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Amazon HLD: The Cart That Never Says No (Dynamo, Quorums, Merge)

An Amazon system design on the always-available shopping cart: AP over CP, a Dynamo-style replicated key-value store, quorums, consistent hashing, and merging divergent carts.

By fiveyearsdevJune 14, 202618 min read

You're on the metro, no signal, and you tap Add to cart on a book you've been eyeing. The app doesn't spin, doesn't warn you, doesn't quietly queue it for "when you're back online" — it just says Added, instantly. You get home, open the laptop, and there's the book, sitting next to the phone charger you added from your desk an hour earlier. Nothing was lost, nothing was refused, and at no point did the cart make you wait. That tiny, boring reliability is one of the hardest promises in distributed systems, and it's the humblest corner of "Design Amazon" — the one that rewrote the field. The cat…

What’s inside

Let's start nowhere near a computer

Where this exact shape shows up

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Nouns, the data model, and which datastore(s)

Step 4 — Replication and quorums

Step 5 — Consistent hashing

Step 6 — Conflict resolution: merge, don't pick

Step 7 — Trade-offs (each one keeping an NFR)

The complete implementation

Step 8 — Only now, the boxes

Step 9 — Scaling the design, one bottleneck at a time

Step 10 — When a piece fails: designing for failure

The interview corner

Where to go from here

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