# Assignment 2 - Kubernetes

## Application description
This assignment demonstrates how to deploy a simple application in Kubernetes.

The application consists of:
- a frontend
- a backend
- a PostgreSQL database

The frontend displays a simple web page.
The backend provides a simple Flask service.
The database runs in PostgreSQL with persistent storage.

## Kubernetes objects used

### Namespace
- `z2app`

The namespace is used to isolate all resources of the application.

### Deployments
- `frontend-deployment`
- `backend-deployment`

The frontend and backend are deployed using `Deployment` because they are stateless components.

### StatefulSet
- `postgres`

The PostgreSQL database is deployed using `StatefulSet` because it requires persistent storage.

### Services
- `frontend-service`
- `backend-service`
- `postgres-service`

Services are used for communication between components and for exposing the frontend.

### Persistent storage
- `PersistentVolume` (`postgres-pv`)
- `PersistentVolumeClaim` (`postgres-pvc`)

These resources are used to store PostgreSQL data persistently.

## Container images used
- `nginx:alpine` for frontend
- `python:3.11-slim` for backend
- `postgres:15` for database

## Application architecture
- Frontend runs on port 80
- Backend runs on port 5000
- PostgreSQL runs on port 5432

The frontend is exposed through `frontend-service`.
The backend is available through `backend-service`.
The database is available through `postgres-service`.

## Files included
- `deployment.yaml`
- `service.yaml`
- `statefulset.yaml`
- `prepare-app.sh`
- `start-app.sh`
- `stop-app.sh`
- `README.md`

## How to prepare the application

Run:

./prepare-app.sh

## How to start the application

Run:

./start-app.sh

## How to stop the application

Run:

./stop-app.sh

## How to verify that the application is running

# Check the pods:

kubectl get pods -n z2app

# Check the services:

kubectl get svc -n z2app

# Check persistent storage:

kubectl get pv
kubectl get pvc -n z2app

## How to access the frontend

# Use port-forward:

kubectl port-forward -n z2app service/frontend-service 8080:80

# Then open in browser:

http://localhost:8080

## How to access the backend

# Use port-forward:

kubectl port-forward -n z2app service/backend-service 5000:5000

# Then test:

curl http://localhost:5000
curl http://localhost:5000/health

## Notes

When the application starts, pods may first appear as ContainerCreating or Pending.
After a few seconds they should become Running.

## Summary

This assignment shows how to deploy a multi-component application in Kubernetes using:

# Namespace
# Deployment
# StatefulSet
# Service
# PersistentVolume
# PersistentVolumeClaim