🧠 MetaData Server for Distributed Databases

⚡ A simple distributed load balancer with consistent hashing and Redis-powered persistence for smart user-server mapping.
Mimics distributed system routing, supports server updates without full remapping.

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🏗️ Project Structure

loadbalancer/
├── src/                  # Java source files
│   ├── Main.java         # Entry point
│   ├── Router.java       # Routes HTTP requests
│   ├── Server.java       # TCP server logic
│   ├── ServerHandler.java# /addServer, /replaceServer endpoints
│   ├── DataHandler.java  # /registerUser, /cachereport endpoints
│   └── Redis.java        # Redis client setup via Redisson
├── Tester.sh             # Simulate server/user operations
├── request_test.sh       # Basic CURL testing
├── build.sh              # Compile + run helper
├── client_requests.log   # Logs requests
├── pom.xml               # Maven config
└── README.md             # This file

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⚙️ Prerequisites

• Java 17+
• Maven
• Redis (local or Docker)
• Internet (to download dependencies)

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🚀 Setup & Run Instructions

🔴 Step 1: Start Redis Locally

You can install Redis or use Docker:

Option A: System Install

sudo apt update
sudo apt install redis-server
redis-server

Option B: Using Docker

docker run -d -p 6379:6379 --name redis-server redis

🔨 Step 2: Compile & Run the Server

chmod +x build.sh
./build.sh
# Server starts on localhost:4000

📦 Step 3: Simulate Requests

chmod +x Tester.sh
./Tester.sh
# Adds 3 servers, registers users, replaces a server, prints Redis cache reports, saves logs to report-x.txt

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🔁 Redis Data Stored

• ServerCache: HashMap of <userHash, ServerNode>
• ServerList: Redis list of all active servers (for recovery after restart)

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📥 API Endpoints

Method	Endpoint	Description
POST	/addServer	Add a new backend server
POST	/replaceServer	Replace an existing server
POST	/registerUser	Assign user to a server
GET	/cachereport	Get all user-to-server mappings

Use userId in header when calling /registerUser

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🧠 Example CURL Commands

# Add a server
curl -X POST http://localhost:4000/addServer -H "Content-Type: text/plain" \
     -d "127.0.0.1:3001,Server1,user1,pass1"

# Register a user
curl -X POST http://localhost:4000/registerUser -H "userId: 101" -d ""

# View current cache report
curl http://localhost:4000/cachereport

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💾 Persistence Strategy

• 🗃️ Redis stores all mappings persistently.
• On server restart, the consistent hashing ring is rehydrated from Redis ServerList.
• Ensures user mappings remain stable across server crashes and restarts.

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🧪 Testing & Debugging

• Logs are printed to the console
• Each test run using Tester.sh generates a report-x.txt under the root directory
• Use redis-cli to view actual Redis data:

redis-cli
> HGETALL ServerCache
> LRANGE ServerList 0 -1

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🧰 Useful Tools

• RedisInsight (GUI for Redis)
• Postman or HTTPie for API testing

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🧠 Learning Concepts

• ✅ Consistent Hashing
• ✅ Fault-tolerant Redis mapping
• ✅ Server-aware routing
• ✅ Persistent recovery without database

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📜 License

MIT – Feel free to build on this foundation.

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✨ Author

Prem Sai – Backend & Distributed Systems Enthusiast
💡 For any questions, improvements or collabs – feel free to reach out!