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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, 202610 min read

"Design Amazon." The catalog, search, and checkout are real subsystems, but the one that made Amazon rewrite distributed-systems history is the humblest: the shopping cart. Amazon's rule is that "add to cart" must never fail — not during a deploy, not during a datacenter hiccup, not during a network partition. A cart write that errors is a sale that didn't happen. So the cart deliberately gives up something most systems cling to — strong consistency — to buy something it values more: always-on availability. That trade, and how you survive its consequences, is this whole interview, and it's the…

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 — The CAP choice, and which datastore

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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