I once was involved in a bigger rewrite/refactoring of a backend where in a few areas we didn't have enough tests and I found goreplay to be very helpful as you

I once was involved in a bigger rewrite/refactoring of a backend where in a few areas we didn't have enough tests and I found goreplay to be very helpful as you do not need to explicitly write into two backends but the request data will be "doubled". Still stateful requests were a bit problematic but you can write so called "Middleware" for goreplay (in different languages) and I was able to solve it this way.