Design a mobile a/b testing SDK
PremiumMobile systemMedium
- Deterministic bucketing
- Feature flags
- Exposure logging
Quick answer
This mobile system interview question asks you to design a mobile a/b testing SDK. 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 A/B testing SDK for iOS and Android, like the experiment tools in Firebase or Optimizely. App teams use it to show different variants to users and to switch features on and off remotely. The hard parts are stable assignment, no screen changing in front of the user, correct exposure logging and turning off a bad feature fast.
What must a mobile a/b testing SDK do?
- Read a variant: getVariant("checkout_button") returns the user's variant, or a default if anything fails.
- Feature flags: isEnabled("new_onboarding") switches features on or off without a release.
How well must it do it?
- Stable: A user keeps the same variant across sessions and restarts unless the experiment changes.
- No flicker: A screen never switches variants while the user is looking at it.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Deterministic bucketing
- Feature flags
- Exposure logging
- Config caching
- Flicker free activation
- Kill switch
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 a/b testing SDK 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