WhatsApp Behind the Scenes: Key Components, Data Flow and Design Trade-offs
Company: Meta
Role: Product Manager
Category: System Design
Difficulty: medium
Interview Round: Other
Design the high-level system architecture behind WhatsApp, the mobile messaging app. This is a technical round for a product manager, so focus on the main components, how data moves between them and the trade-offs behind your choices, rather than on code.
```hint Follow one message
Trace a single message from the moment the sender taps send to the moment the recipient's phone shows it as read, and ask what has to happen if the recipient's phone is switched off.
```
### Clarifying Questions
- Which features are in scope: one-to-one and group text only, or also photos and videos, voice and video calls, and status updates?
- What scale should the design support: today's global user base or an early version of the product?
- Must messages be end-to-end encrypted, so that the servers never see their content?
- Should chat history survive a lost or broken phone, and if so, where should it be kept?
### What a Strong Answer Covers
- Functional and non-functional requirements, with a rough estimate of scale
- The core components and the full path of a message, including receipts and recipients who are offline
- Choices for connections, storage, group delivery and media, each with its trade-off
- The security and privacy model: what the servers can see and how chat history is kept
- Reliability and growth: failures, ordering, duplicates, multiple regions, and the signals used to watch system health
### Follow-up Questions
- A user's message shows as sent but not delivered for hours, although the recipient is online. How would you track down the cause?
- How does the design change for a group with several hundred members compared with a one-to-one chat?
- As the PM for messaging, which metrics would you put on a dashboard to know the service is healthy?
- Suppose the product is Facebook Messenger instead of WhatsApp: full chat history has to stay in sync across phone, desktop and web, and end-to-end encryption is not assumed by default. What changes in storage, sync and the security model?
Overview: A technical system design question for product managers: sketch the architecture behind WhatsApp and follow a message from sender to recipient, including when the recipient is offline. It tests scoping and scale estimates, component choices and their trade-offs, security and privacy, failure handling and the health metrics a PM should watch.