Art of Codeartofcode
Lesson 5 of 16

42. Design E-commerce Cart and Checkout (Amazon / Flipkart)

1
The interview prompt
2
42.1 What do you ask first?
3
42.2 How big is this, really?
4
42.3 What are the doors?
5
42.4 What do we store?
6
42.5 What does the whole system look like?
7
42.6 How is the cart stored?
8
42.7 How do you reserve inventory?
9
42.8 How do you make order creation idempotent?
10
42.9 How does the payment hand-off work?
11
42.10 How does the checkout saga work?
12
42.11 What would you say in the last five minutes?
13
Score yourself
14
Variations to practise
15
Sources
16
Code lab
42. Design E-commerce Cart and Checkout (Amazon / Flipkart)

42.4 What do we store?

3 min read+10 XP
carts (key-value, by user_id or guest_id) items: {sku: qty, price_at_add}, updated_at inventory (SQL, by sku + warehouse) available, reserved reservations order_id, sku, warehouse, qty, expires_at orders (SQL, sharded) order_id, user_id, state, total, idempotency_key (unique) order_items order_id, sku, qty, price_paid payments payment_id, order_id, provider_ref, state, amount outbox order events to publish (Chapter 23)

Interviewer pushback:

  • "Why store price_paid on order items?" → Prices change; the order must record what the customer actually agreed to pay, for refunds, invoices and disputes.

Memory code: "Cart by user, stock by warehouse, orders with a key that can't repeat."