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System Design Interview Secrets: A Step-by-Step Guide with Real-World Examples


What is a System Design Interview?




A system design interview is a type of technical interview that evaluates a candidate's ability to design a large-scale distributed system that meets certain requirements. It is usually conducted by senior engineers or architects who have experience in building complex systems.




[PDF] System Design Interview



In a system design interview, the interviewer presents a high-level problem statement (e.g., design Twitter, design Netflix, design Uber) and expects the candidate to propose a solution that covers various aspects of the system, such as:


  • The architecture and components of the system



  • The data model and storage strategy of the system



  • The communication and integration methods of the system



  • The scalability and reliability of the system



  • The performance and cost optimization of the system



  • The security and privacy considerations of the system



The interviewer may also ask follow-up questions or challenge some assumptions or decisions made by the candidate. The goal is to simulate a realistic scenario where the candidate and the interviewer are working together to determine the best design decision.


Why are System Design Interviews important?




System design interviews are important for several reasons:


  • They test the candidate's knowledge and skills in designing large-scale systems that are relevant for many software engineering roles.



  • They assess the candidate's ability to think critically, creatively, and systematically about complex problems that have no definitive answers.



  • They evaluate the candidate's communication and collaboration skills in explaining their ideas, reasoning, and trade-offs to others.



  • They provide an opportunity for the candidate to showcase their domain expertise, industry experience, and personal interests in building systems.



System design interviews are also important for companies because they help them identify candidates who can:


  • Deliver high-quality solutions that meet business needs and user expectations.



  • Work effectively with other engineers, product managers, designers, and stakeholders on cross-functional teams.



  • Learn new technologies, tools, and frameworks quickly and adapt to changing requirements and environments.



  • Handle the complexity, ambiguity, and uncertainty of real-world systems.



How to prepare for a System Design Interview?




Preparing for a system design interview can be challenging, especially if you have never worked on large-scale systems before. However, with the right mindset and preparation, you can feel confident and ready to tackle any system design question that comes your way. Here are some steps you can take to prepare for a system design interview:


Understand the requirements and scope of the problem




The first step in solving any system design question is to understand the requirements and scope of the problem. You should ask clarifying questions to the interviewer to make sure you have a clear picture of what the system is supposed to do, who are the users, what are the features, and what are the constraints. For example, you can ask questions like:


  • What are the main functionalities of the system?



  • How many users are expected to use the system?



  • What are the latency and availability requirements of the system?



  • What are the security and privacy requirements of the system?



  • Are there any existing systems or platforms that the system needs to integrate with?



By asking these questions, you can narrow down the scope of the problem and focus on the most important aspects of the system. You can also use these questions to make some reasonable assumptions and trade-offs based on the given information.


Choose an appropriate architecture and design principles




The next step in solving any system design question is to choose an appropriate architecture and design principles for the system. You should consider different factors such as scalability, reliability, security, performance, cost, etc. when choosing an architecture and design principles. For example, you can consider questions like:


  • What are the advantages and disadvantages of different architecture styles (e.g., monolithic, microservices, serverless, etc.) for the system?



  • What are the best practices and patterns for designing distributed systems (e.g., load balancing, caching, replication, sharding, etc.)?



  • How can you ensure consistency, availability, and fault tolerance of the system?



  • How can you protect the system from malicious attacks and unauthorized access?



  • How can you optimize the performance and efficiency of the system?



  • How can you minimize the cost and complexity of the system?



By choosing an appropriate architecture and design principles, you can ensure that your solution meets the requirements and constraints of the problem. You can also explain your rationale and trade-offs to the interviewer and demonstrate your understanding of different concepts and technologies.


Break down the problem into components and interfaces




The third step in solving any system design question is to break down the problem into components and interfaces. You should identify the main components of the system and define their interfaces and interactions. For example, you can use questions like:


  • What are the core components of the system (e.g., web servers, application servers, database servers, etc.)?



  • What are the responsibilities and functionalities of each component?



  • How do these components communicate with each other (e.g., RESTful APIs, message queues, RPCs, etc.)?



  • How do these components scale horizontally and vertically?



  • How do these components handle failures and errors?



By breaking down the problem into components and interfaces, you can simplify the complexity of the system and modularize your solution. You can also use diagrams or sketches to illustrate your design and make it easier for the interviewer to follow.


Estimate the resources and trade-offs of the system




The fourth step in solving any system design question is to estimate the resources and trade-offs of the system. You should estimate the capacity, performance, cost, and trade-offs of your solution based on some assumptions or benchmarks. For example, you can use questions like:


  • How much data will be stored in the system (e.g., size, growth rate, retention policy, etc.)?



  • How much traffic will be handled by the system (e.g., requests per second, peak hours, etc.)?



  • How much latency will be experienced by the users (e.g., average response time, percentile response time, etc.)?



  • How much bandwidth will be consumed by the system (e.g., upload/download speed, data compression, etc.)?



Test and optimize the system




The fifth and final step in solving any system design question is to test and optimize the system. You should test the functionality, availability, and performance of your solution and optimize it for efficiency and quality. For example, you can use questions like:


  • How will you test the functionality and correctness of the system (e.g., unit testing, integration testing, end-to-end testing, etc.)?



  • How will you monitor the health and status of the system (e.g., logging, metrics, alerts, dashboards, etc.)?



  • How will you troubleshoot and debug the system (e.g., tracing, profiling, debugging tools, etc.)?



  • How will you improve the scalability and reliability of the system (e.g., load testing, stress testing, chaos engineering, etc.)?



  • How will you enhance the performance and efficiency of the system (e.g., caching, compression, batching, parallelism, etc.)?



By testing and optimizing the system, you can ensure that your solution meets the quality standards and expectations of the users and stakeholders. You can also demonstrate your ability to handle real-world scenarios and challenges.


What are some common System Design Interview questions?




There are many possible system design interview questions that can be asked in different companies and roles. However, some questions are more popular and frequently asked than others. Here are some examples of common system design interview questions with brief descriptions:


  • Design a chat service: This question asks you to design a system that allows users to send and receive messages in real-time. You need to consider aspects such as scalability, reliability, security, latency, etc.



  • Design a ride-sharing service: This question asks you to design a system that matches drivers and riders based on their locations and preferences. You need to consider aspects such as geolocation, routing, pricing, availability, etc.



  • Design a URL-shortening service: This question asks you to design a system that generates short and unique URLs for long URLs. You need to consider aspects such as encoding, decoding, hashing, collision handling, etc.



  • Design a social media newsfeed: This question asks you to design a system that displays relevant and personalized posts for users based on their interests and activities. You need to consider aspects such as ranking, filtering, caching, updating, etc.



  • Design a social message board: This question asks you to design a system that allows users to create and join groups, post messages, comment on messages, and vote on messages. You need to consider aspects such as data model, storage strategy, concurrency control, etc.



transcoding strategy, etc.


  • Design a file sharing service: This question asks you to design a system that allows users to upload and download files from a cloud storage. You need to consider aspects such as authentication, authorization, encryption, synchronization, etc.



  • Design Google Docs: This question asks you to design a system that allows multiple users to edit and collaborate on a document in real-time. You need to consider aspects such as concurrency control, conflict resolution, versioning, etc.



  • Design a video streaming service: This question asks you to design a system that allows users to watch live or recorded videos. You need to consider aspects such as streaming strategy, buffering strategy, adaptive bitrate, etc.



  • Design an API Rate Limiter: This question asks you to design a system that limits the number of requests that can be made to an API within a certain time window. You need to consider aspects such as rate limiting algorithms, data structures, distributed counters, etc.



  • Design a web crawler: This question asks you to design a system that crawls the web and extracts useful information from web pages. You need to consider aspects such as crawling strategy, scheduling strategy, parsing strategy, deduplication strategy, etc.



  • Design a proximity service: This question asks you to design a system that finds nearby users or places based on their locations. You need to consider aspects such as geohashing, spatial indexing, distance calculation, etc.



  • Design typeahead: This question asks you to design a system that provides autocomplete suggestions for user queries. You need to consider aspects such as trie data structure, prefix matching, ranking algorithm, etc.



  • Design Google Maps: This question asks you to design a system that provides maps and navigation services for users. You need to consider aspects such as map data model, map rendering, routing algorithm, traffic estimation, etc.



How to answer a System Design Interview question?




To answer a system design interview question effectively, you need to follow a systematic approach that covers all the steps and aspects of the problem. You also need to communicate your thoughts clearly and concisely with the interviewer and justify your decisions and trade-offs. To illustrate this process, let's take an example of a common system design interview question: Design a URL shortening service.


Example: Design a URL shortening service




A URL shortening service is a system that generates short and unique URLs for long URLs. For example, https://www.educative.io/blog/top-10-system-design-interview-questions can be shortened to https://bit.ly/3FQjwXs. The main functionalities of a URL shortening service are:


  • Given a long URL, generate a short URL and store the mapping in the database.



  • Given a short URL, redirect the user to the corresponding long URL.



Here are the steps to design a URL shortening service:


Step 1: Understand the requirements and scope of the problem




The first step is to ask clarifying questions to understand the requirements and scope of the problem. For example:


What are the expected features of the system? For example:


  • Custom URLs: Users can specify their own short URLs instead of using randomly generated ones.



  • Analytics: Users can view statistics such as click counts and traffic sources for their short URLs.



  • Expiration: Users can set an expiration date for their short URLs after which they will be deleted.




What are the expected constraints of the system? For example:


  • Traffic: How many requests per second (RPS) are expected for generating and redirecting short URLs?



  • Data: How many short URLs are expected to be stored in the system? How long are they expected to be valid?



  • Performance: What is the expected latency for generating and redirecting short URLs?



  • Availability: What is the expected availability of the system? How can we handle failures and errors?




Step 2: Choose an appropriate architecture and design principles




The next step is to choose an appropriate architecture and design principles for the system. For example:


  • Architecture: We can use a microservice architecture to separate the functionality of the system into different services. For example, we can have a URL service that handles the generation and redirection of short URLs, an analytics service that handles the statistics and reporting of short URLs, and an expiration service that handles the deletion of expired short URLs.



Design principles: We can apply some design principles to ensure the scalability, reliability, security, performance, and cost of the system. For example:


  • Scalability: We can use load balancers to distribute the traffic among multiple instances of each service. We can also use horizontal scaling to add or remove instances as needed.



  • Reliability: We can use replication and redundancy to ensure the availability and fault tolerance of each service. We can also use backup and restore mechanisms to recover from data loss or corruption.



  • Security: We can use encryption and hashing to protect the data in transit and at rest. We can also use authentication and authorization mechanisms to control access to the system.



  • Performance: We can use caching and compression to improve the latency and bandwidth of the system. We can also use batching and parallelism to improve the throughput and efficiency of the system.



  • Cost: We can use cloud services and platforms to reduce the operational and maintenance costs of the system. We can also use monitoring and alerting tools to optimize the resource utilization and performance of the system.




Step 3: Break down the problem into components and interfaces




The third step is to break down the problem into components and interfaces. For example:


Components: We can identify the main components of the system and their responsibilities. For example:


  • URL service: This component is responsible for generating and redirecting short URLs. It consists of a web server that handles HTTP requests from users, a URL generator that generates short URLs using a hashing algorithm, a URL database that stores the mapping between short URLs and long URLs, and a cache that stores frequently accessed short URLs for faster redirection.



  • Analytics service: This component is responsible for collecting and analyzing statistics for short URLs. It consists of a web server that handles HTTP requests from users, an analytics engine that processes and aggregates data from various sources, an analytics database that stores the statistics for short URLs, and a dashboard that displays the reports and charts for users.



  • Expiration service: This component is responsible for deleting expired short URLs. It consists of a scheduler that periodically scans the URL database for expired short URLs, a deleter that deletes them from the database and cache, and a notifier that notifies users about their expired short URLs.




Interfaces: We can define the interfaces and interactions between different components. For example:


The URL service exposes a RESTful API for generating and redirecting short URLs. For example:


  • POST /shorten: This endpoint takes a long URL as input and returns a short URL as output.



  • GET /shortUrl: This endpoint takes a short URL as input and redirects the user to the corresponding long URL as output.





The analytics service exposes a RESTful API for viewing statistics for short URLs. For example:


  • GET /stats/shortUrl: This endpoint takes a short URL as input and returns its statistics as output.





The expiration service communicates with the URL service using message queues or RPCs. For example:


  • The expiration service sends a message or makes a call to the URL service to delete an expired short URL from the database and cache.






Step 4: Estimate the resources and trade-offs of the system




the system. For example:


Capacity: We can estimate the amount of data and traffic that the system needs to handle. For example:


Data: Assuming we have 1 billion short URLs, each short URL is 7 characters long, each character is 1 byte, and each mapping entry has some metadata (e.g., creation date, expiration date, etc.), we can estimate the total data size as follows:


  • Data size = 1 billion * (7 bytes + metadata)



  • Assuming the metadata is 10 bytes, we get:



  • Data size = 1 billion * (7 + 10) bytes



  • Data size = 17 GB





Traffic: Assuming we have 100 million daily active users (DAU), each user generates and redirects 10 short URLs per day on average, and each request is 500 bytes on average, we can estimate the total traffic as follows:


  • Traffic = 100 million * 10 * 2 * 500 bytes



  • Traffic = 100 GB/day



  • Traffic = 1.2 KB/s






Performance: We can estimate the latency and throughput of the system. For example:


Latency: Assuming we have a round-trip time (RTT) of 100 ms between the user and the server, and a processing time of 50 ms on the server, we can estimate the average latency as follows:


  • Latency = RTT + processing time



  • Latency = 100 ms + 50 ms



  • Latency = 150 ms





Throughput: Assuming we have


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