Course Introduction

Welcome to Grokking Modern Mobile System Design Interview.

This course teaches you how to design mobile apps and explain your decisions clearly in a system design interview. You will learn how to turn a broad question into a manageable design, connect the app's main components, and describe what happens during normal use and failure.

The focus is the app running on the user's device. You will also define the communication rules and server behavior that the app depends on. A reliable mobile design needs both: clear responsibilities inside the app and clear agreements with the backend.

What this course will help you do

Consider a question such as:

Design a mobile app where users can browse restaurants, place an order, and track its delivery.

A complete answer needs more than a list of screens. What can the user do with a weak connection? Where is the cart saved? What happens if the app closes while an order is being submitted? Can the app safely repeat a request when the response is missing?

This course helps you work through those questions in a clear order.

You will practice how to:

• Clarify the required features, system boundary, and important constraints.
• Divide responsibilities among screens, state owners, repositories, storage, and network components.
• Define an API contract: the requests, responses, and rules used to communicate with a service.
• Design local storage, offline behavior, synchronization, and recovery after interruption.
• Keep scrolling, image loading, startup, and live updates within device limits.
• Include security, testing, monitoring, and safe releases in the design.
• Explain why you chose an approach, what it costs, and when you would change it.

The goal is to build a method you can apply to a new question, not to memorize one architecture for every app.

Who this course is for

This course is intended for:

• iOS and Android developers preparing for mobile system design interviews.
• Cross-platform developers who want to strengthen their understanding of device limits, data ownership, and platform integration.
• Engineers with experience building features who want to explain the wider design, including failure recovery and changes across components.
• Experienced engineers who want structured practice discussing technical choices, operational risks, and release decisions.

It can also help you review designs in your current work. Questions about saved user input, stale data, request retries, and account changes matter outside interviews too.

You do not need previous system design interview experience. If you are new to programming or have not yet built a small app, start with basic app development first. That experience will make the examples easier to understand.

What you should know before starting

You should be comfortable with the basics of one mobile development environment. You do not need to know both iOS and Android.

Useful starting knowledge includes:

• Writing and reading code in a language used for app development.
• Building a simple screen and responding to user actions.
• Sending a basic network request and reading a response, such as JSON data.
• Understanding that data in memory can be lost when a process ends, while saved data can be read later.

You do not need to be an expert in databases, distributed systems, or mobile architecture patterns. The lessons explain the concepts needed for their design questions. Platform-specific details appear where they change the behavior or limits of a design.

How the course is organized

The course has ten modules. Earlier modules introduce the design method and technical foundations. Later modules apply them to apps, reusable libraries, and interview practice.

An SDK, or software development kit, provides tools and interfaces that another developer uses in an app. These design questions require clear behavior and resource limits even when the component has no screen of its own.

The method used in the case studies

The app case studies use the same five-step walkthrough:

1. Requirements: agree on the features, users, and important conditions.
2. API and data model: define communication rules and the records the app needs.
3. Client architecture: assign responsibilities and explain how data moves between components.
4. Deep dive: examine the most important technical decisions in detail.
5. Failure, offline, and shipping: complete the recovery, testing, and release plan.

These steps organize the explanation. They do not mean you should ignore failure until the final step. For example, a requirement to recover an unsent message affects the data model and architecture from the beginning.

The method is a practice framework, not a format that every company requires. Adapt your answer to the confirmed interview scope and the interviewer's questions.

How to study effectively

If the topic is new to you, begin with the modules in order. Try the small exercises as you go, then apply the ideas to the larger case studies.

For each case study:

1. Read the question before reading the walkthrough.
2. Write down the scope and a few questions that could change the design.
3. Draw the main components and explain one user action aloud.
4. Introduce an interruption, such as a timeout, account change, or app restart.
5. Read the walkthrough and identify what your answer was missing.
6. Explain the improved design again without reading the solution.

Use the diagrams to follow ownership, dependencies, and state changes. Do not memorize box names without understanding what each component does.

The capstone lessons combine several topics in one exercise. Practice questions and module assessments help you identify gaps. Review the explanations for incorrect answers, then return to the relevant lesson. A score alone does not show that you can explain a design clearly.

If you already have design experience, attempt a case study first. Use the result to choose which foundation lessons to revisit. Then practice follow-up questions that change a requirement or introduce a new failure.

What to expect from the examples

This is a design and interview-preparation course. It is not a step-by-step tutorial for building complete production apps, and it does not replace a full backend system design course.

Examples use component diagrams, data models, request contracts, and small code or pseudocode samples where useful. Some questions allow several valid designs. Compare them against the requirements and explain the consequences of your choice.

Numbers such as page sizes, memory limits, and practice durations are examples unless stated otherwise. Treat them as assumptions to test, not values that every app or interview should use.

Before you continue

By the end of the course, you should be better able to explain a mobile design as a connected system: what the user needs, which component owns each responsibility, where important work is saved, and how the app recovers when something goes wrong.

Start with Introduction: The Mobile System Design Round to see what a mobile system design discussion includes.