Case study — e-commerce checkout and inventory — System Design — High-Level Design (HLD) from Basics to Case Studies

12. Case study — e-commerce checkout and inventory

Advanced★ Must knowLesson 12 of 14

Requirements: browse catalog, cart, checkout, payment, inventory must not oversell, order history; flash sales with traffic spikes.

text

CDN (images, SPA) → API gateway → Catalog svc (read replicas + cache + search index)
                               → Cart svc (Redis)
                               → Order svc (SQL) ─ outbox ─▶ Service Bus ─▶ Payment svc / Inventory svc / Email svc

Key decisions:

Inventory consistency: reserve stock in a transaction with a conditional update (UPDATE Stock SET Qty = Qty - @n WHERE ProductId = @id AND Qty >= @n); release reservations that aren't paid within N minutes.
Payment idempotency: idempotency key per checkout attempt; payment provider webhooks update order status.
Saga: order created (pending) → payment → inventory confirm → order confirmed; compensate (refund/release) on failure.
Flash sales: queue-based admission (virtual waiting room), pre-warmed caches, rate limiting, autoscaling.
Read scaling: catalog is read-heavy → cache + read replicas + CDN.