Set node_readiness_label default to an empty value. (#204)

Previously, it was set to the lifecycle-status:ready, breaking a
lot of minikube deployments. Also it was not possible befor to run
with this label set to an empty value.

Document the effect of the label in the new section of the
documentation.

README.md

@@ -148,6 +148,27 @@ spec:
 Please be aware that the taint and toleration only ensures that no other pod gets scheduled to a PostgreSQL node 
 but not that PostgreSQL pods are placed on such a node. This can be achieved by setting a node affinity rule in the ConfigMap.
 
+### Using the operator to minimize the amount of failovers during the cluster upgrade
+
+Postgres operator moves master pods out of to be decommissioned Kubernetes nodes. The decommission status of the node is derived
+from the presence of the set of labels defined by the `node_readiness_label` parameter. The operator makes sure that the Postgres
+master pods are moved elsewhere from the node that is pending to be decommissioned , but not on another node that is also
+about to be shut down. It achieves that via a combination of several properties set on the postgres pods:
+
+* [nodeAffinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#node-affinity-beta-feature) is configured to avoid scheduling the pod on nodes without all labels from the `node_readiness_label` set.
+* [PodDisruptionBudget](https://kubernetes.io/docs/concepts/workloads/pods/disruptions/#how-disruption-budgets-work) is defined to keep the master pods running until they are moved out by the operator.
+
+The operator starts moving master pods when the node is drained and doesn't have all labels from the `node_readiness_label` set.
+By default this parameter is set to an empty string, disabling this feature altogether. It can be set to a string containing one
+or more key:value parameters, i.e:
+```
+node_readiness_label: "lifecycle-status:ready,disagnostic-checks:ok"
+
+```
+
+when multiple labels are set the operator will require all of them to be present on a node (and set to the specified value) in order to consider
+it ready. 
+
 #### Custom Pod Environment Variables
 
 It is possible to configure a config map which is used by the Postgres pods as an additional provider for environment variables.

manifests/configmap.yaml

@@ -39,5 +39,5 @@ data:
   pod_terminate_grace_period: 5m
   pdb_name_format: "postgres-{cluster}-pdb"
   node_eol_label: "lifecycle-status:pending-decommission"
-  node_readiness_label: "lifecycle-status:ready"
+  node_readiness_label: ""
   team_api_role_configuration: "log_statement:all"

pkg/cluster/k8sres.go

@@ -238,6 +238,9 @@ PatroniInitDBParams:
 
 func (c *Cluster) nodeAffinity() *v1.Affinity {
 	matchExpressions := make([]v1.NodeSelectorRequirement, 0)
+	if len(c.OpConfig.NodeReadinessLabel) == 0 {
+		return nil
+	}
 	for k, v := range c.OpConfig.NodeReadinessLabel {
 		matchExpressions = append(matchExpressions, v1.NodeSelectorRequirement{
 			Key:      k,
@@ -431,10 +434,13 @@ func (c *Cluster) generatePodTemplate(
 		ServiceAccountName:            c.OpConfig.ServiceAccountName,
 		TerminationGracePeriodSeconds: &terminateGracePeriodSeconds,
 		Containers:                    []v1.Container{container},
-		Affinity:                      c.nodeAffinity(),
 		Tolerations:                   c.tolerations(tolerationsSpec),
 	}
 
+	if affinity := c.nodeAffinity(); affinity != nil {
+		podSpec.Affinity = affinity
+	}
+
 	if c.OpConfig.ScalyrAPIKey != "" && c.OpConfig.ScalyrImage != "" {
 		podSpec.Containers = append(
 			podSpec.Containers,

pkg/util/config/config.go

@@ -32,7 +32,7 @@ type Resources struct {
 	DefaultCPULimit         string            `name:"default_cpu_limit" default:"3"`
 	DefaultMemoryLimit      string            `name:"default_memory_limit" default:"1Gi"`
 	PodEnvironmentConfigMap string            `name:"pod_environment_configmap" default:""`
-	NodeReadinessLabel      map[string]string `name:"node_readiness_label" default:"lifecycle-status:ready"`
+	NodeReadinessLabel      map[string]string `name:"node_readiness_label" default:""`
 	MaxInstances            int32             `name:"max_instances" default:"-1"`
 	MinInstances            int32             `name:"min_instances" default:"-1"`
 }