It is used to ensure the pods are scheduled on the right nodes. It allows us to use complex logic to decide which node the pod should be scheduled on. It uses the concept of labelling nodes as seen in Node Selector.

Node Affinity Types

It defines the behavior of the node affinity when scheduling a new pod (DuringScheduling) and when a pod has been running (DuringExecution).

These are of types:

• requiredDuringSchedulingIgnoredDuringExecution
  • During pod scheduling, the node must satisfy the node affinity requirements. If none of the nodes satisfy the requirements, the pod will not be scheduled.
  • A pod already running should be allowed to keep running.
• preferredDuringSchedulingIgnoredDuringExecution
  • During pod scheduling, the node should ideally satisfy the node affinity requirements. If none of the nodes satisfy the requirements, the pod will still be scheduled on a node.
  • A pod already running should be allowed to keep running.
• requiredDuringSchedulingRequiredDuringExecution (planned to be released)
  • During pod scheduling, the node must satisfy the node affinity requirements. If none of the nodes satisfy the requirements, the pod will not be scheduled.
  • A pod already running on a node that does not satisfy the node affinity requirements will be evicted from that node.

Schedule a pod on a node with compute label high or medium

apiVersion: v1
kind: Pod
metadata:
	name: web-pod
spec:
	containers:
		- name: nginx
			image: nginx
	affinity:
		nodeAffinity:
			requiredDuringSchedulingIgnoredDuringExecution:
				nodeSelectorTerms:
					- matchExpressions:
						- key: compute
							operator: In
							values:
							- high
							- medium

Schedule a pod on a node with compute label not low

apiVersion: v1
kind: Pod
metadata:
	name: web-pod
spec:
	containers:
		- name: nginx
			image: nginx
	affinity:
		nodeAffinity:
			requiredDuringSchedulingIgnoredDuringExecution:
				nodeSelectorTerms:
					- matchExpressions:
						- key: compute
							operator: NotIn
							values:
							- low

Schedule a pod on a node where compute label exists

apiVersion: v1
kind: Pod
metadata:
	name: web-pod
spec:
	containers:
		- name: nginx
			image: nginx
	affinity:
		nodeAffinity:
			requiredDuringSchedulingIgnoredDuringExecution:
				nodeSelectorTerms:
					- matchExpressions:
						- key: compute
							operator: Exists