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Lesson 7 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)

42.6 How is the cart stored?

3 min read+10 XP

Step 5: Deep dives (10 minutes)

  • Key-value store keyed by user ID (or a guest ID in a cookie for signed-out shoppers), sharded and replicated. A cart write should never fail: if one replica is down, accept the write elsewhere (Chapter 16's availability-first choice).
  • Merging at login: add the guest cart's items into the user's cart, with sensible caps. In the chapter's code, {dune: 1} plus a guest cart {dune: 2, atomic-habits: 1} becomes {dune: 3, atomic-habits: 1}, and quantities never exceed the per-item limit.
  • Prices in the cart are hints. Store the price when added, but re-price at checkout and show the shopper what changed. In the chapter's code, a book that dropped from ₹499 to ₹449 is flagged.
  • Stock in the cart is a hint too: "only 3 left" is shown, but nothing is reserved until checkout.
  • Abandoned carts feed reminder emails (Chapter 33) and expire after weeks.

Memory code: "Never refuse an item; re-price at the till."