The logs that don't fit the box
A team must store activity logs from millions of devices. The logs pour in by the billions per day, every device sends a slightly different set of fields, and the shape keeps changing as new device types ship. They're trying to force these into a strict table with fixed columns, and every new field means a painful schema change while the write rate keeps outgrowing one machine. (Their banking app, with strict accounts and transfers, is a totally different story.)
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?
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?