Differential Diagnosis

The measurement that separates two causes of the same symptom — the column a field guide is actually for.

Why it matters in a memory layer

Listing causes is easy and nearly useless when a symptom has three of them. What resolves a case is a cheap check that discriminates: replay turn by turn and diff against an eager store; compare the belief exam with the context exam; re-scan every structure by id; compare the eligible pool before and after a write.

Some checks are ordered rather than parallel. "Never recalled" is: is it in the store; if so does it claim a slot; if so is it in the eligible pool. Each answer makes the next question meaningful, and a guide that lists the three causes without the order sends you to the wrong module first.

And a guide is only trustworthy if its entries were observed. A predicted failure has no tell, because nobody has had to tell it apart from anything.

Connections

Taught in: The Failure Field Guide

Do not confuse with: Failure Class