One of the problems with attempting to write interface is the opaque source/consumer problem. E.g. I'm working on a system that I could hypothetically abstract

One of the problems with attempting to write interface is the opaque source/consumer problem. E.g. I'm working on a system that I could hypothetically abstract (I've got access to it, can poke with enough tests and test data, etc). However. What I don't have is access to code or test injection into any of my sources / consumers. Both of which are expecting all the corner case quirks to be exactly identical and may actually have accreted software that depends on a specific quirk. A specific quirk that I have no way of knowing about. Or may send me something I've never seen and am not expecting because it's a 1:1,000,000 corner case, and we don't have any logging of an example that came through production. I haven't worked on too much of the heavyweight stuff like you have, but I tend to take the perspective that "a 100% compatible rewrite is impossible." 95%+ maybe, but we're going to have to deal with the <= 5% after it goes to production. Did you ever pursue writing and dropping a new tailored load balancer / router type application on the incoming data stream such that you could divert a specific portion onto new system(s)?