System Design Questions

Design a URL Shortener service like bit.ly

Pastebin lets users store and share plain text through unique URLs. Developers use it for code snippets and configuration files. The core challenge: generate unique short URLs and serve content with low latency at scale.

Design a webhook service that processes incoming requests and ensures efficient, secure handling of real-time events, such as payment notifications from services like Stripe or Shopify. If you prefer the video format, here's a walkthrough of the problem: <iframe width="560" height="315" src="https://www.youtube.com/embed/4C9SVQVmUxs?si=VLTgs_oXCJf3TbI7" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>

A typeahead system predicts search queries as users type, showing relevant suggestions in real-time. The core challenge: serve suggestions within milliseconds while handling millions of concurrent users.

Design a location-based dating app where users discover nearby profiles, swipe right or left, and form mutual matches with real-time notifications.

Design a coding contest platform like LeetCode. <div class="responsive-iframe"> <iframe width="560" height="315" src="https://www.youtube.com/embed/NPg2iRtCr-I" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe> </div>

Design a distributed rate limiter to protect API availability and prevent abuse

Design a calendar system like Google Calendar with event management, invitations, RSVP, and notifications

Design a simplified version of Twitter where users can post tweets, follow/unfollow other users and is able to see the tweets of its followings.

Design a local business discovery platform with geo-based search and crowd-sourced reviews.

Design a retail stock-trading app — users watch live prices tick in real time and place orders that must move cash and shares exactly once, never twice, against an external exchange.

Design a file hosting service such as Dropbox or Google Drive. A cloud file hosting service allows users to upload, download, and share files across different platforms. It should support multiple file types including text, images, and multimedia formats. For this problem let's focus on the backend system that stores and syncs them between different devices.

Design a collaborative document editor where many people type into the same document at once, every keystroke shows up for others within a moment, and everyone converges to the identical text with no edit ever silently lost.

Design a event ticketing system like TicketMaster

Design live comments on a live broadcast — millions of concurrent viewers must see new comments stream in near-real-time, while comments at peak arrive faster than any person can read.

Design a news aggregator that crawls thousands of sources continuously and presents a fresh, ranked feed — where the real challenge is recognizing that twenty articles from twenty outlets are all one story.

Design full-text search over a social product's posts — a user types a multi-word query and gets back the most relevant posts, ranked, out of a corpus of billions that grows every second.

Design local, on-demand delivery of physical goods — a shopper sees what a nearby store actually has in stock, orders it, and watches a courier bring it, with two shoppers never both getting the last unit.

Design a real-time auction system where many bidders compete on an item, the price climbs live in front of thousands of watchers, and the auction closes at a deadline with an unambiguous winner.

Design a metrics monitoring system that ingests numeric measurements from millions of hosts, stores them as time-series, and evaluates alert rules — where the real scaling wall is cardinality, not raw write volume.

Design a distributed job scheduler that lets clients define jobs to run at a specific time or on a recurring schedule, dispatches them to workers with controlled resource usage, and provides secure visibility and control over execution.

Design a system that moves money between a payer and a payee, correctly, even though every step runs over networks that fail and an external payment provider you don't control.

Design a system that crawls the web at scale, starting from seed URLs, and keeps discovering and fetching new pages without looping or getting banned.

Design a pipeline that counts ad clicks accurately for billing while giving advertisers near-real-time aggregates.

Design a system to find the top k heavy hitters in a stream of data. This could be - the top k most played songs in a music streaming service like Spotify - the top k most viewed videos on a video streaming service like YouTube - the top k most bought items in an e-commerce service like Amazon - the top k most frequent items in a social media service like Twitter - the top k most accessed IP addresses on a network <div class="responsive-iframe"> <iframe width="560" height="315" src="https://www.youtube.com/embed/HjazbLlrWxI" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe> </div>

Design a ride-hailing service that matches riders with nearby drivers in real-time, handles hundreds of thousands of location updates per second, and manages the full trip lifecycle.

Design a distributed message queue that acts as a durable buffer between producers and consumers, supporting topics, partitions, consumer groups, and configurable retention.

Design a chat app like WhatsApp.

YouTube is an online video-sharing platform where users can upload, share, and view videos. It supports various types of videos, including TV show clips, music videos, short and documentary films, movie trailers, and other content such as video blogging (vlogging), short original videos, and educational videos.

Design the home feed for a social product — a ranked, near-real-time stream of posts from the accounts a user follows. The hard parts are fan-out and ranking, and they are coupled.

Design a photo-sharing service — upload images, follow accounts, and see a timeline of recent posts. The read path is a News Feed variant; the new problem is the media write path — upload, transcode, durable blob storage, and CDN delivery.

Design an in-memory caching layer that serves reads at sub-millisecond latency for tens of millions of requests per second, shards across many nodes, survives node loss without taking the database down, and has a defensible answer for stale data.

Design a horizontally-scalable key-value store that stays available and low-latency across hundreds of nodes and multiple datacenters, survives node and network failures, and has a defensible answer for what a read returns after a write that raced a partition. The Dynamo / Cassandra / Riak lineage.

Design a mapping service like Google Maps