Design A WhatsApp-Style Messaging System
Company: Whatnot
Role: Software Engineer
Category: System Design
Difficulty: medium
Interview Round: Technical Screen
Design a messaging system similar to WhatsApp. Users should send one-to-one and group messages, receive messages in near real time, and recover message history after reconnecting.
```hint Hint 1
Start by naming the core entities, constraints, and success criteria.
```
```hint Hint 2
Make the trade-offs explicit before going deep on implementation details.
```
### Constraints & Assumptions
- The design should cover core messaging, not every social feature.
- Messages must be durable.
- Online users should receive low-latency delivery.
- Offline users should receive messages when they reconnect.
### Clarifying Questions to Ask
- Are end-to-end encryption and media attachments required?
- What scale should be assumed for daily active users and messages per second?
- Are read receipts and typing indicators required?
- What ordering guarantees are needed within a conversation?
- How long should message history be retained?
### What a Strong Answer Covers
```premium-lock What a Strong Answer Covers
```
### Follow-up Questions
- How would you add read receipts?
- How would you handle a user sending the same message twice?
- How would you scale a group with thousands of members?
- What changes for end-to-end encryption?
Overview: Study a WhatsApp-style messaging system design prompt covering durable message storage and near real-time delivery. The question targets conversation models, WebSocket fanout, offline sync, ordering, idempotency, group messaging, and operational metrics.