Messaging App (WhatsApp)
Difficulty: Medium
What It Tests
Real-time messaging, delivery receipts, group chat, media sharing.
Topics Covered
- Real-time Updates
- Message Queues & Async Processing
- Database Design: SQL vs NoSQL
- Blob / Object Storage
- Networking Essentials
- Wide-Column Stores (Cassandra)
Hello Interview Breakdown
Read the full Hello Interview breakdown →
Approach Hints
- Persistent WebSocket connection per client
- Store messages in Cassandra (partitioned by conversation_id + timestamp)
- Media uploaded to S3 and linked in the message
- Delivery states (sent/delivered/read) stored in Redis, push notifications via APNs/FCM for offline users