# Battleship Application on Kubernetes

## Overview
This application implements a Battleship game system composed of three main components:
- **Backend:** Handles game logic and communicates with the database.
- **Frontend:** Provides the web user interface for playing the game.
- **MySQL Database:** Stores game data persistently.

The application is deployed in a Kubernetes cluster using Deployments, a StatefulSet with persistent storage, and Services for network exposure.

## Containers Used
- **battleship-backend:**  
  - **Description:** Contains the backend logic for the game.  
  - **Port:** Listens on port 4000.  
  - **Configuration:** Uses environment variables to connect to the MySQL database.

- **battleship-frontend:**  
  - **Description:** Hosts the web user interface for the game.  
  - **Port:** Listens on port 80.  
  - **Exposure:** Exposed via a NodePort (30001).

- **MySQL (battleship-mysql):**  
  - **Description:** Runs a MySQL database (version 8.0.30) in a StatefulSet.  
  - **Port:** Uses port 3306.  
  - **Persistence:** Uses a PersistentVolume (PV) and PersistentVolumeClaim (PVC) for data storage.

## Kubernetes Objects
- **Namespace:**  
  - **battleship-app**  
    All objects (Deployments, StatefulSet, Services, PV, PVC) are created within this namespace.

- **Deployments:**  
  - **battleship-backend:** Manages the backend pods.  
  - **battleship-frontend:** Manages the frontend pods.

- **StatefulSet:**  
  - **battleship-mysql:** Manages the MySQL pods with stable identities and persistent storage. It includes:  
    - **PersistentVolume (mysql-pv):** Provides 1Gi storage (using a hostPath at `/mnt/data/mysql`).  
    - **PersistentVolumeClaim (mysql-pvc):** Claims the persistent storage for MySQL.

- **Services:**  
  - **MySQL Service:** Exposes the MySQL StatefulSet on port 3306.  
  - **Backend Service:** Exposes the backend on port 4000 (via NodePort 30000).  
  - **Frontend Service:** Exposes the frontend on port 80 (via NodePort 30001).

## Virtual Networks and Named Volumes
- **Virtual Networks:**  
  Kubernetes provides internal networking for the cluster, and Services offer stable endpoints for communication between pods.  
  *Note:* In the original Docker Compose configuration, a network named `app-net` was defined; in Kubernetes, the cluster’s internal network is used.

- **Named Volumes:**  
  The MySQL StatefulSet uses a PersistentVolume and a PersistentVolumeClaim to ensure data persistence. This setup guarantees that data (e.g., the game database) survives pod restarts or rescheduling.

## Container Configuration
- **Environment Variables:**  
  - **Backend Container:**  
    Configured with variables such as `DB_HOST`, `DB_PORT`, `DB_USER`, `DB_PASSWORD`, and `DB_NAME` to connect to the MySQL database.  
  - **MySQL Container:**  
    Configured with variables `MYSQL_ROOT_PASSWORD` and `MYSQL_DATABASE` to initialize the database.

- **Ports:**  
  - **Backend:** Container port 4000, exposed on NodePort 30000.  
  - **Frontend:** Container port 80, exposed on NodePort 30001.  
  - **MySQL:** Container port 3306, exposed internally on port 3306.

- **Image Pull Policy:**  
  The backend and frontend Deployments use `imagePullPolicy: Never` assuming images are built locally.

## How to Prepare, Run, Pause, and Remove the Application

### Preparation
1. **Build Docker Images:**  
   Execute the `prepare-app.sh` script to build the Docker images for the backend and frontend:
   ```bash
   ./prepare-app.sh
   ```

### Running the Application
1. **Deploy the Application:**  
   Use the `start-app.sh` script to deploy the namespace, Deployments, StatefulSet, and Services:
   ```bash
   ./start-app.sh
   ```

### Pausing the Application
1. **Scale Down the Application:**  
   If you need to temporarily pause the application without deleting objects (thus preserving the data), use:
   ```bash
   ./stop-app.sh
   ```
   *Note:* This script scales the Deployments and StatefulSet to 0 replicas.

### Removing the Application
1. **Delete All Kubernetes Objects:**  
   To completely remove the application—including Deployments, StatefulSet, Services, and the Namespace—execute:
   ```bash
   ./remove-app.sh
   ```
   *Important:* Deleting the objects may also remove the associated PersistentVolume depending on its reclaim policy. To preserve data, consider setting the PersistentVolume's reclaim policy to `Retain`.

## How to Access the Application in a Web Browser
- **Frontend Access:**  
  The frontend is exposed via a NodePort on port **30001**.  
  Open a web browser and navigate to:
  ```
  http://<Node-IP>:30001
  ```
  Replace `<Node-IP>` with the IP address of one of your Kubernetes nodes.