When looking for resources to answer Tim's question at
<https://www.mediawiki.org/wiki/Architecture_guidelines#Clear_separation_of_concerns>,
I found a very nice and concise overview of principles to follow for writing
testable (and extendable, and maintainable) code:
"Writing Testable Code" by Miško Hevery
<http://googletesting.blogspot.de/2008/08/by-miko-hevery-so-you-decided-to.html>.
It's just 10 short and easy points, not some rambling discussion of code philosophy.
As far as I am concerned, these points can be our architecture guidelines.
Beyond that, all we need is some best practices for dealing with legacy code.
MediaWiki violates at least half of these principles in pretty much every class.
I'm not saying we should rewrite MediaWiki to conform. But I'd wish that it was
recommended for all new code to follow these principles, and that (local) "just
in time" refactoring of old code in accordance with these guidelines was encouraged.
-- daniel