System design questions.

72 system design questions from the bank, ready to practice out loud.

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All 72 questions

URL shortener design clarificationsDesign a URL shortener that needs to handle 50K writes per day and 5M reads. What are your first three clarifying questions?ArchitectureMid–senior

Real-time presence system designDesign a system that supports real-time presence — knowing within seconds whether each of 10 million users is online.ScalingSenior–staff+

Global airline seat-selection back-endDesign the back-end for a global airline seat-selection service. What's harder than it looks here?ConsistencySenior–staff+

Event log ingestion and queryingSketch a system that ingests structured event logs from millions of devices and lets analysts run ad-hoc queries within minutes.ArchitectureSenior–staff+

500M user feed fanoutDesign 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?ScalingSenior–staff+

Multi-channel notification serviceDesign the API and back-end for a notification service that fans messages out via email, push, and SMS, respecting user preferences.Api DesignSenior–staff+

API usage tracking and billingDesign a system to track and bill API usage for a developer platform. Bills go out monthly and customers care about real-time visibility.TradeoffsSenior–staff+

Ride-matching during peak hoursDesign a ride-matching service that pairs drivers and riders within a city. Calls per second peaks during commute hours.ArchitectureSenior–staff+

Flash-sale inventory enforcementDesign the back-end for a flash-sale system that has to sell exactly 10,000 units and not one more.ConsistencySenior–staff+

High-resolution metrics dashboardDesign a metrics dashboard that ingests 10K events per second and shows 1-second-resolution graphs back to thousands of users.ArchitectureSenior–staff+

Video upload and playback serviceDesign a service that lets users upload videos and watch them within seconds, even on a phone over 3G.TradeoffsSenior–staff+

Idempotent payment processingDesign the back-end for a payment-processing API that needs to be idempotent and survive partial failures.Api DesignSenior–staff+

Fraud detection under 200msDesign a system to detect and block fraudulent transactions in under 200 milliseconds at peak traffic.ScalingSenior–staff+

Collaborative document editorDesign a collaborative document editor where multiple users see each other's edits within a second. What's your conflict-resolution story?ConsistencySenior–staff+

Chat application with message historySketch a back-end for a chat application supporting one-on-one and group conversations, with full message history.ArchitectureMid–senior

Slow AI image descriptionsDesign a system that lets users upload images and get back AI-generated descriptions. The model is slow — how do you keep the UI responsive?TradeoffsSenior–staff+

Distributed cron job schedulerDesign the back-end for a system that schedules and runs millions of cron-like jobs across a fleet. What goes wrong at scale?ScalingSenior–staff+

Highly available authentication serviceDesign 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?Api DesignSenior–staff+

Real-time stock price trackingDesign a system that lets users follow stock prices in real time. What components are non-negotiable, and where do you cut?ArchitectureSenior–staff+

Product recommendation systemDesign a system that recommends products to users based on their past purchases. Walk me through both online and offline components.ScalingSenior–staff+

Money transfer between usersDesign the back-end for a money transfer between two users on the same platform. What invariants must always hold?ConsistencySenior–staff+

Search typeahead with 50M itemsDesign a search-typeahead service that returns suggestions within 100ms. Inventory has 50 million items.TradeoffsSenior–staff+

IoT sensor data ingestionSketch a system that ingests millions of small IoT sensor readings per second from a global fleet of devices.ArchitectureSenior–staff+

Real-time post view countingDesign 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.ScalingSenior–staff+

Calendar service API integrationDesign the API surface for an external developer to integrate a calendar service into their app. What endpoints, what auth, what rate limits?Api DesignSenior–staff+

Photo feed with global latencyDesign a system that lets users post photos and have them appear in followers' feeds globally within seconds. Latency budgets matter.TradeoffsSenior–staff+

Marketplace with service boundariesDesign a system for a marketplace where sellers list products and buyers purchase them. Where are the natural service boundaries?ArchitectureSenior–staff+

Global session store with replicationDesign a global session store for an app with users in multiple regions. What's your replication and consistency story?ConsistencySenior–staff+

Custom alerts on data streamsDesign a system that lets each user define custom alerts on a stream of data and notifies them when conditions are met.ScalingSenior–staff+

Personalized autocomplete searchDesign the back-end for an autocomplete search box that suggests popular queries personalized to the user.TradeoffsSenior–staff+

Video calls with screen sharingSketch a system that handles 100K concurrent video calls with screen sharing and chat. Calls last 30 minutes on average.ArchitectureSenior–staff+

Webhook delivery guaranteesDesign a webhook delivery system that guarantees at-least-once delivery to customer endpoints, including handling slow or down receivers.Api DesignSenior–staff+

Daily activity uploads and summariesDesign a system that lets a million users each upload daily activity data and view weekly summaries. Cost per user matters.ScalingSenior–staff+

Feature flags with fast propagationDesign a system that lets developers feature-flag changes for subsets of users with sub-second propagation of flag changes.ConsistencySenior–staff+

Nightly ML predictions at scaleDesign a system to compute and serve nightly machine-learning predictions for every active user. Some predictions are stale-tolerant and some are not.TradeoffsSenior–staff+

Article bookmarking and retrievalDesign 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?ArchitectureEntry–mid

Mobile game leaderboardDesign 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?ScalingEntry–mid

Live polling with vote countsDesign 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?ConsistencyEntry–mid

Task management REST APIDesign 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?Api DesignEntry–mid

Daily email digest generationDesign 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.TradeoffsEntry–mid

Post likes and user checksDesign 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?ArchitectureEntry–mid

Nested comment threadsDesign 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?ArchitectureEntry–mid

Page view counting at scaleDesign 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?ScalingEntry–mid

External weather API resilienceDesign 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?TradeoffsEntry–mid

Profile picture resizingDesign 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?TradeoffsEntry–mid

E-commerce inventory reservationDesign 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?ConsistencyEntry–mid

Note-taking API versioningDesign 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?Api DesignEntry–mid

Distributed rate limitingDesign 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?ArchitectureSenior–leadership

Live auction platformDesign the backend for a live auction platform where 100K concurrent users bid on items that close at precise times. How do you prevent overselling?ConsistencySenior–staff+

Real-time stock price aggregationDesign a service that aggregates real-time stock prices from 20 exchanges and delivers sub-100ms updates to trading terminals. What's your data model?ScalingSenior–leadership

E-commerce search autocompleteDesign a search autocomplete system for an e-commerce site with 100M products. How do you balance freshness, relevance, and latency?TradeoffsMid–senior

Collaborative document editingDesign the backend for a collaborative document editor like Google Docs. How do you handle concurrent edits from 50 users on the same paragraph?ConsistencySenior–leadership

Recommendation engine computation timingDesign a recommendation engine API that personalizes results for 10M users. When do you compute recommendations: query time, write time, or scheduled batch?TradeoffsMid–senior

Real-time credit card fraud detectionDesign 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?ArchitectureSenior–leadership

Distributed lock service maintenanceDesign a distributed lock service that other systems depend on for leader election and coordination. What happens when the lock service itself needs maintenance?ArchitectureStaff+–leadership

Multi-tenant data isolationDesign the API for a multi-tenant SaaS product where each customer expects total data isolation. How does this impact your schema and query patterns?Api DesignMid–senior

Delayed job scheduling at scaleDesign 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?ArchitectureMid–staff+

Content moderation queue prioritizationDesign 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?TradeoffsMid–senior

Global CDN cache invalidationDesign a global CDN that caches and serves static assets. A customer updates a file — how quickly can you invalidate it across 200 edge locations?ConsistencySenior–staff+

Food delivery matching and ETADesign 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?ScalingMid–senior

API versioning and deprecationDesign an API versioning strategy for a public platform with 10K external developers. How do you deprecate old versions without breaking existing integrations?Api DesignSenior–leadership

Petabyte-scale database backupDesign 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?TradeoffsSenior–leadership

Mobile game leaderboard updatesDesign 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.ScalingMid–staff+

Large-scale A/B testing infrastructureDesign 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?ArchitectureSenior–leadership

Financial transaction reconciliationDesign a service that reconciles financial transactions across multiple systems overnight. If you find a discrepancy, what's your resolution workflow?ConsistencyMid–senior

Chat system message historyDesign 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?ScalingMid–senior

API gateway for microservicesDesign an API gateway that sits in front of 100 microservices. How do you handle authentication, rate limiting, and request routing without becoming a bottleneck?ArchitectureSenior–staff+

User consent and privacy trackingDesign 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.Api DesignMid–senior

Session management cookies vs tokensDesign a session management system for a web application. When should you use cookies vs. tokens, and how do you handle session expiration and renewal?Api DesignEntry–mid

Insurance claims processing consistencyDesign a system that processes insurance claims: intake, fraud detection, adjuster assignment, and payout. Each claim touches 3-5 systems. How do you ensure consistency?ConsistencySenior–leadership

Feature flag rollout and rollbackDesign 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?ArchitectureMid–staff+

Marketplace escrow and inventoryDesign the backend for a marketplace that connects buyers and sellers. How do you handle escrow, dispute resolution, and preventing double-spending of seller inventory?TradeoffsMid–senior