Introduction
System design interviews are a critical part of the hiring process for senior and mid-level engineering roles. They test your ability to build large-scale, fault-tolerant, and performant systems from the ground up. Mastering these interviews requires not only technical knowledge but also the ability to articulate your thought process, justify trade-offs, and consider real-world constraints. To help you prepare, we’ve compiled detailed answers to the most frequently asked system design interview questions in 2025.
1. Design a URL Shortening Service like bit.ly
One of the most common system design interview questions involves creating a URL shortening service. This service takes a long URL and generates a short, unique key for it.
- Key Components:
- Short Key Generation: The most efficient way to generate a short key is to use a unique ID (from a database auto-increment or a UUID) and convert it to a smaller base, like Base62 (a combination of a-z, A-Z, and 0-9). This allows for a massive number of unique keys.
- Data Storage: A scalable database is essential for storing the mapping of
short_keytolong_url. A NoSQL database like DynamoDB or Cassandra is a good choice due to its high write and read throughput, which is crucial for handling redirects. - Caching: To handle the high read volume for redirects, use a distributed cache like Redis. Caching the most frequently accessed
short_keytolong_urlmappings is a key strategy for this system design question.
- Scaling & Reliability: Your design should include load balancers to distribute traffic, sharding for your database, and data replication for durability and failover.
2. Design a Social Media Feed
Designing a social media feed is a popular system design interview question that tests your knowledge of data delivery at scale. This system must handle a high volume of posts and efficiently deliver them to users.
- Key Components:
- Data Storage: Store users, posts, and followers in a scalable database. A relational database might be suitable for user data, while a NoSQL database could handle the high-volume posts.
- Feed Generation: There are two main approaches to this system design problem:
- Fan-out-on-write: When a user posts, immediately push the new post to the feeds of all their followers. This is great for a smaller user base, as reading is fast.
- Fan-out-on-read: When a user opens their feed, fetch the posts from all the people they follow and merge them. This is better for a large-scale system with users who have millions of followers (e.g., celebrities).
- Caching & Ranking: Cache user feeds for fast access. Implement a ranking algorithm to order posts by relevance (e.g., using factors like likes, comments, and post recency).
3. Design a Messaging Service like WhatsApp
In system design interviews, questions about messaging services are common because they involve real-time communication and robust data delivery.
- Key Components:
- Message Delivery: Use a persistent message queue (e.g., RabbitMQ, Kafka) to ensure messages are delivered reliably.
- End-to-End Encryption: Implement end-to-end encryption so that only the sender and receiver can read messages.
- Presence Services: Use a dedicated service to show the online status of users.
- Offline Support: Messages should be stored on the server until the recipient is back online.
- Notifications: Use push notification services to alert users of new messages when the app is in the background.
4. Design an Online Bookstore
An online bookstore (like Amazon) is a classic e-commerce system design problem that touches on multiple components. When tackling this system design interview question, be sure to consider all the major services.
- Key Components:
- Catalog Service: Manages a large catalog of books, including metadata like title, author, price, and images.
- Search Service: A specialized search engine, often powered by Elasticsearch or Lucene, to provide fast, full-text searches across the catalog.
- E-commerce Modules: Services for user accounts, order processing, inventory management, and payment processing.
- Recommendation Engine: An engine that uses machine learning to provide personalized book suggestions based on a user’s history and behavior.
5. Design a File Storage Service
A service like Dropbox or Google Drive requires a distributed system to handle massive amounts of data reliably and securely. This is a challenging but insightful system design question.
- Key Components:
- File Chunks: Files are broken down into smaller chunks and stored across a distributed file system like HDFS or S3. This improves upload/download speed and resilience.
- Metadata Service: A database that tracks file information, chunk locations, access permissions, and version history.
- Replication: Chunks are replicated across multiple servers to ensure durability.
- Consistency: The system must handle consistency, using either a strong consistency model (for metadata) or an eventual consistency model (for file replication).
6. Design a Ride-Sharing Service
This popular system design interview question tests your ability to handle real-time data and dynamic matching. A ride-sharing service like Uber is a complex real-time system with many moving parts.
- Key Components:
- Location Tracking: A system that tracks the real-time location of both drivers and riders using GPS.
- Matching Engine: A service that efficiently pairs nearby riders and drivers. This often uses algorithms to optimize for distance, driver rating, and time.
- Pricing: A dynamic pricing service that implements surge pricing based on real-time demand and supply.
- Notifications: A notification service to alert users of ride status, driver arrival, etc.
- Payment & Ratings: Services to handle payments, driver ratings, and user feedback.
7. Design a Web Crawler
Designing a web crawler is an excellent system design question for assessing your understanding of distributed systems and data processing.
- Key Components:
- URL Frontier: A queue that manages all the URLs to be crawled. This queue is prioritized to crawl important pages first.
- Fetcher Workers: A pool of workers that download the HTML content of the pages from the URL frontier.
- Parser: A module that extracts new URLs from the downloaded pages and adds them to the URL frontier.
- Deduplication: A mechanism to ensure the same page is not crawled repeatedly.
- Indexer: A component that processes the content and stores it in a searchable format for an index.
8. Design a Video Streaming Service
This classic system design interview question requires you to think about how to deliver video content efficiently and reliably to a global audience.
- Key Components:
- Content Delivery Network (CDN): A network of servers distributed globally that cache and serve video content to users from the nearest location. This significantly reduces latency and bandwidth costs.
- Adaptive Bitrate Streaming (ABS): The video is encoded into multiple quality levels. The player automatically switches between these levels based on the user’s network conditions to ensure smooth playback.
- Transcoding Pipeline: A workflow that converts raw video files into various formats and bitrates for different devices and network conditions.
- Player: The client-side application that handles video playback, buffering, and playback control.
9. Design a Cache System
This is a fundamental system design interview question that assesses your knowledge of data storage optimization. A cache is a high-speed data storage layer that stores a subset of data to serve future requests faster.
- Key Components:
- Eviction Policy: An algorithm that determines which data to remove when the cache is full. Common policies include LRU (Least Recently Used) and LFU (Least Frequently Used).
- Consistent Hashing: A technique used in distributed caches to distribute keys evenly among cache nodes, minimizing key re-distribution when nodes are added or removed.
- Invalidation: A strategy to ensure cached data remains fresh, such as write-through or write-back.
- Replication & Failover: Replicating cache nodes ensures that data is not lost and the system remains available if a node fails.
10. Design a Real-Time Chat Application
This system design question is similar to a messaging service but focuses more on live, synchronous communication, making it a great way to showcase your skills.
- Key Components:
- WebSockets: The primary protocol for real-time, bi-directional communication between the client and server.
- Message Queues: Used for reliable message delivery and to handle backpressure when the server is overwhelmed.
- Persistent Storage: A database to store chat history, ensuring that messages are not lost and can be retrieved later.
- Presence Detection: A system to track and broadcast user statuses, such as “online,” “offline,” or “typing.”
By thoroughly reviewing these system design interview questions, you will be well-prepared to demonstrate a comprehensive understanding of how to build robust, scalable, and reliable software systems.
This article is part of our Interview Prep series.
Good luck with your system design interview preparation!
