Add Pod Anti-Affinity

 1apiVersion: kyverno.io/v1
 2kind: ClusterPolicy
 3metadata:
 4  name: insert-pod-antiaffinity
 5  annotations:
 6    policies.kyverno.io/title: Add Pod Anti-Affinity
 7    policies.kyverno.io/category: Sample
 8    policies.kyverno.io/subject: Deployment, Pod
 9    policies.kyverno.io/description: >-
10      Applications may involve multiple replicas of the same Pod for availability as well as scale
11      purposes, yet Kubernetes does not by default provide a solution for availability. This policy
12      sets a Pod anti-affinity configuration on Deployments which contain an `app` label if it is
13      not already present.      
14spec:
15  rules:
16    - name: insert-pod-antiaffinity
17      match:
18        resources:
19          kinds:
20            - Deployment
21      preconditions:
22        # This precondition selects Pods with the label `app`
23      - key: "{{request.object.spec.template.metadata.labels.app}}"
24        operator: NotEquals
25        value: ""
26      # Mutates the Deployment resource to add fields.
27      mutate:
28        patchStrategicMerge:
29          spec:
30            template:
31              spec:
32                # Add the `affinity`if not already specified.
33                +(affinity):
34                  +(podAntiAffinity):
35                    +(preferredDuringSchedulingIgnoredDuringExecution):
36                      - weight: 1
37                        podAffinityTerm:
38                          topologyKey: "kubernetes.io/hostname"
39                          labelSelector:
40                            matchExpressions:
41                            - key: app
42                              operator: In
43                              values:
44                              - "{{request.object.spec.template.metadata.labels.app}}"