System Design Interview Overview for Software Engineers

If you have looked into the software engineering interview process, you may have come across something called a system design interview. These interviews tend to be very different from the usual coding interviews.

In this post we will look at why interviewers hold system design interviews, the expectations they have for job candidates, and how to prepare for this type of interview.

But first, we need to understand what functional and non-functional requirements are in a software system and how they should be considered in the design process.

Functional and Non-Functional Requirements

Let’s say you’re building an app where users can post and share images. What functions can users perform on the app?

A user can register and sign in on the app. They can post, share, and comment on images. All of these functionalities are user-focused.

Functional requirements define the specific functionalities, features, and capabilities that the app must provide to meet the needs of its users.

But in addition to these functionalities, the app needs to perform well, be reliable, and securely store user information. Non-functional requirements describe the qualities and characteristics of the system that are not directly related to specific behaviors but are critical for its overall effectiveness.

It’s critical to consider both functional and non-functional requirements when designing and building software.

What is System Design?

System design refers to the process of designing an application while taking into account both the functional and non-functional requirements. It involves making high-level decisions and choices regarding the structure, components, interactions, and behavior of the system.

The System Design Interview Process

In system design interviews, you will generally be asked open-ended questions with vague requirements. You’ll have to collaborate with your interviewer to figure out which qualities and characteristics to prioritize while designing your system.

Unlike coding interviews—where you may have to do algorithm challenges or build apps to specifications—system design interviews have no correct solutions. You will have to justify all of your design decisions in the context of the functional and non-functional requirements while also comparing alternatives.

A typical system design interview will usually follow this format:

• The interviewer asks the candidate to design a system
• The candidate then does the following:
  • Clarifies requirements
  • Does back-of-the-envelope calculations
  • Establishes scope of the design
  • Defines a data model
  • Proposes a high level design
  • Defines the APIs for the system
• The candidate and interviewer discuss details, trade-offs, and extensions

Generally, the interview is 45-60 minutes long so you won’t have time to dive deep into every aspect of the system. This is why it’s important to communicate with your interviewer about the areas you should be focusing on. For certain problems, they may ask you to focus more on the data model and what kind of database you’d use. For others, they may ask you to define your APIs in detail and describe how they would communicate with each other.

Interviewers are generally looking to assess the following skills:

• Fundamental system design concepts
• Requirements clarification and analysis
• Well-reasoned decisions
• Trade-off considerations
• Collaboration
• Communication

These aren’t in order of importance since different interviewers may prioritize different skills based on the job description, role level (IC3 will get easier problems than IC6), and their own experiences.

System Design Study Resources

A system design interview may seem impossible to study for since there are so many topics that can be covered. But thankfully, there are some fundamental concepts that will help you to get through most of the junior to mid-level interviews. Once you’ve got those down, you can read up on real system design on company blogs or research papers.

Below are some additional resources to help you learn how to excel in the interview:

• Watch the video 20 System Design Concepts Explained in 10 Minutes 
• Watch CS50’s Scalability and Security Lecture video
• Watch the video System Design Interview: A Step-by-Step Guide  
• Read A Senior Engineer’s Guide to System Design Interview (it is a four-part series and I recommend reading in order)

For practice problems:

• Watch a system design walkthrough
• Consider reading Volume 1 and Volume 2 of Alex Yu’s book series System Design Interview: An Insider’s Guide (both books are also available as an online course)

You should ideally try designing and building some complex systems yourself to see how all the pieces fit together. Starting with simple data pipelines or simple microservices may be helpful.

But the free resources (except for the books) mentioned above should give you a solid system design foundation which will allow you to understand most of the system design interview questions and walkthroughs you may encounter.

It takes time to get really good at designing systems. You will learn new concepts, tools, and techniques throughout your career which will influence your unique way of designing systems. 

For your interviews, focus on understanding the fundamentals, prepare in advance, and do practice interviews on sites like Pramp or Interviewing.io. Best of luck!

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Disclaimer: The information in this blog is current as of February 11, 2024. Current policies, offerings, procedures, and programs may differ.