Most commenters are focused on the optimizations made, but I actually think the custom routing and verification mechanism is the interesting bit.

That kind of a tool could be handy in lots of scenarios (comparing the same service written in two different languages or with different dependencies, etc).

But how does their verifier mechanism deal with changes in the production database between responses? If the response of the legacy service comes first and the response of the new service comes after, in between both responses (the request being the same) couldn't the data from the database change and thus result in the responses not passing verification when they otherwise should have? How do they manuever around that issue?

Great write-up by the way! I really liked it :)