Practice briefStart with your own approach
14. Sell a limited drop without overselling
Separate reservation, payment and confirmed purchase during a 50,000-person launch.
Product & backendMedium
Guided practice
Start with a template. Work through each step. Ask Coach when you need a second opinion.
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33. 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 & backendMedium52. Design a Payment System like Stripe
Charge cards through the networks exactly once: idempotency keys, a card vault, timeouts as unknowns, signed webhooks, a double-entry ledger and reconciliation. One-hour boards for junior, senior and staff, with the theory behind them.
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.
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.
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.
ClassicMedium