4. Design a Notification System
Deliver email, push and SMS with preferences, retries and honest delivery status.
ClassicMedium
Guided practice
Start with a template. Work through each step. Ask Coach when you need a second opinion.
Classic designs 14
Ordered by design numberCompany tags are community-reported. Counts on cards show how many people reported that design.
QueuesClear filters
8. Design a Video Streaming Platform
Turn uploads into playable renditions and serve viewers through unreliable networks.
ClassicHard39. 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.
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.
ClassicMedium53. Design a Stock Trading App like Robinhood
Live prices to five million phones through per-symbol pub/sub, and market and limit orders over FIX that are never lost, sent twice or overspent.
ClassicMedium63. Design Real-Time Updates for Millions of Clients
Every goal on 25 million open apps within two seconds: SSE or WebSockets, pub/sub to the right server, resume by seq, hot topics, reconnect storms and push.
ClassicMedium69. Design Long-Running Tasks: Async Jobs with Progress and Results
Accept in milliseconds, work in the background: leases and heartbeats, retries and dead letters, progress by SSE and webhooks, fairness across tenants.
ClassicMedium80. Design a Message Queue like Amazon SQS
Build a queue around visibility leases and explicit acknowledgement, then add replicated partitions, FIFO groups, tenant fairness and bounded redrive: three self-contained interview boards.
ClassicMedium85. Design a Change Data Capture Pipeline
Every committed change in Aurora and DynamoDB, read from the log and delivered in order per row to search, caches, Redshift and an S3 lake.
ClassicMedium89. Design a Donations Website for a Charity Event
Ten charities, three days, $100 M: charge through a third-party processor exactly once, queue the broadcast spikes, keep live totals on sharded counters. One-hour boards for junior, senior and staff, with the theory behind them.
ClassicMedium91. Design a CI/CD System like GitHub Actions
Turn 10 pushes a second into job graphs on ephemeral runners, with fair queues per pool, leases fenced by attempt and live logs.
ClassicMedium101. Design a Webhook Delivery System
Signed HTTPS callbacks for a billion events a day, at least once, retried for three days, with no endpoint able to slow another.
ClassicMedium