Distributed OOD• Patterns: Observer, Strategy

WhatsApp / Chat System

Hard

Problem Summary

Design a chat application managing real-time connections, chat history, and dynamic user statuses.

Functional Scope

Deliver messages in real-time when recipient is online.
Queue messages for offline users and deliver them once they connect.
Support read receipts (Sent -> Delivered -> Read status).

Entity-Relationship (ER) Schema

User [1] <---> [*] UserConnection
User [1] <---> [*] ChatMessage
ChatMessage [1] <---> [1] MessageStatus

Design Approach

Protect the active connections directory using concurrent collections. Use message queues to isolate offline delivery logic from active socket threads.

Core Classes & Models

User (ID, active status, contacts)Message (Sender, Recipient, timestamp, contents, status)MessageService (Queues, routes messages)

Code Blueprint

public class Message {
    private String id;
    private String sender;
    private String recipient;
}