E-Commerce & Marketplace
ecommerce-marketplace
System Design
Design a Rate Limiter
Design a distributed rate limiter that protects an API platform from abuse and uneven load while staying fast and accurate at 1B requests per day. The interview centerpiece is choosing among the five canonical algorithms (fixed window, sliding window log, sliding window counter, token bucket, leaky bucket) and explaining how to make the chosen one atomic across a Redis cluster. We cover where to place the limiter (edge, gateway, in-process), per-IP vs per-user vs per-API-key keys, returning 429 with Retry-After, the hot key problem, and fail-open vs fail-closed under cache outages.
Design an E-Commerce Platform (Amazon)
Design an Amazon-scale e-commerce platform that lets 200M monthly users browse 100M SKUs, add items to a cart, check out, and have orders fulfilled from regional warehouses. The interview centerpiece is the order lifecycle: how to reserve inventory atomically while a customer is on the checkout page, how to chain cart-to-payment-to-fulfillment as a saga with compensating actions, and how to make checkout idempotent so a flaky network never charges a customer twice. We also cover catalog browse at scale, multi-warehouse fulfillment routing, and the asymmetric read/write workload that makes aggressive catalog caching the right call.
Design a Ticketing System (Ticketmaster)
Design a Ticketmaster-style ticketing platform that sells reserved seats for concerts and sports events, with the central challenge being a flash onsale where 1M users compete for 50K seats in five minutes. The interview centerpiece is the seat reservation lock: each unique seat (Section A, Row 12, Seat 7) cannot be split or sub-bucketed like fungible inventory, so contention is unavoidable. We cover seat-level pessimistic holds with TTL, the virtual waiting room that randomizes queue position to absorb flash demand fairly, anti-bot defenses, dynamic pricing tiers, and the read-replica explosion that interactive seat maps cause.
Design a Payment System (Stripe)
Design a Stripe-style payment platform that processes 100M payments per day across 50 currencies and dozens of payment methods, where the central requirement is financial correctness: never charge a customer twice, never lose a payment, always reconcile to the cent. The interview centerpiece is the trio of idempotency keys, the payment intent state machine, and the immutable double-entry ledger - together they make the system safe in the face of network failures, partial outages, and adversarial retries. We also cover webhook delivery with signing and exponential backoff, PCI scope minimization through tokenization, multi-region availability, and the reconciliation jobs that compare our ledger to the bank's settlement files every night.