🧭 Scenario · Make the call
The like that went missing
The situation
A photo has a 'likes' number stored in one place in memory. Two helpers (threads) both get a like at the exact same instant. Each one reads the current count of 100, adds 1 in its own head, and writes 101 back. Two likes came in, but the count only went up by one — it shows 101 instead of 102. One like just disappeared, and it happens randomly, only when two likes land together.
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?