Art of Codeartofcode
Lesson 14 of 16

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

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

Variations to practise

3 min read+10 XP

Chapters used

Approach (1), Estimation (2), Consistency (16), Transactions, locking and sagas (17), Message queues (22), Pub/sub and outbox (23), APIs, idempotency and webhooks (24), Rate limiting (25), Failures (29), Notifications (33), Flash sales (43).

Try these variations

Each is a common follow-up. Answer in two minutes, then check the sketch.

  1. "Items come from three warehouses." → Reserve per warehouse; the order splits into shipments; the saga releases each hold separately on failure.
  2. "Add a wallet plus card split payment." → Two payment steps in the saga: debit wallet, charge card; on card failure, refund the wallet.
  3. "Price drops after the order." → A separate price-protection job that issues credits; never edit the order's price_paid.
  4. "Sale day: 10× normal traffic." → Pre-scale (Chapter 41), bucket hot stock, waiting room for the flash items (Chapter 43), and a second payment gateway.