60. 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.
ClassicMedium
Guided practice
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Ordered by design numberCompany tags are community-reported. Counts on cards show how many people reported that design.
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61. 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.
ClassicMedium64. Design for Contention: Thousands Racing for the Same Item
Design for contention through a limited sneaker drop: row locks, conditional writes, expiring holds, fencing tokens, sagas and one writer per item.
ClassicMedium65. Design Multi-Step Processes: Sagas, Orchestration and Durable Workflows
An order checkout across payment, inventory and shipping, run as a saga.
ClassicMedium87. Design a Hotel Booking System like Airbnb
Nights as counted inventory, holds that never double-book, search that knows availability, and payments that reconcile.
ClassicMedium88. Design a Reviews and Ratings System
Dish reviews only from customers who ordered them, votes counted from a change stream, reviews ranked by the Wilson bound, payouts made exactly once.
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.
ClassicMedium90. Design a Book Seller Aggregator Platform
Buy the cheapest offer under a maximum price from 1,000 sellers: cached offers, coalesced quotes, a hold-order-capture saga, open requests.
ClassicMedium92. Design Online Chess
Real-time rated chess with server-validated moves, lag-compensated clocks, pairing by rating and a million watchers on one game.
ClassicMedium93. Design a Real-Time Game Leaderboard
Rank 200 million players in real time with sharded Redis sorted sets, count every game exactly once, and close a season fairly.
ClassicMedium95. Design a Team Chat App like Slack
Design a team chat app like Slack: channels and DMs in real time, threads, unread counts and search. One-hour boards for junior, senior and staff, with the theory behind them.
ClassicMedium96. Design a Food Delivery App like Uber Eats
Find restaurants that deliver to you, check out without double charges, time couriers to the kitchen and track them live.
ClassicMedium98. Design a Double-Entry Ledger
Record every money movement as balanced, append-only entries, exactly once, with holds, payouts and reconciliation.
ClassicMedium99. Design a Cross-Product Tagging System
One set of tags across Jira issues, Confluence pages and Bitbucket pull requests: batch renders, tag pages that never leak, suggestions, popular tags.
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.
ClassicMedium148. Design a Text Sharing Service like Pastebin
The URL shortener with a payload: the same id generation and cache-first read path, but the value is kilobytes of text, so it moves out of the database and into object storage.
Product & backendEasy