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Lesson 15 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)

Sources

3 min read+10 XP

Source notes

  • Conditional updates, optimistic locking and sagas: see Chapter 17. Idempotency keys and webhooks: Chapter 24. Outbox: Chapter 23.
  • Order volumes, cart sizes, hold durations and sale-day peaks are interview assumptions agreed in Step 1, not figures for any real store. Arithmetic computed 2026-10-08.
  • Cart merge and re-pricing, oversell vs conditional reservation, expiring holds, idempotent order creation, the payment state machine (including late payments) and the saga with compensations: tested 2026-10-08 (code/ch42_checkout.py, all checks pass).