System design questions.
72 system designquestions from the bank — open to read. Pick one and practice it out loud; a coach note comes back in seconds.
All 72 questions
Design a URL shortener that needs to handle 50K writes per day and 5M reads. What are your first three clarifying questions?Design a system that supports real-time presence — knowing within seconds whether each of 10 million users is online.Design the back-end for a global airline seat-selection service. What's harder than it looks here?Sketch a system that ingests structured event logs from millions of devices and lets analysts run ad-hoc queries within minutes.Design a feed for a social platform with 500 million users. What's the read-time versus write-time fanout decision and where do you land?Design the API and back-end for a notification service that fans messages out via email, push, and SMS, respecting user preferences.Design a system to track and bill API usage for a developer platform. Bills go out monthly and customers care about real-time visibility.Design a ride-matching service that pairs drivers and riders within a city. Calls per second peaks during commute hours.Design the back-end for a flash-sale system that has to sell exactly 10,000 units and not one more.Design a metrics dashboard that ingests 10K events per second and shows 1-second-resolution graphs back to thousands of users.Design a service that lets users upload videos and watch them within seconds, even on a phone over 3G.Design the back-end for a payment-processing API that needs to be idempotent and survive partial failures.Design a system to detect and block fraudulent transactions in under 200 milliseconds at peak traffic.Design a collaborative document editor where multiple users see each other's edits within a second. What's your conflict-resolution story?Sketch a back-end for a chat application supporting one-on-one and group conversations, with full message history.Design a system that lets users upload images and get back AI-generated descriptions. The model is slow — how do you keep the UI responsive?Design the back-end for a system that schedules and runs millions of cron-like jobs across a fleet. What goes wrong at scale?Design an authentication service that other services in your company depend on. What does the API look like and how do you avoid being a single point of failure?Design a system that lets users follow stock prices in real time. What components are non-negotiable, and where do you cut?Design a system that recommends products to users based on their past purchases. Walk me through both online and offline components.Design the back-end for a money transfer between two users on the same platform. What invariants must always hold?Design a search-typeahead service that returns suggestions within 100ms. Inventory has 50 million items.Sketch a system that ingests millions of small IoT sensor readings per second from a global fleet of devices.Design a system that lets users see how many times each of their posts has been viewed, with the count visible within seconds of a view happening.Design the API surface for an external developer to integrate a calendar service into their app. What endpoints, what auth, what rate limits?Design a system that lets users post photos and have them appear in followers' feeds globally within seconds. Latency budgets matter.Design a system for a marketplace where sellers list products and buyers purchase them. Where are the natural service boundaries?Design a global session store for an app with users in multiple regions. What's your replication and consistency story?Design a system that lets each user define custom alerts on a stream of data and notifies them when conditions are met.Design the back-end for an autocomplete search box that suggests popular queries personalized to the user.Sketch a system that handles 100K concurrent video calls with screen sharing and chat. Calls last 30 minutes on average.Design a webhook delivery system that guarantees at-least-once delivery to customer endpoints, including handling slow or down receivers.Design a system that lets a million users each upload daily activity data and view weekly summaries. Cost per user matters.Design a system that lets developers feature-flag changes for subsets of users with sub-second propagation of flag changes.Design a system to compute and serve nightly machine-learning predictions for every active user. Some predictions are stale-tolerant and some are not.Design a system that lets users bookmark articles and retrieve them later. You expect 100K active users who each save about 10 articles per month. What's your data model and how do you handle retrieval?Design a leaderboard system for a mobile game that shows the top 100 players globally. The game has 2 million daily active users. How do you keep the leaderboard fresh without recomputing it on every score update?Design a simple polling system where users vote on yes/no questions and see live results. Each poll gets about 10K votes over 24 hours. Do you update the count synchronously on every vote or use a different approach?Design a REST API for a task management app where users can create, update, complete, and delete tasks. What endpoints do you expose, and how do you handle listing tasks with basic filtering?Design a system that sends daily email digests to users summarizing their activity. You have 500K users. Do you generate emails on-demand when users open them or pre-generate and store them? Walk through your reasoning.Design a system where users can like posts. Each post can have thousands of likes. How do you store likes, and how do you quickly answer 'has user X liked post Y?' for rendering the UI?Design a comment system for blog posts. Comments are nested (replies to replies). How do you model this in your database, and how do you fetch and display a full comment thread efficiently?Design a system that tracks the number of page views for each article on a news site. Articles get 10M views per day total. Do you increment a counter in the database on every view, or batch updates? What breaks first?Design an API for a third-party weather service that your app calls to display forecasts. How do you handle cases where the external API is slow or down? What do you show users?Design a system where users upload profile pictures. Pictures need to be resized to three different dimensions. Do you resize synchronously during upload or asynchronously afterward? What are the pros and cons?Design a simple inventory system for an e-commerce site. When a user adds an item to their cart, do you reserve that inventory immediately or only at checkout? What happens if stock runs out?Design an API for a note-taking app where users create, edit, and delete notes. How do you version the API so that mobile clients don't break when you add new fields or endpoints?Design a distributed rate-limiter that enforces per-user quotas across thousands of API servers. How do you handle clock skew and avoid a single point of failure?Design the backend for a live auction platform where 100K concurrent users bid on items that close at precise times. How do you prevent overselling?Design a service that aggregates real-time stock prices from 20 exchanges and delivers sub-100ms updates to trading terminals. What's your data model?Design a search autocomplete system for an e-commerce site with 100M products. How do you balance freshness, relevance, and latency?Design the backend for a collaborative document editor like Google Docs. How do you handle concurrent edits from 50 users on the same paragraph?Design a recommendation engine API that personalizes results for 10M users. When do you compute recommendations: query time, write time, or scheduled batch?Design a system to detect and prevent credit card fraud in real-time as transactions arrive. You have 200ms to approve or decline. What's your approach?Design a distributed lock service that other systems depend on for leader election and coordination. What happens when the lock service itself needs maintenance?Design the API for a multi-tenant SaaS product where each customer expects total data isolation. How does this impact your schema and query patterns?Design a system that schedules and executes millions of delayed jobs (e.g., send email in 3 days). How do you handle retries and guarantee at-least-once delivery?Design a content moderation system that reviews 1M user-uploaded images per day, combining ML models and human reviewers. How do you prioritize the review queue?Design a global CDN that caches and serves static assets. A customer updates a file — how quickly can you invalidate it across 200 edge locations?Design the backend for a food delivery app that matches orders to couriers and provides real-time ETA updates. What breaks when a city has a local event with 10x traffic?Design an API versioning strategy for a public platform with 10K external developers. How do you deprecate old versions without breaking existing integrations?Design a system to backup and restore petabyte-scale databases with point-in-time recovery. What's your RTO and RPO, and what do they cost?Design a leaderboard system for a mobile game with 50M daily active users. Players expect to see their global rank update within 10 seconds of finishing a match.Design the infrastructure for A/B testing at scale: 1000 concurrent experiments, each with custom targeting rules. How do you assign users and measure statistical significance?Design a service that reconciles financial transactions across multiple systems overnight. If you find a discrepancy, what's your resolution workflow?Design a chat system that supports 1-on-1 and group conversations with message history. How do you handle a user who joins a 10K-member group and scrolls to the beginning?Design an API gateway that sits in front of 100 microservices. How do you handle authentication, rate limiting, and request routing without becoming a bottleneck?Design a system to track user consent and privacy preferences across web, mobile, and email channels. GDPR requires you to honor deletion requests within 30 days.Design a session management system for a web application. When should you use cookies vs. tokens, and how do you handle session expiration and renewal?Design a system that processes insurance claims: intake, fraud detection, adjuster assignment, and payout. Each claim touches 3-5 systems. How do you ensure consistency?Design a feature flag service that controls rollouts across 10K servers. A bad flag takes down production — how do you build in safeguards and fast rollback?Design the backend for a marketplace that connects buyers and sellers. How do you handle escrow, dispute resolution, and preventing double-spending of seller inventory?