Design wallpaper sync across devices
PremiumMobile systemMedium
- Background prefetch
- Resumable downloads
- HTTP range requests
Quick answer
This mobile system interview question asks you to design wallpaper sync across devices. 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 wallpaper sync across devices as a built-in feature of an operating system: the OS vendor's own sync service and on-device client keep a user's phones, tablets and laptops on the same daily wallpaper. The hard parts are downloading large images in the background without draining battery or mobile data, and keeping choices consistent across devices that are often offline.
A user picks a collection, a daily change time and favourites in system settings on any device. Every signed-in device shows the same wallpaper of the day, in a size that fits its screen. Because the client is part of the OS, it hands images straight to the system wallpaper service; there is no third-party app and no store listing.
What must the system do?
- Daily change: The wallpaper changes once a day near the chosen time, even with no network, using images prefetched earlier.
- Cross-device sync: A change of collection, time or favourites on one device appears on the user's other devices within 5 minutes when they are online.
How well must it do it?
- Battery and data: No more than 2 background prefetch runs per day per device, and no prefetch over a metered network unless the user allows it.
- Integrity: A wallpaper is never applied from a partial or corrupt file; a day is never skipped or changed twice.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Background prefetch
- Resumable downloads
- HTTP range requests
- Etag
- OS background scheduling
- Exponential backoff
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 wallpaper sync across devices 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