Live Comments (FB Live)
Difficulty: Medium
What It Tests
High-throughput real-time fan-out, comment ordering, scaling WebSocket connections.
Topics Covered
- Real-time Updates
- Message Queues & Async Processing
- Database Sharding
- Consistent Hashing
- Wide-Column Stores (Cassandra)
- Stream Processing
Hello Interview Breakdown
Read the full Hello Interview breakdown →
Video Walkthroughs
Approach Hints
- Publish comments to Kafka (partitioned by stream_id)
- WebSocket servers subscribe to stream partitions via Kafka consumer groups
- Persist comments to Cassandra (partition by stream_id, cluster by timestamp)
- Use consistent hashing to assign streams to WebSocket server groups; rate-limit comments per user to prevent spam