Cracking System Design Interviews: Answers to Top 25 Interview Questions for Software Developers

Hello guys!

If you're gearing up for technical interviews at FAANG companies or any software developer interviews at startups or tech giants like Spotify, Flipkart, or Zoom, and you're feeling anxious about system design questions, you're not alone. System design can be a daunting topic, demanding extensive study and hands-on experience to design real-world systems effectively.

While it's not rocket science, the knowledge of different system design components and concepts is hard to acquire. In this article, I’ll share 25 System Design Interview Questions, both conceptual and concrete design problems like how to design WhatsApp, YouTube, NetFlix, etc with answers for programmers with zero to 3 years of experience.

System Design interview just like other interviews, requires you to be up to the task. This means that you have to adequately prepare so you can have it easy during the interview.

By the way, if you are serious about cracking System design interviews on the first attempt, then joining a site like ByteByteGo or Exponent, or a course like Grokking the System Design Interview on DesignGuru can really help. They cover essential System design concepts in a guided and structured way and help you prepare faster.

25 System Design Interview Questions and Coding Problems with Answers

The following are the top 25 System Design interview questions you can prepare for your next Interview. These are pretty basic questions, and as a developer, you should already be familiar with them. If not, then make sure you practice before going for any system design interview.

1. What is the difference between Horizontal and Vertical Scalability?

Answer: Horizontal scalability, also known as scale-out, refers to the ability to increase the capacity of a system by adding more hardware or resources such as servers, nodes, or instances, thus distributing the workload across multiple machines. In contrast, vertical scalability, or scale-up, involves increasing the power of existing hardware, such as upgrading the CPU, memory, or storage capacity of a single machine, allowing it to handle a greater volume of work.

Horizontal scalability offers more flexibility and typically better fault tolerance compared to vertical scalability, as it can handle increasing loads by simply adding more inexpensive machines, while vertical scalability may eventually reach hardware limitations and require more expensive upgrades. However, you need to use special software architecture components like Load Balancers and API Gateways to distribute the incoming traffic across multiple servers in case of horizontal scaling as shown below.

2. How do you design a web crawler?

Answer: A web crawler service collects information/crawls from the entire internet and fetches millions of web documents. Things to keep in mind while designing a web crawler are:

• The approach taken to find new web pages

• The approach to prioritizing web pages that can change dynamically

• Ensuring that the web crawler service is bound on the same domain

  
  

You can try solving the question but if you stuck, I recommend you to check out Grokking Modern System Design for Software Engineers & Managers course on Educative, it provides a step-by-step solution to not just this question but also other System design questions.

Here is a nice system design diagram for a web crawler:

3. What is the difference between a Load balancer and an API Gateway?

Answer: This is another popular System design question you will often see at the start. A load balancer is a networking device or software component responsible for distributing incoming network traffic across multiple servers or resources within a server farm or data center.

Its primary function is to improve the availability and reliability of applications by ensuring that no single server is overwhelmed with requests, thus optimizing resource utilization and preventing system downtime.

Load balancers operate at the network or transport layer of the OSI model and are typically used to manage traffic for web servers, application servers, or databases. They focus on efficiently distributing incoming requests based on factors like server health, capacity, and response time.

On the other hand, an API Gateway is a specialized type of server that acts as an intermediary between clients and backend services, providing a unified interface for clients to access multiple APIs or microservices.

API Gateways handle tasks such as authentication, authorization, rate limiting, request transformation, and response caching, abstracting the complexities of the underlying services and simplifying client interactions.

Unlike load balancers, which primarily focus on distributing network traffic, API Gateways operate at a higher level, often providing additional functionality such as API versioning, documentation, and analytics. They are particularly useful in modern microservices architectures, enabling organizations to efficiently manage and secure their APIs while maintaining flexibility and scalability.

Here is also a nice diagram from DesignGuru.io which highlights the difference between API Gateway and Load Balancer by their responsibilities and function:

4. What is the difference between forward proxy and reverse proxy? When do you use forward proxy and reverse proxy in System design?

Answer: This is another common System design question that is asked on the telephonic round of interviews. A forward proxy acts as an intermediary server between a client and the internet, forwarding client requests to the appropriate destination servers on behalf of the client. It is typically used to control and filter outbound internet traffic from within a private network, providing anonymity, caching, and security benefits for users accessing the internet.

Forward proxies are commonly implemented within corporate networks to enforce policies, improve performance through caching, and protect against malicious content.

In contrast, a reverse proxy sits between clients and backend servers, intercepting incoming client requests and routing them to the appropriate backend servers based on various criteria such as load-balancing algorithms or content-based routing rules.

Reverse proxies are often used to enhance security by hiding the internal structure of a network, improving scalability by distributing incoming traffic across multiple servers, and providing additional features such as SSL termination, content caching, and web application firewall (WAF) capabilities.

They are commonly employed in web servers, application servers, and API gateways to optimize performance and ensure the high availability of backend services. Here is also a nice diagram from ByteByteGo which explains the difference between Forward Proxy and Reverse Proxy by showing their functionalities and usage.

5. How do you design YouTube?

Designing a YouTube is an interesting System design question as everyone is familiar with YouTube but a lot goes into designing a video streaming service like YouTube.  There are a lot of challenges related to storing data, indexing data, and providing streaming services.

Scalability is another major challenge as it needs to be fast and should be able to support millions of users.

You can start designing YouTube on your own but if you stuck or need guidance you can see this free tutorial by Alex Xu from his popular System Design Interview Course on ByteByteGo.

Here is a nice flow diagram to explain the video upload process:

6. In the System Design process, what is ‘Requirements Determination’?

Answer: A requirement is most important for a new system which includes processing or capturing data, controlling the activities of a business, producing information, and supporting the management. Requirement determination helps you to study the existing system and to gather details to find out what are the requirements, how it works, and what kind of improvements should be made.

7. How do you design a URL Shortener like bit.ly or goo.gl?

This is another popular System design question that is often asked during FAANG interviews. This question poses a lot of challenges like where the short URL and target URL will be stored. How to create a unique short URL every time for different target URL and return the same short URL for the same target URL.

You can try solving this problem on your own but if you get stuck then you can also check out this step-by-step solution of URL shortener on DesignGuru, one of my favorite portals for preparing System design interviews.

8. How can you design autocomplete functionality?

Answer: To design autocomplete functionality, one can implement a system that stores a dataset of possible completions, such as words, phrases, or search queries, in a data structure optimized for quick retrieval, like a trie or prefix tree. As a user types a query into the search bar or input field, the system can dynamically suggest completions based on the partial input, leveraging efficient search algorithms to quickly locate matching entries in the dataset. Additionally, caching commonly used completions and implementing mechanisms for incremental updates to the dataset can help optimize performance and ensure real-time responsiveness, providing users with a seamless and efficient autocomplete experience.

9. What are the Types of Documentation in System Design?

Answer: The four types of documentation are:

• Program documentation
• System documentation
• Operations documentation
• User documentation

10. How is Horizontal scaling different from Vertical scaling?

Answer:

• Horizontal scaling refers to the addition of more computing machines to the network that share the processing and memory workload across a distributed network of devices. In simple words, more instances of servers are added to the existing pool, and the traffic load is efficiently distributed across these devices.

  
  

• Vertical scaling refers to the concept of upgrading the resource capacity such as increasing RAM, adding efficient processors etc of a single machine or switching to a new machine with more capacity. The capability of the server can be enhanced without the need for code manipulation.

  
  

If you want to learn more about Scalability concepts then I highly recommend you to check out Frank Kane's System Design Course on Udemy where he shared all these key System design concepts in detail.

11. What do you understand by load balancing? Why is it important in system design?

Answer: Load balancing refers to the concept of distributing incoming traffic efficiently across a group of various backend servers. These servers are called server pools. Modern-day websites are designed to serve millions of requests from clients and return the responses in a fast and reliable manner. To serve these requests, the addition of more servers is required.

In such a scenario, it is essential to distribute request traffic efficiently across each server so that they do not face undue loads. Load balancer acts as a traffic police cop facing the requests and routes them across the available servers in a way that not a single server is overwhelmed which could degrade the application performance.

12. What is Sharding? What do you understand about Database Sharding?

Answer: Sharding is a process of splitting a large logical dataset into multiple databases. It also refers to the horizontal partitioning of data as it will be stored on multiple machines. By doing so, a sharded database becomes capable of handling more requests than a single large machine.

Consider an example - assume that we have around 1TB of data present in a database. When we perform sharding, we divide the large 1TB data into smaller chunks of 256GB into partitions called shards.

13. What are the various Consistency patterns available in system design?

Answer:

• Consistency from the CAP theorem states that every read request should get the most recently written data. When there are multiple data copies available, there arises a problem of synchronizing them so that the clients get fresh data consistently. Following are the consistency patterns available:
• Weak consistency: After a data write, the read request may or may not be able to get the new data. This type of consistency works well in real-time use cases like VoIP, video chat, real-time multiplayer games etc. For example, when we are on a phone call, if we lose network for a few seconds, then we lose information about what was spoken during that time.
• Eventual consistency: Post data write, the reads will eventually see the latest data within milliseconds. Here, the data is replicated asynchronously. These are seen in DNS and email systems. This works well in highly available systems.
• Strong consistency: After a data write, the subsequent reads will see the latest data. Here, the data is replicated synchronously. This is seen in RDBMS and file systems and are suitable in systems requiring transactions of data.

14. What are the most important aspects of the System Study?

Answer: The 3 most important aspects of System Study are as follows:

• Identifying current issues and establishing new goals.
• Study of an existing system.
• Documenting the existing system.

15. As a system designer, how you can design universal file-sharing and storage apps like Google Drive or Dropbox?

Answer: The above-mentioned apps are used to store and share files, photos, and other media. We can design things like allowing users to upload/search/view files or photos. It checks permissions for file sharing and enables multiple users to make changes in the same document.

16. What feature allows one class to derive features from another class?

Answer: The Inheritance feature allows one class to derive features from another class. But more often than not you should use Composition to reuse code from another class instead of Inheritance. Composition is more flexible than Inheritance and there are many benefits of using Composition like ease of testing. You can also see my post 5 Reasons to Choose Composition over Inheritance for more details.

17. Which language was the first to be developed as a purely object-oriented programming language?

Answer: Smalltalk was the first programming language to be developed as a purely object-oriented programming language. While many great programming languages support OOP like Java, C++, JavaScript, Python, and C#, none of them are pure object-oriented programming languages, if we’re wearing purist shoes at least.

18. How do you design a Trade position aggregator or Portfolio Manager?

You need to design a system that can accept Trade and then show the position for each symbol, much like a Portfolio Manager. Your system needs to support multiple symbols and it should be fast enough to calculate position in real time.

To test your system, you can input a set of trades, both buy and sell sides and then query the system to see the live position. You can first try solving this problem yourself but if you are stuck you can see my solution on implementing the Trade position aggregator in Java for guidance.

19. What considerations would you take into account when designing a scalable video streaming service?

Answer: Java is an OOPS programming language, but it does not support all inheritance types.

20. How would you design a system for processing and analyzing large-scale data in real time?

Answer: There are three types of constructors in C++, which are as follows:

• Default Constructors: It does not take any argument and has zero parameters.
• Parameterized Constructors: These types of constructors take some argument.
• Copy Constructors: It is a member function, and its basic function is to initialize the object using another object of the same class.

21. What is RAID?

Answer: RAID stands for Redundant Array of Independent Disks. RAID is the technology that specializes in data storage, combining various physical disk drive components within one or numerous logical units for data redundancy and performance improvement.

22. How do you design a Vending Machine in Java?

This is a common object-oriented analysis and design question that is also asked in System design interviews. You need to design a Vending Machine that can vend a couple of products like Coke, Biscuits, Chocolates,  and Cake. It can accept coins (Nickle, Dime, Pence, and Cent), and small denomination notes.

There are multiple ways to solve this problem but the solution using State Design Pattern is the simplest one. You can try yourself but if you are stuck you can also see my post How to Design a Vending Machine in Java for a step-by-step solution to this OOP Design problem.

23. Can a Java application be created without implementing the OOPs concept?

Answer: A Java application cannot be created without implementing the OOPs concept. Java applications are completely based on the Object-oriented programming concept.

24. Describe the design of a ticketing system for a large event or transportation service like TicketMaster for Taylor Swift ERAS Tour

Designing a ticketing system for a large event like Taylor Swift's ERAS Tour involves several key components to ensure smooth ticket sales, handling of high traffic volumes, and prevention of ticket scalping. Firstly, the system needs a user-facing interface accessible through web and mobile platforms where users can browse available tickets, select seats, and make purchases securely.

This interface should provide real-time updates on ticket availability and pricing, as well as interactive seating maps to allow users to choose their preferred seats. Additionally, the system should incorporate a robust authentication and authorization mechanism to prevent fraud and ensure fairness in ticket sales. This could involve implementing CAPTCHA, two-factor authentication, or anti-bot measures to deter automated ticket purchasing.

Behind the scenes, the ticketing system requires a scalable and fault-tolerant backend infrastructure capable of handling high volumes of concurrent transactions. This backend should include components such as databases to store event details, ticket inventory, and user information, as well as payment processing systems to facilitate secure transactions.

Load balancing and caching mechanisms can help distribute traffic across multiple servers and optimize performance during peak ticket sale periods. Furthermore, the system should incorporate comprehensive logging and monitoring functionalities to track ticket sales, detect unusual activity, and identify potential issues in real time.

Overall, a well-designed ticketing system for a large event like Taylor Swift's ERAS Tour should prioritize user experience, security, scalability, and reliability to ensure a seamless ticket-buying process for fans. Here is a nice diagram from Educative.io, another great resource for system design interview prep which gives a glimpse of a possible Ticketmaster architecture.

25. Describe the components and interactions in a content delivery network (CDN)

A Content Delivery Network (CDN) comprises several key components that work together to efficiently deliver content to users around the world. At the core of a CDN are multiple points of presence (PoPs) strategically distributed across different geographical locations. These PoPs consist of caching servers that store copies of static content, such as images, videos, CSS files, and JavaScript, closer to end-users. '

When a user requests content, the CDN's DNS servers direct the request to the nearest PoP based on factors like network proximity and server availability. The PoP then serves the content to the user, reducing latency and improving performance. Additionally, CDNs often include load balancers, security features like DDoS protection and Web Application Firewalls (WAF), and analytics tools to monitor traffic and performance.

The interactions within a CDN involve various components collaborating to efficiently deliver content to users. When a user requests content, the CDN's edge servers at the nearest PoP determine whether the requested content is already cached locally. If the content is cached, it is served directly from the edge server, minimizing latency.

If the content is not cached or has expired, the edge server retrieves it from the origin server, which could be a web server, storage service, or content management system.

The CDN then caches the content at the edge server for future requests, optimizing delivery for subsequent users. This caching mechanism, combined with intelligent routing and load balancing, ensures fast and reliable content delivery while reducing the load on origin servers and improving scalability.

Here is a nice diagram from exponent that shows how CDN works and interacts with different systems:

Best Books and Courses to Prepare for System Design Interviews in 2024

Now that you have a fair idea of what to expect in System design interviews, not just theory and conceptual questions but also System Design problems that require you to design the solution, its time to see resources to further improve your understanding and better prepare for interviews. let's see which book you can read and which online courses you can join to prepare well for your System design interview in 2024.

• Grokking the System Design Interview on DesignGuru
• Software Design Interview Course on Exponent
• Mastering the System Design Interview by Frank Kane (Ex Amazon Hiring Manager)
• Pragmatic System Design [Udemy Course]
• Grokking Modern System Design Interview for Engineers & Managers
• System Design Interview — An Insider’s Guide by Alex Xu

That's all about System Design Interview Questions and Answers for Coding Interviews. You will surely do better in your interview if you have mastered all the questions with answers in this article. That is the best thing you can do to yourself because this interview is about to change your life in some way. Just be ready for that day and you will see for yourself that everything can be easy and perfect if you do nothing but prepare.

Thanks for reading this article so far. All the best for your System Design and Coding interviews and if you have any questions that you don't know the answer to or any doubts feel free to ask me in the comments.

P. S. - If you struggle to solve System Design Questions then I highly recommend you to go through a comprehensive System Design course like Grokking the System Design Interview course on DeisgnGuru.io to not just learn essential System design concepts but also practice frequently asked System Design Problems.