Design a mobile banking app
PremiumMobile systemMedium
- Biometric authentication
- Android keystore
- Keychain secure enclave
Quick answer
This mobile system interview question asks you to design a mobile banking app. 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 banking app for a retail bank on iOS and Android. Customers check balances, view statements, add payees and send money. The hard parts are fast biometric login with keys bound to the phone, short sessions, extra checks for risky actions, transfers that never run twice and fast security fixes.
The core banking system and its API are a given. Design the app.
What must a mobile banking app do?
- Quick login: After a one-time password login, customers log in with fingerprint or face in under 2 seconds.
- Accounts: View balances, recent transactions and statements.
How well must it do it?
- Device-bound keys: Keys never leave the phone's secure hardware; a stolen token alone is useless on another device.
- Never transfer twice: Retries and app kills never send money twice.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Biometric authentication
- Android keystore
- Keychain secure enclave
- Oauth 2.0 refresh tokens
- Certificate pinning
- Step up authentication
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 banking app 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