Design a Grocery Checkout System

A grocery checkout is the software behind the till. It looks up what was scanned, works out the price after every deal and coupon, takes the money and prints the receipt. At Susquehanna it's one of the most commonly reported design prompts for software engineers. It shows up in the final-stage object-oriented round, which runs 45 to 60 minutes, and candidates from intern to senior describe getting some version of it.

This is a low-level design round, so there are no load balancers to draw. The interviewer starts from the overall shape and then pushes down into class structure and real code in a shared editor, usually C++ or Python. The coding section has the phone-screen version, a Premium question that prints a receipt, which fits in about 30 minutes. Here we build the onsite scope on top of it, adding weighed goods, a promotion engine, payments that can fail, lanes that share stock, and the inventory and profit extension.

The Susquehanna angle

SIG tends to grade this round the way it would review trading code. The firm builds its own low-latency trading systems, and a checkout raises the same kinds of problems at a small scale. Money has to be exact to the cent, and a finished sale can't be quietly edited later. The points reported for this prompt match that. Integer money and a FIFO cost history for stock come up. So does whether a checkout commits every product in the basket or none of them, and what happens when several lanes sell from the same inventory. Expect questions on payment retries that must not charge twice, and on sale records that never change once written. If you've seen how a trading system works out realized profit by matching sells against the earliest buys, the inventory half of this problem is the same calculation with bananas.

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