Interview practice · 33 complete cases
System design interview questions you can actually practice
Move beyond prompt lists. Scope requirements, calculate capacity, defend an architecture, survive a failure injection, and repair the exact concept your answer missed.
Practice the whole interview, not a memorized diagram
Most system-design question lists stop at the prompt. Knowing that “design a rate limiter” is common does not prove that you can clarify its contract, estimate load, choose a consistency model, defend the hot path, or respond when the interviewer changes one constraint. These cases turn each question into a repeatable interview with observable evidence.
Every case follows six stages: scope the problem, estimate capacity, draw the high-level design, defend one critical path, respond to a failure, and review the result. The reference answer stays hidden during closed-book practice. After submission, scores cite your own words, missing dimensions link to focused concepts and drills, and already-demonstrated dimensions are not penalized by an unrelated gap.
If the topic is new, learn the linked mechanisms first and study an approved worked guide. Then wait before attempting the case without notes. If an interview is close, begin closed-book and use the review as diagnosis. A strong answer is not one remembered picture; it is a chain of explicit assumptions, unit-carrying calculations, named trade-offs, and operational responses.
AI systems
advanced · 45 minutes
Design LLM inference at 10K RPS
Design a production LLM inference service that sustains 10,000 requests per second. Explain how you turn that headline into capacity, architecture, overload, and reliability decisions.
advanced · 45 minutes
Design production RAG
Design retrieval-augmented generation over a large, frequently changing document corpus with permissions and citations.
advanced · 45 minutes
Design a multi-tenant LLM gateway
Design an API gateway in front of multiple LLM models and providers for thousands of tenants.
advanced · 45 minutes
Design real-time recommendations
Design a real-time recommendation service for a large consumer application.
Social and real-time
advanced · 45 minutes
Design real-time chat
Design real-time one-to-one and group chat for 50 million daily active users.
advanced · 45 minutes
Design a ranked news feed
Design a ranked home feed for 100 million users with celebrity accounts, freshness, and moderation.
core · 45 minutes
Design a photo-sharing platform
Design an Instagram-like photo-sharing service with uploads, transformations, privacy, feeds, likes, and safe deletion. Focus on media and social-delivery boundaries.
core · 45 minutes
Design a notification delivery service
Design push, email, SMS, and in-app notification delivery with user preferences, priorities, quotas, retries, provider failure, and duplicate control.
core · 45 minutes
Design a proximity search service
Design a service that returns open businesses within a radius, filtered by category and ranked by distance, for a global consumer application.
advanced · 45 minutes
Design a nearby friends service
Design an opt-in feature that shows which friends are nearby and updates within seconds as people move, while making privacy and battery cost first-class constraints.
core · 45 minutes
Design a real-time gaming leaderboard
Design a leaderboard for a global game with seasonal, regional, and friends-only rankings, real-time score updates, anti-cheat review, and stable end-of-season results.
Media and collaboration
core · 45 minutes
Design a video streaming platform
Design an on-demand video platform for creator uploads and global playback. Focus on the reusable upload, transcode, package, authorize, and CDN-delivery pattern rather than recommendations.
advanced · 45 minutes
Design a collaborative document editor
Design a Google Docs-like editor with real-time multi-user edits, offline work, presence, history, permissions, and deterministic convergence.
Infrastructure and storage
core · 45 minutes
Design a URL shortener
Design a global URL-shortening service with custom aliases, analytics, expiration, and abuse controls.
core · 40 minutes
Design a distributed rate limiter
Design a global, multi-tenant rate limiter for an API gateway with burst allowances and regional enforcement.
advanced · 45 minutes
Design a web crawler
Design a web-scale crawler that discovers, fetches, deduplicates, stores, and recrawls useful pages while obeying robots policy and host politeness.
advanced · 45 minutes
Design a distributed cache
Design a low-latency distributed cache with partitioning, replication, TTL, eviction, invalidation, node failure, hot keys, and stampede protection.
core · 45 minutes
Design search autocomplete
Design typeahead suggestions for a large search product with prefix lookup, ranking, trends, personalization, freshness, moderation, and low latency.
advanced · 45 minutes
Design cloud file storage
Design Dropbox or Google Drive-style file synchronization with resumable uploads, folders, sharing, versions, offline conflicts, durability, and efficient transfer.
advanced · 45 minutes
Design a distributed key-value store
Design a Dynamo-style distributed key-value store with partitioning, replication, tunable consistency, durability, rebalancing, repair, and compaction.
core · 45 minutes
Design a distributed unique ID generator
Design a service that issues globally unique, roughly time-sortable 64-bit IDs to thousands of application servers across regions without a database round trip per ID.
advanced · 45 minutes
Design a maps and routing platform
Design a driving-directions service that returns routes and ETAs across a continent, incorporates live traffic, and can serve map tiles separately from route computation.
advanced · 45 minutes
Design a distributed message queue
Design a multi-tenant distributed queue for asynchronous jobs and event streams with durable retention, consumer groups, backpressure, and at-least-once delivery.
advanced · 45 minutes
Design a metrics monitoring platform
Design a multi-tenant metrics platform that ingests counters, gauges, and histograms, supports dashboard queries and recording rules, and evaluates reliable alerts.
advanced · 45 minutes
Design a distributed email service
Design a transactional email platform that accepts API requests, renders templates, delivers through SMTP, handles retries and bounces, and protects sender reputation.
advanced · 45 minutes
Design an object storage service
Design an S3-like object storage service with buckets, immutable object versions, multipart uploads, ranged reads, checksums, lifecycle policies, and regional durability.
Commerce and marketplaces
advanced · 45 minutes
Design a ride-sharing platform
Design an Uber or Lyft-style marketplace with driver locations, nearby matching, dispatch offers, atomic trip assignment, live trip state, ETA, and regional spikes.
advanced · 45 minutes
Design a ticket-booking platform
Design Ticketmaster-style reserved-seat booking with event browse, a virtual waiting room, expiring holds, payments, oversell prevention, bots, and flash crowds.
advanced · 45 minutes
Design a payment-processing system
Design a payment system with authorization, capture, refunds, idempotency, webhooks, an auditable ledger, processor timeouts, reconciliation, and fraud controls.
advanced · 45 minutes
Design an ad click aggregation system
Design an ad click pipeline that powers near-real-time campaign dashboards and produces auditable daily billing aggregates despite duplicates, retries, bots, and late events.
core · 45 minutes
Design a hotel reservation system
Design a hotel booking platform that searches room availability and prevents overselling across multi-night stays, retries, payment delays, and expiring holds.
advanced · 45 minutes
Design a digital wallet
Design a digital wallet that supports deposits, peer-to-peer transfers, merchant payments, refunds, and withdrawals while maintaining auditable balances across retries and external settlement.
advanced · 45 minutes
Design a stock exchange
Design the core of a stock exchange that accepts limit and market orders, matches by price-time priority, publishes market data, and recovers without reordering or duplicating trades.
What the review measures
Cases score requirements, capacity, architecture, critical-path judgment, and reliability as separate dimensions. The deterministic rubric looks for declared evidence and remains available without an AI provider. When optional AI critique is configured, it can judge phrasing against fixed anchors, but it cannot add requirements, dimensions, facts, or out-of-range scores.
7 source-reviewed worked guides are public. Practice-only cases receive guides only after their calculations, primary sources, and editorial explanations pass the same review. That prevents generic, templated pages from diluting the curriculum.