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Mobile System Design

How real apps are built — layers, local storage, sync, networking, media and push — and how to talk through it in a 45-minute interview. Every case study ends in the lab, where you sketch it and run it.

Enroll free Try case 01 without an account
English6 modules8 case studiesAndroid & iOS friendly

What you'll learn

Six modules, from the shape of a mobile app to the trade-offs interviewers push on.

Module 1

App architecture

Layers, unidirectional data flow, state holders, dependency boundaries.

Module 2

Local data & caching

Single source of truth, memory vs disk caches, invalidation, migrations.

Module 3

Networking

REST vs GraphQL vs WebSockets, pagination, retries, idempotency.

Module 4

Offline & sync

Write queues, background work, conflict resolution, eventual consistency.

Module 5

Media & performance

Image loading, CDNs, prefetching, startup time, battery and memory.

Module 6

The interview

The 5-step framework, timing yourself, and the trade-offs worth saying out loud.

The 5-step framework

Every case study is solved in this order — the same order you'll use in the interview.

5–10 min1 · Understand the problemFunctional and non-functional requirements, scale, offline needs, what's out of scope.
5–10 min2 · API designEndpoints and payloads, pagination, real-time vs request/response.
10–15 min3 · High-level architectureBoxes and arrows: client layers, backend services, how data flows.
15–20 min4 · Design deep diveCaching, sync, storage, performance and the tricky edge cases.
0–5 min5 · Wrap-upTrade-offs, what you'd monitor, what you'd do with more time.
EN / বাংলাALGONURMobile System Designthe 5-step way
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The Mobile System Design Handbook

Everything from this course in one PDF you can read offline. Join the newsletter and it lands in your inbox in a minute, along with one email whenever a new lesson, case study or post is out.

  1. The 5-step interview framework
  2. App architecture & state
  3. Local data & caching
  4. Networking & API design
  5. Offline-first & sync
  6. Media & performance
  7. Case study walkthroughs
  8. Trade-off cheat sheet
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Case studies

Each one has a brief, a reference design, a video, and a lab you can break.

Real-time chat01

Real-time chat

Sockets, ordering, receipts

Try free in Lab →Infinite news feed02

Infinite news feed

Pagination, prefetch, caching

Try free in Lab →Offline-first notes03

Offline-first notes

Sync queue, conflicts

Try free in Lab →
Image-heavy gallery04

Image-heavy gallery

Caches, downsampling, CDN

Coming soon
Live location tracking05

Live location tracking

Background limits, battery

Coming soon
Video streaming06

Video streaming

Adaptive bitrate, buffering

Coming soon
Push notifications07

Push notifications

Fan-out, tokens, deep links

Coming soon
Checkout & payments08

Checkout & payments

Idempotency, retries

Coming soon