System design interviews are where companies test whether you can build software that survives the real world.

Not “can you write code,” but: can you design a service that stays fast when traffic spikes, keeps working when servers fail, stores data safely, and still feels simple to evolve six months later?

System design interviews are different from coding interviews. There's no single correct answer. No test cases to pass. Instead, you're evaluated on how you think about building large-scale systems.

In this chapter, I'll explain:

1. What is a System Design Interview?

A system design interview is a technical interview where you're asked to design the architecture of a large-scale software system. Think "Design Instagram" or "Design a rate limiter."

In a coding round, you're given a well-defined problem with clear inputs, outputs, and constraints. You write code, run it against test cases, and either pass or fail. It's binary.

System design is messier. The problem is intentionally vague. The constraints are whatever you and the interviewer decide they should be. There's no code to run. Instead, you're drawing boxes and arrows on a whiteboard while explaining your thought process out loud.

These interviews typically last 45-60 minutes and involve designing systems like:

The goal is not to produce a production-ready design in 45 minutes. That's impossible. Real systems take teams of engineers months or years to build.

The goal is to demonstrate that you can think through complex problems systematically, make reasonable trade-offs when there's no perfect answer, and communicate technical ideas in a way that others can follow and build upon.

2. Why Do Companies Conduct System Design Interviews?

Coding interviews answer one question: can this person write correct code? That's necessary, but it's not sufficient. Building software at scale requires a completely different set of skills.

The Gap Between Code and Systems

Consider what happens when you write a function that works perfectly on your laptop. Now imagine that function needs to:

Suddenly, the algorithm isn't the hard part anymore. The hard part is everything around it: how data flows through the system, where bottlenecks will appear, what happens when things fail.

System design interviews exist to filter for engineers who can think beyond the function level.

How Expectations Change with Seniority

The weight given to system design increases dramatically as you move up the ladder.

This makes sense when you think about what each level does day-to-day. Junior engineers implement features that someone else designed. Senior engineers design those features.

Staff engineers design systems that span multiple teams. At each level, the scope of your design responsibility expands.

What Companies Learn About You

System design interviews reveal more than whether you know the “right” architecture. They show how you think, how you make decisions, and how you communicate:

3. What Do Interviewers Look For?

System design interviews are subjective. Different interviewers value different things, and different problems call for different approaches. That said, most interviews still rely on a common set of evaluation criteria.

1. Requirements Gathering

The first few minutes of a system design interview reveal more than most candidates realize. When you get a problem like “Design a URL shortener,” you’re expected to pause and ask clarifying questions before you start drawing boxes and arrows.

For a URL shortener, a few high-signal clarifying questions are:

2. High-Level Design

Once requirements are clear and you have identified the core entities/apis of the system, you can move on to the high-level design. The goal here is to sketch the major components of the system and how they interact.

A typical high-level design includes:

The goal isn't to include every possible component. It's to include the right components for your specific requirements and be able to explain why each one is there.

3. Deep Dives

Once you’ve sketched the high-level design, the interviewer will usually zoom in on one or two components and ask you to go deeper.

Common deep-dive areas include:

4. Trade-off Analysis

Every design choice comes with a downside, and you are expected to recognize it.

If you say, “I'll use a NoSQL database”, the next question could be: “Why?” A weak answer is “because it scales.” A strong answer ties the choice to the requirements and names the trade-off.

5. Communication

Technical knowledge matters, but how you communicate it matters just as much. A system design interview isn’t a quiz, it’s closer to a design discussion with a teammate. You're expected to explain your thinking in a way that's easy to follow.

Here are few signs of clear communication: