Design WhatsApp
PremiumProduct architectureMedium
- End to end encryption
- WebSocket
- Delivery receipts
Quick answer
This product architecture interview question asks you to design WhatsApp. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Design WhatsApp, a messaging app that sends end-to-end encrypted messages between phones, shows sent, delivered and read ticks and delivers to people who were offline. The hard parts are holding hundreds of millions of open connections and never losing or duplicating a message while the server only sees ciphertext.
What must the system do?
- 1:1 messages: A user sends a text to a contact; it arrives within a second when both are online, in the order it was sent.
- Ticks: One tick when the server has the message, two when the recipient's device has it, blue when it is read (if read receipts are on).
How well must it do it?
- No loss, no duplicates: An acknowledged message reaches every recipient device and is shown exactly once.
- Low latency: p99 delivery under 1 second when both devices are online in the same region.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- End to end encryption
- WebSocket
- Delivery receipts
- Offline message queue
- Group fan out
- Push notifications
How you’ll practise
- Requirements
- Core entities
- API design
- Client and server flow
- Scale and edge cases
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any product architecture design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a product architecture question. You design WhatsApp and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team