How to use Kubernetes

Scaling a Flask App (Docker vs Kubernetes)

Step 1: Create Simple Flask App

Create file: app.py

from flask import Flask
import socket

app = Flask(__name__)

@app.route("/")
def hello():
    return f"Hello from {socket.gethostname()}"

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000)

Step 2: Dockerfile

Create file: Dockerfile

FROM python:3.9
WORKDIR /app
COPY . .
RUN pip install flask
CMD ["python", "app.py"]

Build image:

docker build -t flask-demo .

Run container:

docker run -d -p 5000:5000 flask-demo

Open browser:

http://localhost:5000

To scale we must manually run:

docker run -d -p 5001:5000 flask-demo
docker run -d -p 5002:5000 flask-demo

Problems:

• Manual scaling

• Manual load balancing

• Manual restart if crash

Step 3: Kubernetes Deployment

Enable Kubernetes (Docker Desktop or Minikube)

Create deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: flask-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: flask
  template:
    metadata:
      labels:
        app: flask
    spec:
      containers:
      - name: flask-container
        image: flask-demo
        imagePullPolicy: Never
        ports:
        - containerPort: 5000

Apply:

kubectl apply -f deployment.yaml

Check:

kubectl get pods

You’ll see 3 pods automatically created

Step 4: Create Service

Create service.yaml

apiVersion: v1
kind: Service
metadata:
  name: flask-service
spec:
  type: NodePort
  selector:
    app: flask
  ports:
    - port: 80
      targetPort: 5000
      nodePort: 30007

Apply:

kubectl apply -f service.yaml

Now open:

http://localhost:30007

Refresh multiple times → You’ll see different hostnames

That proves: Load balancing working automatically

Step 5: Auto Scaling Demo

Increase replicas:

kubectl scale deployment flask-deployment --replicas=5

Check:

kubectl get pods

Now 5 pods running

Docker cannot do this automatically.

Practical Example 2

Self-Healing Demo

Delete one pod manually:

kubectl delete pod <pod-name>

Now run:

kubectl get pods

You will see:Kubernetes automatically creates new pod.