inject-node-affinity.yaml

apiVersion: generic-webhook/v1beta1
kind: GenericWebhookConfig
webhooks:
  - name: patch-node-affinity
    path: /patch-node-affinity
    actions:
      # If the pod doesn't have any node affinity that involves the `myorg.io/instance-cost`
      # label, then we inject a preferred node affinity for the nodes that have
      # the value `cheap` for the `myorg.io/instance-cost` label
      - condition:
          and:
            # We are analysing a Pod
            - .kind == "Pod"
            # {key: myorg.io/instance-cost} doesn't appear in preferredDuringSchedulingIgnoredDuringExecution
            - all: .spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution.*.preference.matchExpressions.* -> .key != "myorg.io/instance-cost"
            # {key: myorg.io/instance-cost} doesn't appear in requiredDuringSchedulingIgnoredDuringExecution
            - all: .spec.affinity.nodeAffinity.requiredDuringSchedulingIgnoredDuringExecution.nodeSelectorTerms.*.matchExpressions.* -> .key != "myorg.io/instance-cost"

        patch:
          # Inject a preferred node affinity for the nodes that have the label
          # `myorg.io/instance-cost = cheap`
          - op: add
            path: .spec.affinity.nodeAffinity.preferredDuringSchedulingIgnoredDuringExecution
            value:
              preference:
                matchExpressions:
                  - key: myorg.io/instance-cost
                    operator: In
                    values:
                      - cheap
              weight: 1