Decommission a Specific Pod in KubeBlocks-Managed PostgreSQL Clusters

This guide explains how to decommission (take offline) specific Pods in PostgreSQL clusters managed by KubeBlocks. Decommissioning provides precise control over cluster resources while maintaining availability. Use this for workload rebalancing, node maintenance, or addressing failures.

Why Decommission Pods with KubeBlocks?

In traditional StatefulSet-based deployments, Kubernetes lacks the ability to decommission specific Pods. StatefulSets ensure the order and identity of Pods, and scaling down always removes the Pod with the highest ordinal number (e.g., scaling down from 3 replicas removes Pod-2 first). This limitation prevents precise control over which Pod to take offline, which can complicate maintenance, workload distribution, or failure handling.

KubeBlocks overcomes this limitation by enabling administrators to decommission specific Pods directly. This fine-grained control ensures high availability and allows better resource management without disrupting the entire cluster.

Prerequisites

Before proceeding, ensure the following:

• Environment Setup:
  • A Kubernetes cluster is up and running.
  • The kubectl CLI tool is configured to communicate with your cluster.
  • KubeBlocks CLI and KubeBlocks Operator are installed. Follow the installation instructions here.
• Namespace Preparation: To keep resources isolated, create a dedicated namespace for this tutorial:

kubectl create ns demo
namespace/demo created

Deploy a PostgreSQL Cluster

KubeBlocks uses a declarative approach for managing PostgreSQL clusters. Below is an example configuration for deploying a PostgreSQL cluster with 2 replicas (1 primary, 1 replicas).

Apply the following YAML configuration to deploy the cluster:

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
metadata:
  name: pg-cluster
  namespace: demo
spec:
  terminationPolicy: Delete
  clusterDef: postgresql
  topology: replication
  componentSpecs:
    - name: postgresql
      serviceVersion: 16.4.0
      disableExporter: true
      replicas: 2
      resources:
        limits:
          cpu: "0.5"
          memory: "0.5Gi"
        requests:
          cpu: "0.5"
          memory: "0.5Gi"
      volumeClaimTemplates:
        - name: data
          spec:
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 20Gi

Verifying the Deployment

Monitor the cluster status until it transitions to the Running state:

kubectl get cluster pg-cluster -n demo -w

Expected Output:

NAME         CLUSTER-DEFINITION   TERMINATION-POLICY   STATUS     AGE
pg-cluster   postgresql           Delete               Creating   50s
pg-cluster   postgresql           Delete               Running    4m2s

Once the cluster status becomes Running, your PostgreSQL cluster is ready for use.

Decommission a Pod

Expected Workflow:

1. Replica specified in onlineInstancesToOffline is removed
2. Pod terminates gracefully
3. Cluster transitions from Updating to Running

To decommission a specific Pod (e.g., 'pg-cluster-postgresql-1'), you can use one of the following methods:

apiVersion: operations.kubeblocks.io/v1alpha1
kind: OpsRequest
metadata:
  name: pg-cluster-decommission-ops
  namespace: demo
spec:
  clusterName: pg-cluster
  type: HorizontalScaling
  horizontalScaling:
  - componentName: postgresql
    scaleIn:
      onlineInstancesToOffline:
        - 'pg-cluster-postgresql-1'  # Specifies the instance names that need to be taken offline

kubectl get ops pg-cluster-decommission-ops -n demo -w

NAME                          TYPE                CLUSTER      STATUS    PROGRESS   AGE
pg-cluster-decommission-ops   HorizontalScaling   pg-cluster   Succeed   1/1        33s

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
metadata:
  name: pg-cluster
  namespace: demo
spec:
  terminationPolicy: Delete
  clusterDef: postgresql
  topology: replication
  componentSpecs:
    - name: postgresql
      serviceVersion: 16.4.0
      disableExporter: true
      replicas: 1
      offlineInstances:
        - pg-cluster-postgresql-1   # <----- Specify Pod to be decommissioned
      resources:
        limits:
          cpu: '0.5'
          memory: 0.5Gi
        requests:
          cpu: '0.5'
          memory: 0.5Gi
      volumeClaimTemplates:
        - name: data
          spec:
            storageClassName: ""
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 20Gi

Verify the Decommissioning

After applying the updated configuration, verify the remaining Pods in the cluster:

kubectl get pods -n demo -l app.kubernetes.io/instance=pg-cluster

Example Output:

NAME                      READY   STATUS    RESTARTS   AGE
pg-cluster-postgresql-0   4/4     Running   0          6m12s

Summary

Key takeaways:

• Traditional StatefulSets lack precise Pod removal control
• KubeBlocks enables targeted Pod decommissioning
• Two implementation methods: OpsRequest or Cluster API

This provides granular cluster management while maintaining availability.