Affinity and anti-affinity

Affinity and anti-affinity

nodeSelector provides a very simple way to constrain pods to nodes with particular labels. The affinity/anti-affinity feature greatly extends the types of constraints you can express. The key enhancements are:

  • The language is more expressive (not just “AND of exact match”)
  • You can indicate that the rule is “soft”/“preference” rather than a hard requirement, so if the scheduler can’t satisfy it, the pod will still be scheduled
  • You can constrain against labels on other pods running on the node (or other topological domain), rather than against labels on the node itself, which allows rules about which pods can and cannot be co-located

The affinity feature consists of two types of affinity, “node affinity” and “inter-pod affinity/anti-affinity”. Node affinity is like the existing nodeSelector (but with the first two benefits listed above), while inter-pod affinity/anti-affinity constrains against pod labels rather than node labels, as described in the third item listed above, in addition to having the first and second properties listed above.

Node affinity

Node affinity was introduced as alpha in Kubernetes 1.2. 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.

There are currently two types of node affinity, called requiredDuringSchedulingIgnoredDuringExecution and preferredDuringSchedulingIgnoredDuringExecution.

You can think of them as “hard” and “soft” respectively, in the sense that the former specifies rules that must be met for a pod to be scheduled onto a node (just like nodeSelector but using a more expressive syntax), while the latter specifies preferences that the scheduler will try to enforce but will not guarantee. The “IgnoredDuringExecution” part of the names means that, similar to how nodeSelector works, if labels on a node change at runtime such that the affinity rules on a pod are no longer met, the pod will still continue to run on the node.

Thus an example of requiredDuringSchedulingIgnoredDuringExecution would be “only run the pod on nodes with Intel CPUs” and an example preferredDuringSchedulingIgnoredDuringExecution would be “try to run this set of pods in availability zone XYZ, but if it’s not possible, then allow some to run elsewhere”.

Node affinity is specified as field nodeAffinity of field affinity in the PodSpec.

Let’s see an example of a pod that uses node affinity:

We are going to create another label in the same node that in the last example:

kubectl label nodes azname=az1

And create an affinity:

cat <<EoF > ~/environment/pod-with-node-affinity.yaml
apiVersion: v1
kind: Pod
  name: with-node-affinity
        - matchExpressions:
          - key: azname
            operator: In
            - az1
            - az2
      - weight: 1
          - key: another-node-label-key
            operator: In
            - another-node-label-value
  - name: with-node-affinity

This node affinity rule says the pod can only be placed on a node with a label whose key is azname and whose value is either az1 or az2. In addition, among nodes that meet that criteria, nodes with a label whose key is another-node-label-key and whose value is another-node-label-value should be preferred.

Let’s apply this

kubectl apply -f ~/environment/pod-with-node-affinity.yaml

And check if it worked with kubectl get pods -o wide

NAME                 READY     STATUS    RESTARTS   AGE       IP               NODE                                          NOMINATED NODE
nginx                1/1       Running   0          35m   <none>
with-node-affinity   1/1       Running   0          29s   <none>
Now let’s try to put the affinity in another node We are going to put the label in a different node so first, let’s clean the label and delete the Pod.

kubectl delete -f ~/environment/pod-with-node-affinity.yaml
kubectl label nodes azname-

We are putting the label to the node now

kubectl label nodes azname=az1
kubectl apply -f ~/environment/pod-with-node-affinity.yaml

And check if it works with kubectl get pods -o wide

NAME                 READY     STATUS    RESTARTS   AGE       IP               NODE                                           NOMINATED NODE
nginx                1/1       Running   0          43m    <none>
with-node-affinity   1/1       Running   0          42s   <none>

You can see the operator In being used in the example. The new node affinity syntax supports the following operators: In, NotIn, Exists, DoesNotExist, Gt, Lt. You can use NotIn and DoesNotExist to achieve node anti-affinity behavior.

  • If you specify both nodeSelector and nodeAffinity, both must be satisfied for the pod to be scheduled onto a candidate node.
  • If you specify multiple nodeSelectorTerms associated with nodeAffinity types, then the pod can be scheduled onto a node if one of the nodeSelectorTerms is satisfied.
  • If you specify multiple matchExpressions associated with nodeSelectorTerms, then the pod can be scheduled onto a node only if all matchExpressions can be satisfied.
  • If you remove or change the label of the node where the pod is scheduled, the pod won’t be removed. In other words, the affinity selection works only at the time of scheduling the pod.

The weight field in preferredDuringSchedulingIgnoredDuringExecution is in the range 1-100. For each node that meets all of the scheduling requirements (resource request, RequiredDuringScheduling affinity expressions, etc.), the scheduler will compute a sum by iterating through the elements of this field and adding “weight” to the sum if the node matches the corresponding MatchExpressions. This score is then combined with the scores of other priority functions for the node. The node(s) with the highest total score are the most preferred.