What Questions Are Asked in a Mobile System Design Interview?

Mobile system design questions come in two families. One family asks you to design a single feature of an app, such as a feed, a chat screen, or photo upload. The other asks you to design a library that other app teams call, such as image loading or a file downloader.

Candidates report both families. At Meta, Uber and Flipkart, candidates report library questions about as often as feature questions. Other companies lean toward app features.

Every question has the same shape. You design one feature, running as one process, on one device. The network is bad, the battery is small, and the operating system can stop your app at any moment.

Quick Overview

The full ranked list of twenty questions, with a sheet you can download, is in the companion post on mobile system design interview questions. This answer gives the shape and the top of each group.

App Feature Questions

Ordered by how often candidates report them.

• News feed. New items arrive while the user scrolls. Page numbers shift, so cursor paging is expected.
• Chat. Messages must be sent while the phone is offline. That needs a queue and client generated IDs.
• Photo upload and sync. An upload outlives the screen that started it. Background time is limited by the operating system.
• Stories. Media must be prefetched without wasting the user's data plan. Memory is the limit, not bandwidth.
• Ride hailing or food delivery tracking. Location updates are frequent. Each update costs battery.
• Booking and checkout. A retry must never charge the user twice. Idempotency belongs on the client too.
• Email client. Mail must be readable with no network. The local store becomes the source of truth.
• Video player. Buffering, quality switching and preloading the next item all compete for memory.

Each of these can be scoped three ways. Client only, client plus the API, or full stack. Ask which one before you draw anything.

Library and SDK Questions

These ask you to design something other engineers call. The grader watches your public API as closely as your internals.

• Image loading library. Decode to the size of the view. A large photo decoded at full size can use tens of megabytes.
• File downloader and uploader. Resume after the app is killed. Report progress without holding a reference to a dead screen.
• Networking layer. Retries, timeouts and cancellation in one place. One place to add headers and logging too.
• Pagination library. Hide cursors from the screen. Keep scroll position when the data refreshes underneath.
• Caching library. Two levels, memory and disk. Decide what to evict and when an entry goes stale.
• Analytics SDK. Batch events, store them on disk, send them later. Nothing may be lost when the app closes.
• Feature flag SDK. Read a value with no network. Decide when a changed value takes effect.
• Push notification client. Push is not reliable delivery. The app must still fetch on open.

Library questions reward a clear contract. Say what the caller passes in, what comes back, and what happens on error. Then say what the caller must never do.

What Every Question Is Really Testing

The market grades this round in five steps. Requirements come first, then the API and data model.

Client architecture is third. The hardest part in depth is fourth. Failure, offline behaviour and shipping close the round.

A common budget in a forty five minute round is about five, eight, ten, seventeen and five minutes. The fourth block is the largest. That is the part the interviewer goes deepest on.

Your level shows in the last block. A mid level answer stops when the design works. A senior answer adds what breaks, what the user sees when it breaks, and what each choice costs.

How to Prepare

• Work the questions, not just the list. Grokking Modern Mobile System Design Interview works sixteen of them as full case studies, app features and libraries both.
• Learn one method, then apply it to every question. The five steps are the same for a feed and for a caching library. Practice the order until you never have to think about it.
• Pick your platform answer and say it in one breath. Read what the iOS round is like or what the Android round is like.
• Build a study order with time on it. The plan is in how to prepare for a mobile system design interview.
• If you came from backend preparation, read what changes. Much of your method still applies. See mobile vs backend system design.
• If you are interviewing at Meta or Apple, read those two first. See the Meta Product Architecture round and Apple iOS expectations.
• Learn the server half properly. Grokking the System Design Interview teaches the backend side of a full stack question, which is the half the course above does not cover.