Add Pod Anti Affinity (#489)

* Add Pod Anti Affinity

docs/administrator.md

@@ -151,6 +151,36 @@ Postgres pods by default receive tolerations for `unreachable` and `noExecute` t
 Depending on your setup, you may want to adjust these parameters to prevent master pods from being evicted by the Kubernetes runtime.
 To prevent eviction completely, specify the toleration by leaving out the `tolerationSeconds` value (similar to how Kubernetes' own DaemonSets are configured)
 
+### Enable pod anti affinity
+
+To ensure Postgres pods are running on different topologies, you can use [pod anti affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/)
+and configure the required topology in the operator ConfigMap.
+
+Enable pod anti affinity by adding following line to the operator ConfigMap:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: postgres-operator
+data:
+  enable_pod_antiaffinity: "true"
+```
+
+By default the topology key for the pod anti affinity is set to `kubernetes.io/hostname`,
+you can set another topology key e.g. `failure-domain.beta.kubernetes.io/zone` by adding following line
+to the operator ConfigMap, see [built-in node labels](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#interlude-built-in-node-labels) for available topology keys:
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: postgres-operator
+data:
+  enable_pod_antiaffinity: "true"
+  pod_antiaffinity_topology_key: "failure-domain.beta.kubernetes.io/zone"
+```
+
 ### Add cluster-specific labels
 
 In some cases, you might want to add `labels` that are specific to a given

docs/reference/operator_parameters.md

@@ -213,6 +213,14 @@ configuration they are grouped under the `kubernetes` key.
   that should be assigned to the Postgres pods. The priority class itself must be defined in advance.
   Default is empty (use the default priority class).
 
+* **enable_pod_antiaffinity**
+  toggles [pod anti affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/) on the Postgres pods, to avoid multiple pods
+  of the same Postgres cluster in the same topology , e.g. node. The default is `false`.
+
+* **pod_antiaffinity_topology_key**
+  override
+  [topology key](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#interlude-built-in-node-labels)
+  for pod anti affinity. The default is `kubernetes.io/hostname`.
 
 ## Kubernetes resource requests