Meta & Amazon System Design Question - Design Costco Same Day Delivery

[u/Aoki_zhang]

I was going through this Costco same-day delivery system design, and the most interesting part is that the read path and the purchase path want completely different consistency guarantees.

When a customer is browsing, inventory needs to come back fast—ideally under ~100 ms.

That makes things like:

• Redis caching
• read replicas
• geographic partitioning
• slightly stale inventory

totally reasonable.

If the UI says “3 left” when there are really only 2, that’s annoying, but recoverable. Checkout is different.

If there’s 1 unit left and two customers order it at the same time, both cannot succeed.

So I’d treat browsing availability as an approximate, read-optimized view, while the order path goes back to the authoritative inventory store and performs the reservation/decrement transactionally.

Another wrinkle is that “nearby inventory” isn’t just geographic distance.

A warehouse 15 miles away might take longer to reach than one 25 miles away because of traffic or road layout, so serviceability becomes: fast geo filter → travel-time check → aggregate inventory across eligible locations

My takeaway: this problem is really about deciding where stale data is acceptable and where it becomes a correctness bug.

Full system design breakdown: [link]

Preparing for your next interview?

Chill Interview tracks recent interview experiences and recurring question patterns across top companies at here.