How would you design an e-commerce platform?

Short explanation:

An e-commerce system coordinates browsing, stock, orders and payments, which have different consistency needs. Catalog reads can often be cached, while stock reservation and order/payment transitions need stronger protection. External payments introduce uncertain outcomes that require durable state and reconciliation.

Key points:

• Separate catalog, cart, order, inventory and payment responsibilities.

• Reserve stock safely and make checkout/payment requests idempotent.

• Verify payment events and reconcile partial failures; do not trust only the browser's success page.

• Model order state transitions explicitly and give checkout/payment attempts stable idempotency keys.

• Release expired reservations safely, and verify provider events before changing an order to paid.

Interview point:

• Walk through successful payment followed by a lost callback; the design should recover without charging the customer twice.