A distributed shopping prototype implemented in Java using UDP, TCP, and multithreading.
The system allows clients to register, search for items, negotiate prices, and complete purchases through a custom communication protocol. A persistent central server coordinates interactions between buyers and sellers.

• Custom peer-to-peer shopping protocol over UDP (control messages) and TCP (transactions)
• Asynchronous message delivery using separate sending/receiving threads
• Fully custom message format (CSV-like), serialization, and parsing utilities
• Hardcoded server and client ports for simplified discovery

• Server spawns a new thread for each incoming UDP message
• Client uses independent threads for:
  • CLI interaction
  • UDP reception
  • TCP inbound/outbound communication
  • Timers and expiration checks
• Concurrency control using synchronized blocks and volatile fields

• Multiple item lifecycle states:
  • lookingFor
  • negotiating
  • reserved
  • found
  • offering
  • buying
• Server-side persistence of:
  • Registered clients
  • Item lists
  • Current request number
• Custom serialization/deserialization for Items, Offers, and RegisteredClient

• Two-way TCP transaction between buyer and seller:
  • INFORM_REQ / INFORM_RES exchange
  • Payment validation
  • Shipping confirmation
• Independent sender/receiver threads for reliability

• Java
• UDP + TCP sockets
• Java concurrency (threads, synchronized, volatile)
• File-based persistence

├── src/
│   ├── Client.java
│   ├── Server.java
│   ├── Item.java
│   ├── Offer.java
│   ├── RegisteredClient.java
│   ├── OneShotUDPClient.java
│   ├── TCPClient.java
│   ├── TCPServer.java
│   └── Utils.java
├── project.jar
└── README.txt

• REGISTER
• DEREGISTER
• LOOKING_FOR
• SEARCH
• OFFER
• ACCEPT
• REFUSE
• FOUND
• RESERVE
• BUY
• CANCEL
• CANCEL_BUY
• NOT_AVAILABLE

• INFORM_REQ
• INFORM_RES
• PAYMENT_SUCCESSFUL
• SHIPPING_INFO

javac src/*.java -d out
jar cfe project.jar Main -C out .

java -jar project.jar

Choose server when prompted.

java -jar project.jar

Provide:

• Server IP
• Desired client UDP port
• Desired client TCP port

Available through the client CLI:

• Register
• Search for items
• Make or respond to offers
• Buy items
• Cancel transactions

Testing performed across:

1. Localhost (multiple terminals)
2. Home LAN (multiple computers)
3. University lab (separate physical machines)

Validated scenarios:

• Successful negotiation and purchase
• Offer timeout expiration
• Negotiation timeouts
• Concurrent buyers/sellers
• Cancelled transactions
• TCP binding issues on multi-homed systems
• Input interrupt handling using buffer + busy state

All test cases passed during the final demonstration.

• TCP client and server modules
• Inbound message handlers
• Timer logic (client & server)

• Outbound client message functions (register, offer, buy, cancel)
• Major server logic (message execution, negotiation, reservation, buy sequence)
• OneShotUDPClient implementation

• Item, Offer, RegisteredClient classes
• Serialization & persistency mechanisms
• Migration to global constants for protocol strings