Understanding Kubernetes: Part 7 -StatefulSet

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Understanding Kubernetes: Part 7 -StatefulSet

If you’ve been following our Kubernetes series 2025, welcome back! For new readers, check out Part 6:

What is a StatefulSet in Kubernetes?

A StatefulSet in Kubernetes is a resource used to manage stateful applications that require stable, unique network identities and persistent storage. Unlike a Deployment or ReplicaSet, a StatefulSet ensures the pods are created and scaled in a predictable order, making it ideal for applications like databases, distributed systems, and queues.

For example:

If you are deploying a distributed database like Cassandra or a message queue like RabbitMQ, each pod in the StatefulSet needs a unique identity and persistent storage to maintain its state even if the pod restarts. A StatefulSet ensures that each pod gets a consistent hostname and access to its own storage volume.

StatefulSet Capabilities:

Stable Network Identity: Each pod has a predictable name (e.g., app-0, app-1).
Persistent Storage: Each pod gets its own persistent volume, which is retained even after the pod is deleted.
Ordered Scaling and Updates: Pods are created, deleted, and updated in a defined sequence.
Resilience: Ensures data consistency and availability in stateful workloads.

In my previous role:

As a Senior DevOps Engineer, I used StatefulSets to deploy a MongoDB cluster in our Kubernetes environment. Each pod required a unique hostname for the replica set configuration and its own persistent volume to store data. The StatefulSet ensured that the MongoDB pods (mongo-0, mongo-1, mongo-2) were created in order, with each pod accessing its dedicated storage. During a scale-up, the StatefulSet added new pods sequentially, maintaining the application’s stability. This setup helped us achieve high availability and data resilience for our backend services.

Here’s a simple YAML for deploying a StatefulSet:

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: mongo
spec:
  serviceName: mongo-service
  replicas: 3
  selector:
    matchLabels:
      app: mongo
  template:
    metadata:
      labels:
        app: mongo
    spec:
      containers:
      - name: mongo
        image: mongo:5.0
        ports:
        - containerPort: 27017
        volumeMounts:
        - name: mongo-data
          mountPath: /data/db
  volumeClaimTemplates:
  - metadata:
      name: mongo-data
    spec:
      accessModes: ["ReadWriteOnce"]
      resources:
        requests:
          storage: 10Gi

This StatefulSet ensures a three-node MongoDB cluster with stable identities and persistent storage for each pod, making it a robust solution for managing stateful applications in Kubernetes.

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