📚 Main Topics

1. Functional Requirements of a Chat Application

   • One-on-one chat
   • Group chat functionality
   • Read receipts (double tick marks)
   • Online status sharing
   • Notification system for new messages
   • Multimedia sharing (images, videos, files)

2. Technical Requirements

   • Low latency for real-time communication
   • High reliability and availability
   • Support for both mobile and desktop clients
   • Storage for chat history
   • Handling large amounts of data (text, photos, videos)
   • End-to-end encryption
   • Client-initiated connections using WebSocket protocol

3. System Design Components

   • Messaging ServiceHandles sending and receiving messages.
   • Relay ServiceTemporarily stores messages for offline users.
   • Session ServiceManages user connections and server mappings.
   • Last Seen ServiceTracks user activity timestamps.
   • Asset ServiceManages multimedia file storage and retrieval.
   • Group Messaging ServiceManages messages sent to groups.

4. Database Schema

   • Users table: user ID, username, contact number
   • Groups table: group ID, user IDs of group members
   • Unsent messages table: message ID, sender ID, receiver ID, content/media URL, timestamp
   • Last seen table: user ID, timestamp
   • Sessions table: user ID, server ID mapping

5. Group Messaging Logic

   • Uses Kafka for message queuing and handling.
   • Group message handler retrieves user connections and sends messages accordingly.

6. Media File Handling

   • Media files are compressed and encrypted on the sender's device.
   • Files are uploaded to a server, and links are sent to the receiver for download.

✨ Key Takeaways

• Real-time chat applications require careful consideration of both functional and technical requirements.
• WebSocket protocol is preferred for real-time communication due to its persistent connection capabilities.
• Proper capacity planning is essential, especially when dealing with large user bases and high message volumes.
• A hybrid database approach may be necessary to accommodate different data types and relationships.

🧠 Lessons Learned

• There is no one-size-fits-all design; flexibility and adaptability to requirements are crucial.
• Understanding the underlying technologies (like WebSockets and Kafka) is important for implementing efficient systems.
• Security measures, such as end-to-end encryption, are vital in protecting user data in chat applications.
• Effective communication and clarification of requirements with stakeholders (like interviewers) can lead to better design outcomes.