Decommission a Specific Pod in KubeBlocks-Managed MongoDB Clusters

This guide explains how to decommission (take offline) specific Pods in MongoDB 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 MongoDB Cluster

KubeBlocks uses a declarative approach for managing MongoDB Replication Clusters. Below is an example configuration for deploying a MongoDB ReplicaSet Cluster with one primary replica and two secondary replicas.

Apply the following YAML configuration to deploy the cluster:

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
metadata:
  name: mongo-cluster
  namespace: demo
spec:
  terminationPolicy: Delete
  clusterDef: mongodb
  topology: replicaset
  componentSpecs:
    - name: mongodb
      serviceVersion: "6.0.16"
      replicas: 3
      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

Verifying the Deployment

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

kubectl get cluster mongo-cluster -n demo -w

Expected Output:

kubectl get cluster mongo-cluster -n demo
NAME            CLUSTER-DEFINITION   TERMINATION-POLICY   STATUS     AGE
mongo-cluster   mongodb              Delete               Creating   49s
mongo-cluster   mongodb              Delete               Running    62s

Check the pod status and roles:

kubectl get pods -l app.kubernetes.io/instance=mongo-cluster -L  kubeblocks.io/role -n demo

Expected Output:

NAME                      READY   STATUS    RESTARTS   AGE   ROLE
mongo-cluster-mongodb-0   2/2     Running   0          78s   primary
mongo-cluster-mongodb-1   2/2     Running   0          63s   secondary
mongo-cluster-mongodb-2   2/2     Running   0          48s   secondary

Once the cluster status becomes Running, your MongoDB 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., 'mongo-cluster-mongodb-1'), you can use one of the following methods:

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

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

NAME                             TYPE                CLUSTER         STATUS    PROGRESS   AGE
mongo-cluster-decommission-ops   HorizontalScaling   mongo-cluster   Succeed   1/1        5s

apiVersion: apps.kubeblocks.io/v1
kind: Cluster
spec:
  componentSpecs:
    - name: mongodb
      replicas: 1       # explected replicas after decommission
      offlineInstances:
        - mongo-cluster-mongodb-1   # <----- Specify Pod to be decommissioned
 ...

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=mongo-cluster

Example Output:

NAME                      READY   STATUS    RESTARTS   AGE
mongo-cluster-mongodb-0   2/2     Running   0          25m
mongo-cluster-mongodb-2   2/2     Running   0          24m

Login to MongoDB replica and check :

# login to any mongodb replica:
mongo-cluster-mongodb [direct: secondary] admin> rs.status()

Verify the change in members.

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.