Design a mobile image caching library
PremiumMobile systemEasy
- Memory LRU cache
- Disk cache
- Image downsampling
Quick answer
This mobile system interview question asks you to design a mobile image caching library. 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 image caching library, like Glide or SDWebImage, that app teams use to show remote images in lists and screens on iOS and Android. It downloads, decodes, caches and displays images. The hard parts are memory use on small phones, smooth scrolling and not downloading the same image twice.
What must a mobile image caching library do?
- Simple API: load(url).resize(width, height).placeholder(p).into(view), with a callback form for non-view targets.
- Two caches: A memory cache of decoded images and a disk cache of downloaded files that survives app restarts.
How well must it do it?
- No jank: No network, disk or decode work on the main thread; 60 fps scrolling in a grid of thumbnails.
- Bounded memory: The memory cache never exceeds its byte budget and shrinks when the OS reports memory pressure.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Memory LRU cache
- Disk cache
- Image downsampling
- Request coalescing
- Request cancellation
- HTTP cache headers
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 image caching library and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Easy, so it suits engineers who are new to mobile system interviews. It is also a good warm-up before harder questions.
- 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