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.
Designs with worked examples 130
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.
ClassicMedium4. Design a Notification System
Deliver email, push and SMS with preferences, retries and honest delivery status.
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.
ClassicMedium8. Design a Video Streaming Platform
Turn uploads into playable renditions and serve viewers through unreliable networks.
ClassicHard10. Design an API Gateway
Route API traffic with authentication, tenant limits and safe configuration rollout.
ClassicMedium11. Design a Container Orchestrator
Reconcile desired workloads, schedule containers and recover from failed nodes safely.
ClassicHard29. 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 & SREMedium30. Transactional Outbox
A service commits to its database and then publishes an event. If it dies in between, the row exists and no one hears about it. Publishing first has the mirror problem: an event for a write that never happened. There is no ordering of two independent systems that makes this safe.
Product & backendMedium31. 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 & SREMedium33. Idempotency Keys
Any request can time out after the server has already acted. The client cannot tell that from a request that never arrived, and the only safe response to uncertainty is to retry — which, without help, charges the card twice.
Product & backendMedium34. Delay Queue Scheduler
Millions of future actions — reminders, retries, expiries — must fire near their due time. Scanning everything every minute does not scale, and an in-process timer dies with the process.
Infrastructure & SREMedium35. WebSocket Gateway and Session Registry
To push a message to someone you must find the one server out of hundreds that currently holds their connection — and connections are long-lived, unevenly distributed, and disappear without warning.
Product & backendMedium36. 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 & backendMedium38. Pre-Signed Direct Upload
Streaming multi-gigabyte uploads through application servers wastes the most expensive capacity in the system on copying bytes, and ties up a request for minutes on a connection that may drop.
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.
ClassicMedium41. Ad Click Aggregator: Low-Level Design
The classes and code behind the ad click aggregator: signed click tokens, a redirect that never waits on the log, and a stream counter that de-duplicates and handles late clicks. Tests run all of it.
Data engineeringMedium42. 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.
ClassicMedium46. Design a Web Crawler
Ten billion pages in five days, politely, without losing progress: fetchers and parsers, domain locks, bandwidth math, deduplication and a crawl that stays fresh. One-hour boards for junior, senior and staff.
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.
ClassicMedium48. Design a Ticket Booking System like Ticketmaster
Hold seats with one conditional write, pay by authorize and capture, and put ten million fans in a fair waiting room without selling a seat twice.
ClassicMedium