9 Common System Design Interview Questions and How to Answer Them

System designers are IT specialists who design or update a computing system tailored to a company's needs. In this role, you consider software, hardware, telecommunications, and databases and figure out how to work together for the end users. 

As you prepare for an upcoming interview in this IT role, review what the interviewer is looking for by practicing possible system-design interview questions, exploring questions you can ask the interviewer, and getting more tips on having a successful interview for the job you want. 

9 system design interview questions

Preparing for this interview requires reviewing your skills and typical job duties in the role you’re applying for. To help, you can become familiar with these commonly asked questions and the subsequent system design interview responses.   

1. How would you design a recommendation system?

What they're really asking: What's your knowledge and experience designing recommendation systems?

Recommendation systems (or recommender systems), as the name suggests, recommend new shows to users based on their behavior. They're a type of information filtering system that has become very popular on e-commerce sites and streaming platforms. Netflix, for example, has a strong recommendation system, as it recommends new shows to users based on their viewing habits. 

To show your understanding of designing a content-based recommendation system, talk about these high-level components of a recommendation system:

• Candidate generation

• Scoring

• Reranking

If you can, tailor your answer to the company. If, for example, the company plans to recommend additional products at the time of purchase, speak to that situation. 

Other forms this question could take:

• What kind of recommendation system would you suggest for our site?

• What's your experience creating recommendation systems?

2. Explain the difference between NoSQL systems and SQL systems.

What they're really asking: Do you know data queries and the systems used to make them work? 

SQL stands for Structured Query Language, a well-known programming language dating back to the 1970s that helps manage data. Its best use case is querying relational data. While SQL still has its place in system design, the evolving need to gather and mine data has made way for NoSQL, a non-relational database that offers different structures and more flexibility. 

SQL systems:

• Are relational

• Use SQL and have a predefined schema

• Are vertically scalable

• Are table-based

• Best for multi-row transactions

NoSQL systems:

• Are non-relational

• Use dynamic schemas and unstructured data

• Are horizontally scalable

• Are documents, key-value, graphs, or wide-column

• Best for unstructured data

Other forms this question might take:

• Explain when you'd use SQL or NoSQL.

• What would you suggest for our company's needs? SQL or NoSQL?

3. Explain how web crawlers are designed.

What they're really asking: How much do you know about the inner workings of web crawlers?

To start, explain what a web crawler is. A web crawler indexes content and helps return relevant information for a search. Big-name search engines like Google and Bing use web crawlers. 

To design a web crawler, you can:

• Write a design spec based on the requirements

• Write an implementation spec

• Consider the crawler's data structures and operations

• Implement the code to create the crawler

Other forms this question might take:

• How would you design a web crawler?

• What kind of architecture would you use to design a web crawler?

4. What is caching?

What they're really asking: What is your technical experience with caching, and what are the different types?

Caching is a site's way of temporarily storing copies of files to improve speed or functionality. A site could cache HTML files to load sites quickly, for example. 

Several different kinds of caching exist, including CDN and DNS caching. CDN, or content delivery network, caches content in proxy servers close by to deliver content faster. Domain name system, or DNS, caching occurs on DNS servers that store lookups to instantly deliver IP addresses.  

Other forms this question might take:

• Why is caching important?

• What's the difference between CDN and DNS caching?

5. Explain the difference between horizontal and vertical scaling.

What they're really asking: Do you have the ability to access our scaling needs?

Horizontal and vertical scaling enhance cloud computing power to better serve customers. Horizontal scaling adds more machines to the network, so processing and memory workloads are distributed over more machines, improving efficiency. 

Vertical scaling upgrades existing machines to improve efficiency, like upgrading RAM. These updates eliminate the need to update code, making it easier to implement than horizontal scaling. 

Other forms this question might take:

• Which is easiest to perform, horizontal or vertical scaling?

• We're thinking of vertically scaling our cloud computing power. What are the pros and cons of this?

6. What is sharding?

What they're really asking: What's your competency level in scaling databases?

Data sharding splits a data set into smaller databases. For example, if you take a 1T database and break it into four databases, each holding 256MB, you're sharding the system. Creating these partitions increases throughput, storage capacity, and availability.

Other forms this question might take:

• Why would you shard a database?

• How would you shard our single-machine database?

7. What are the most common types of system documentation in system design?

What they're really asking: What kind of documentation have you done in the past?

System development documentation is akin to a handbook of a company's system. It overviews the system's features, functions, design, and implementation. These documents inform everyone and ensure new people can step in as needed. 

You can work with more than a dozen types of system documentation, but some of the most common are:

• Process documentation: Records development and maintenance 

• Standards documentation: Established norms

• System documentation: Lists capabilities, design, limitations, and operations

Other forms this question may take:

• What system documents do you find most helpful?

• If selected for this job, what kind of system documentation would you update or implement?

8. Explain the different design issues found in distributed systems.

What they're really asking: Are you familiar with distributed systems?

Distributed systems are a group of components on different systems that work together. Design challenges with this type of system include:

• The lack of global knowledge

• Scalability

• Compatibility

• Process synchronization 

• Security

Other forms this question might take:

• What challenges do you see with a distributed system?

• List the foreseeable issues you'd have with a distributed system.

9. Why is a rate limiter used?

What they're really asking: What is your experience with rate limiting?

Rate limiting helps maintain good service and often helps accomplish several things, including:

• Resource starvation: Improve availability of services

• Manage quotas: Shares the service with multiple users

• Control flow of data: Better distribution of data

• Security: Defend against attacks

Other forms this question might take:

• Why would you implement a rate limiter?

• What are the advantages of rate limiting?

Questions to ask the interviewer

At the end of the interview, you can ask your own questions. It's a good time to access the company and see if it fits you. Questions to consider include:

• What would a typical day as a system designer at your company look like?

• What expectations do you have of me in this role?

• In the first 90 days, what goals should I plan to hit?

• How is my performance in this role accessed?

Preparing for the interview

Before your interview, consider setting aside several hours to prepare. During this time, improve your interviewing techniques and research the company. More specifically, split your time into these three sections:

• Research: Study the company online, review articles, and browse social media.

• Identify questions: Search for a list of possible interview questions that pertain to the job.

• Practice responses: Run responses for each question.

In addition, consider asking a friend to conduct a mock interview to help you practice. Dress the part, watch your body language, and practice everything from an introductory handshake to answering questions.

Industry outlook

According to the US Bureau of Labor Statistics [1], jobs for computer network architects will likely grow 13 percent between 2023 and 2033, with about 10,200 openings in the field expected each year. Typical requirements for obtaining this type of position with a company are a bachelor's degree and several years of IT experience.  

Improve your interviewing skills with Coursera

System design interview questions are usually quite specific to the job and focus on the technical aspects of the role. By reviewing these common interview questions and forming intellectual answers, you can confidently enter your interview. If you'd like to further your interview prep, consider taking an online course, such as Successful Interviewing on Coursera. You'll have the opportunity to learn how to research the market, make a strong first impression, and confidently answer interview questions.