Art of Codeartofcode
Lesson 9 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)

42.8 How do you make order creation idempotent?

3 min read+10 XP

Shoppers double-tap "Place order", networks drop responses, and apps retry (Chapter 24):

  • The app creates an idempotency key per checkout attempt and sends it with POST /v1/orders.
  • The orders table has a unique constraint on it; a repeat returns the existing order instead of creating another. In the chapter's code, the second tap returns the same order and the table still has one order.
  • The payment service passes the order ID to the gateway as the gateway's idempotency key, so a retried charge can't charge twice (Chapter 24).
  • Warehouse and email consumers de-duplicate by event ID (Chapters 22 and 23).

Memory code: "One key per attempt, one order per key, one charge per order."