1. Design a URL Shortener
Create short links, resolve them quickly and handle expiry without sending users to the wrong destination.
ClassicEasy
Designs
Pick a system. Work through the problem. Compare your approach.
All designs 44
Ordered by design numberCompany tags are community-reported. Counts on cards show how many people reported that design.
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2. Design a Distributed Rate Limiter
Enforce tenant quotas across servers while making burst behavior and outage policy explicit.
ClassicMedium3. Design a Distributed Cache
Partition cached data, survive node loss and keep cache misses from overwhelming the source of truth.
ClassicMedium5. Design a Chat System
Send durable messages, reconnect devices and explain ordering and delivery receipts.
ClassicMedium6. Design a News Feed
Build a paginated feed that handles high-fan-out authors, fresh posts and visibility changes.
ClassicMedium7. Design a File Storage Service
Upload and share large files with resumable transfer, versions and safe metadata changes.
ClassicMedium29. Cache-Aside (Lazy Loading)
A read-heavy service sends every request to a database that is slower and more expensive per read than memory, and the same few rows are asked for over and over.
Infrastructure & SREMedium31. Hybrid Fan-Out (Push and Pull)
Building a timeline when someone opens the app means reading from everyone they follow and merging — slow, and slowest for the most active users. Building it when someone posts makes reads a single lookup, but one post by an account with fifty million followers becomes fifty million writes.
Product & backendMedium32. Token Bucket Rate Limiter
A limit enforced per server is not a limit: ten servers each allowing a hundred requests a minute allow a thousand. And a counter per fixed window lets twice the limit through across a window boundary.
Infrastructure & SREMedium36. Geospatial Index with TTL
Finding the nearby members of a set that is constantly moving. A table scan with a distance function is hopeless at any scale, and a stale position is worse than none — it sends a car to someone who left ten minutes ago.
Data engineeringMedium37. Reservation with a TTL, and a Unique Constraint
Scarce inventory — a seat, a room, the last unit — with far more buyers than units. Holding a database row for the minutes someone takes to pay does not scale, and checking availability before writing is a race that sells the same seat twice.
Product & backendMedium39. Design a Calendar System
Google Calendar-style events with recurrence, RSVPs and on-time reminders, using CDC for change notifications and a delay queue for reminders.
ClassicMedium40. Design an Ad Click Aggregator
Count every ad click once, fast enough to chart live and exactly enough to bill. One-hour interview boards for junior, senior and staff: requirements, data layer, low-level design and what goes wrong at every component.
ClassicMedium42. Design a Metrics Monitoring Platform
Collect 5 million samples a second from 500,000 hosts, store them as time series, chart them and page people.
ClassicMedium43. Design YouTube Top-K
The K most-viewed videos for the last hour, day, month and all time from 700,000 views a second, exactly and in milliseconds.
ClassicMedium44. Design a Price Tracking Service
Watch prices on 500 million products with a polite crawler and a million browsers, verify what you are told, and notify subscribers within minutes of a drop.
ClassicMedium45. Design a Job Scheduler
Run 10,000 jobs a second within two seconds of their time, at least once, with retries, fairness between tenants and exactly-once effects.
ClassicMedium47. Design an Online Judge like LeetCode
Run strangers' code safely in single-use microVMs, return verdicts within 5 seconds, and rank 100,000 contestants live.
ClassicMedium50. Design a Ride-Sharing Service like Uber
Price a trip, match a rider to a nearby driver who has ten seconds to accept, and never give one driver two rides.
ClassicMedium54. Design a Photo Sharing App like Instagram
Photos and videos for 500 million daily users: presigned multipart uploads, a processing pipeline, CloudFront and a hybrid fan-out feed.
ClassicMedium56. Design a Local Business Review Site like Yelp
Search 10 million businesses by place, words and category, keep every average rating exact, and keep fake reviews out.
ClassicMedium57. Design Post Search like Facebook
Search a billion posts a day by keyword, newest or most liked first, with an inverted index you build yourself.
ClassicMedium58. Design a Dating App like Tinder
Stacks of nearby people in under 300 ms, 2 billion swipes a day, and a match never missed even when both like at once.
ClassicMedium59. Design a Fitness Tracking App like Strava
Record runs and rides on the phone without signal, upload each once, show them to friends and rank athletes by week, month and year.
ClassicMedium60. Design Live Comments like Facebook Live
Live comments for millions of viewers: SSE streams, a Redis channel per video with co-located viewers, sampling and CDN snapshots for hot videos.
ClassicMedium61. Design a Collaborative Editor like Google Docs
A hundred people typing in one document, every copy converging, nothing acknowledged lost: one owner per document, an op log with snapshots, and fenced failover.
ClassicMedium62. Design a Flash Sale System
Sell 10,000 units to a million people at noon with a lottery waiting room, a Redis gate before an Aurora ledger and holds that expire.
ClassicMedium66. Design for Read Scale: A Product Page Read a Million Times a Second
Serve a product page a million times a second: a cheap query, replicas, Redis and the edge, with hot keys, stampedes and invalidation handled.
ClassicMedium67. Design for Write Scale: A Million Writes a Second
A live TV vote at a million writes a second: spread by key, buffer in Kafka, aggregate the hot counter in two stages, shed what can wait.
ClassicMedium70. Design an In-Memory Data Store like Redis
An in-memory data store like Redis, from one event loop to Redis Cluster to a durable platform.
ClassicMedium