Design a mobile chat app like WhatsApp
PremiumMobile systemMedium
- Offline first messaging
- Message outbox
- Notification service extension
Quick answer
This mobile system interview question asks you to design a mobile chat app like 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 a mobile chat app like WhatsApp for iOS and Android. It keeps every chat on the phone, sends messages typed offline as soon as the network returns and shows sent, delivered and read ticks. The hard parts are reliable delivery on flaky networks and receiving messages while the OS keeps the app suspended.
Messages are end-to-end encrypted, so the server cannot read them and push payloads carry ciphertext only. The backend is a given API; design the app.
What must a mobile chat app like WhatsApp do?
- Offline send: A message typed with no network shows at once with a clock icon and is sent automatically, in order, when the connection returns.
- Delivery ticks: Each outgoing message moves through pending, sent, delivered and read, updated live.
How well must it do it?
- No lost or duplicate messages: A message survives the app being killed mid-send and appears exactly once for both people, even after retries.
- Instant UI: The chat list renders from local data in under 300 ms; typing and sending never wait for the network.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Offline first messaging
- Message outbox
- Notification service extension
- FCM high priority messages
- End to end encryption
- Sqlite local store
How you’ll practise
- Requirements
- App architecture
- Data and offline sync
- Networking and push
- Performance, battery and releases
- 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 mobile system 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 mobile system question. You design a mobile chat app like 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