Resolved: In Kubernetes, are podAffinity and podAntiAffinity weights compared to each other? Or idependently? What about podAffinity and nodeAffinity?

Question:

In Kubernetes, are podAffinity and podAntiAffinity weights compared to each other? Or independently? What about podAffinity and nodeAffinity? Would podAntiAffinity outweight the podAffinity in the below example? And what if nodeAffinity was added to the mix as well.

 affinity:
  podAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - weight: 50
      podAffinityTerm:
        labelSelector:
          matchExpressions:
          - key: app.kubernetes.io/name
            operator: In
            values:
            - test-1
        topologyKey: kubernetes.io/hostname

  podAntiAffinity:
    preferredDuringSchedulingIgnoredDuringExecution:
    - weight: 100
      podAffinityTerm:
        labelSelector:
          matchExpressions:
          - key: app.kubernetes.io/name
            operator: In
            values:
            - test-2          
        topologyKey: kubernetes.io/hostname

Answer:

All affinity settings for both pods and nodes are weighed the same way if you don’t overwrite the default individual weight of 1.
During execution the occurrence of affinity conditions on a node is multiplied by the weight of the condition.

pod a has an affinity to b
pod a has an anti-affinity to c

new pod of type "a" to be scheduled
---
node1 runs 3 "b" pods and 1 "c" pod --> affinity score: 2 (3 "b" pods - 1 "c" pod)
node2 runs 1 "b" pod --> affinity score 1: 1 (1 "b" pod)

Based on the affinity calculation the pod will be scheduled on node1 despite it already running pod “c” as 3 “b” pods outweigh 1 “c” pod.
In your example the anti-affinity would outweigh the affinity settings if there is a 1:1 ratio of test-1 to test-2 on a node. With a 2:1 ratio of test-1 to test-2 on a node the (anti-)affinities would cancel each other out.
Source

If you have better answer, please add a comment about this, thank you!

Source: Stackoverflow.com

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