The Failures That Make No Noise
Last timeDeciding to Retrain
A broken input, a stale lookup and a confident answer about something never seen are the three production failures that leave every quality number looking normal.
A broken input
Three failures remain, and they share one property that makes them the hardest
things in this course: they produce no bad number anywhere.
Start with the simplest. A field stops arriving for twelve per cent of
requests, because an app release stopped sending it, or an upstream service
started returning an empty string instead of absent, or a currency unit
changed. The model receives a default value for those rows and answers
confidently.
- the share of rows affected
- the error rate on the unaffected rows
- the error rate on the affected rows
- the overall error rate, which is the number on the report
Two properties make this worse than the arithmetic suggests. The affected rows
are systematic rather than random, so the damage lands on one group of users
repeatedly. And the quality number arrives weeks later for all the reasons in
the fourth lesson, so a failure of this kind typically runs for a month before
anybody has a number to be suspicious about.
The only thing that catches it quickly is a check on the input itself, which is
the second row of the very first figure in this course.
Stale values and two code paths
The second quiet failure is more insidious because nothing is broken anywhere.
Every component works. They disagree.
A model uses a customer average spend over ninety days. In training, that
value was computed by a query over the warehouse. In serving, it is read from
a lookup table refreshed by a different job written by a different person at a
different time. The two produce slightly different numbers: one excludes
refunds, one includes them, or one counts ninety days from midnight and the
other from the moment of the request.
The lesson stops here
5 more paragraphs to go
You have read the opening. The rest of the argument, the problems that check whether it landed, and the lines worth keeping at the end all come with a plan.
The first lesson of every course in the library reads the whole way through, free, so you can see exactly what the rest of them are.
See the planThe contentsThis is the reading half
Starting the course gives you your own copy of it. Every idea on every page has problems standing under it, marked with a reason rather than a tick, and any sentence you do not believe can be opened and argued with. None of that can happen on a page nobody owns.
The contents