[In-Depth Guide] iOS App System Design: From concept to architecture

As AI-assisted software development accelerates, code itself is becoming a commodity. What’s growing increasingly valuable is the ability to design robust, scalable systems. If you can architect a system effectively, you can almost certainly build its components—especially with AI as your copilot.

Employers have also taken notice of this trend. System design is now a standard part of technical interviews. It's no longer enough to simply know how to code or use an iOS framework—whether you're seeking a job or building something on your own.

In this post, I’ll walk you through designing a system for an iOS app and share a practical mental model for approaching system design.

Scope down the requirements

The most critical first step in system design—whether in a real-world project or a technical interview—is scoping the requirements. This decision shapes every step that follows.

Clearly defining the scope means identifying what to focus on, but just as importantly, what to exclude. Without this clarity, you risk spending valuable time on unnecessary details that don’t meaningfully contribute to your system.

This is especially important in a technical interview, where you typically have just 45 minutes to an hour to design a system for an iOS app. Use the first five to ten minutes to ask clarifying questions and narrow the scope to something manageable within the time frame.

Even when you're designing for your own project without strict time limits, scoping the requirements helps you focus on the most important parts of your app—what actually delivers value.

Identify external dependencies

Once you’ve scoped your requirements, the next step is to identify external dependencies. Most iOS apps rely on at least a few components outside the app itself—such as backend servers, third-party APIs, or hardware devices.

An external dependency is anything that originates outside your app’s codebase or runtime environment. To design an effective system, you need to be aware of these dependencies and clearly understand how your app will interact with each one.

Clarifying external dependencies now will make it much easier to dive into the next phase.

Focus on a key component of your system

After identifying your external dependencies, the next step is to define how your app will interact with them. This is where your design starts to take shape—mapping out the flow of data: where it starts, how it’s processed, and where (and when) it ends up. This step brings clarity to how your system will actually function.

You’re not getting into implementation-level details like classes or structs just yet. Instead, you're zooming in one level from simply naming external components to explaining, in a structured and logical way, how your app will communicate with them.

In a technical interview, you’ll likely only have time to design one key part of the system. Be sure to ask your interviewer which core process they’d like you to focus on so you can spend your time effectively.

At this stage you will be expected to dive into the details of the core component you are instructed to focus on. This will include defining the exact components and involved as well as how they interact. You will have to look back at any requirements identified in the first step. These can include things like performance, error handling, localization, etc.

Working through an example: Designing a system for music streaming app

Now that we've learned about the steps involved in the system design process, we are now ready to look at an example. This example is from a real world system design interview I had with a company where I was asked to design a system for a music streaming app.

The structure I will use if that of a conversation, with an interviewer and a developer talking back and forth. There will be diagrams, and we will see how the diagrams are updated as the conversation progresses.

Let's begin!

Interviewer: How would you build a system for music streaming app?

Dev: Okay, so first I wish to start by clearly defining the scope and requirements. What is the target user base and what is it's size?

Interviewer: Assume the user base is world wide, so millions of users.

Dev: What core capabilities will this app have?

Interviewer: Great question. Users should be able to see music recommendations, build playlists, search songs, and play music.

Dev: Got it. Now, what are the non-functional requirements? Are we supporting offline mode? Also, since we have an international user base, are we worrying about localization?

Interviewer: The app should be performant and scalable, users should be able to download songs to play offline, and yeah we will need localization.

Notice that the interviewer started with a very open ended prompt. It was up to the dev to ask clarifying questions first, instead of just jumping into a design.

Dev: Okay, so the key components of the system I am identifying is music streaming, playlist management, a recommendation system, and localization. In addition, we will need offline capabilities and possibly some caching. Given that there is a time constraint, is there a specific piece of this system that you would like me to focus on first?

Interviewer: Sure, show me how the playlist management system would work.

Dev: Sure. So there are a few core components of this playlist management system I can identify off the top of my head: a database, a server providing an API to the database, and the iOS app itself.

Top level components of a playlist management system

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