Interesting, I just now know about selective testing after reading this post. But, now I wonder when do you do selective testing and when do you just break up the codebase?
Let's say you have 100 small repos, and make a change to one. How confident are you in the changed repo's test suite that you can guarantee there are no bugs that will affect other related repos? If not, which other repos do you test with your change to get that confidence?
It’s not a purely technical problem.