Stateless vs Stateful Applications in Kubernetes

This blog will show you what makes them different and when to use each type in Kubernetes. Some programs remember things, while others don’t. We call these two kinds “stateful” and “stateless” apps. It’s like choosing the right tool for the job, depending on whether your app needs a good memory or can live in the moment. Let’s explore these ideas together!

Stateless Applications

In Kubernetes, stateless applications are applications that don’t rely on storing persistent data on the nodes they run on. They can be easily replaced or scaled up or down without any impact on their functionality. Kubernetes can simply start a new container to take its place. Stateless applications are designed to be highly scalable, easily replaceable, and resilient to failures. They can be created, terminated, or scaled up or down without concerns about data loss or disruptions to the application’s functionality.

Stateful Applications

On the other hand, Stateful applications need persistent storage to store their data that must persist across instances and even container restarts. Kubernetes provides features like StatefulSets and Persistent Volumes. StatefulSets allow you to maintain a stable identity for pods and ensure ordered scaling and deployment. Persistent Volumes provide a way to manage persistent storage for these applications. Stateful applications are commonly used for databases and other services that need to keep track of data and state over time.

Deploying a Stateful Application in kubernetes

Below is an example of deploying a stateful database application using a StatefulSet and a PersistentVolumeClaim (PVC).

1. Create a Statefulset YAML:

https://kubernetes.io/docs/concepts/workloads/controllers/statefulset.

				
					apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: postgres
spec:
  serviceName: postgres-svc
  selector:
    matchLabels:
      app: postgres
  replicas: 3
  template:
    metadata:
      labels:
        app: postgres
    spec:
      containers:
        - name: postgres
          image: postgres
          imagePullPolicy: IfNotPresent
          env:
            - name: POSTGRES_USER
              valueFrom:
                secretKeyRef:
                  name: postgres
                  key: username
            - name: POSTGRES_PASSWORD
              valueFrom:
                secretKeyRef:
                  name: postgres
                  key: password
          ports:
            - containerPort: 5432
          volumeMounts:
            - name: data
              mountPath: /var/lib/postgres/data
      volumes:
        - name: data
          persistentVolumeClaim:
            claimName: postgres-volume-claim

				
					# Apply the PVC to your kubernetes cluster
kubectl apply -f db-statefulset.yaml

2. Create a PersistentVolumeClaim (PVC) YAML:

				
					apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: postgres-volume-claim
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 8Gi

				
					# Apply the PVC to your kubernetes cluster
kubectl apply -f database-pvc.yaml

3. Create a Service YAML:

				
					apiVersion: v1
kind: Service
metadata:
   name: postgres-SVC
   labels:
     app: postgres
spec:
   selector:
     app: postgres
   ports:
     - protocol: TCP
       name: http
       port: 5432
       targetPort: 5432

				
					# Apply the service file
kubectl apply -f service.yaml

Keep in mind that the actual YAML files may need customization based on your application and storage requirements. Stateful applications often require more detailed configuration to handle data persistence and unique pod identities.

When to Use Stateful & Stateless Applications

1. Stateful Applications:

When your application requires persistent data: If your application needs to store and manage data that should survive container restarts or rescheduling, such as databases, file storage, or messaging systems, you should use a stateful application. StatefulSets in Kubernetes are designed for this purpose.
When your application needs stable network identities: Stateful applications typically have stable network identities, like hostnames, which are important for database replication, clustering, or other distributed systems where each instance has a specific role. StatefulSets provide ordered deployment and scaling for such workloads.

2. Stateless Applications:

When your application doesn’t need to retain data: Stateless applications are designed to be stateless, meaning they don’t rely on saving data locally. They treat each request or transaction independently, and you can lose a container instance without affecting the application’s overall functionality.
When you want easy scaling and load balancing: Stateless applications are more suitable for horizontal scaling. Kubernetes can easily distribute incoming traffic among multiple instances of stateless applications using Services and load balancing.
When you prefer simplicity and flexibility: Stateless applications are simpler to manage and scale in Kubernetes. They are often more flexible and easier to deploy, making them a good choice for web servers, microservices, and other stateless workloads.

Conclusion

Kubernetes provides the flexibility to deploy and manage both stateful and stateless applications, allowing you to make informed choices that align with your needs. In the world of Kubernetes and container orchestration, the choice between stateful and stateless applications is not one-size-fits-all. Instead, it’s a decision that should be carefully considered based on your specific application requirements and use cases. Its flexibility and feature set allow you to tailor your infrastructure to the unique requirements of your workloads, ensuring that your applications run efficiently and reliably in the containerized world.