you've missed any part of the series, you can always go back and check out the previous Answers:

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  • What are taints and tolerations in Kubernetes?
    Gain insights into taints and tolerations and how they control pod scheduling by preventing or allowing specific workloads to run on designated nodes.

  • Introduction to Node Affinity in Kubernetes
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