🧭 Scenario · Make the call
The two sales that ate each other
The situation
A store has 100 units of a hot item in stock. Two orders come in at the exact same instant. Each one reads 'stock = 100', subtracts the amount it sold, and writes the result back. Order A sold 30 and wrote 70; Order B sold 40 and wrote 60. Both read 100 before either had written, so the final stock shows 60 — as if Order A's sale never happened. One of the two updates was silently lost.
You haven't read about this exact system. That's the point — walk the three questions a designer asks: what's really wrong, what fixes it, and what does the fix cost?
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Diagnose
Read past the story. What's the REAL problem — not what the feature does, but what's actually going wrong?
Look past the story. What's the REAL problem here?