For example, suppose a startup has an Express app that made downstream requests to Postgres, Redis, Kafka, other REST APIs, etc. How do the outgoing requests, which may follow different protocols, with varying serialization formats and handshakes, all get intercepted, recorded, and matched to the same outgoing calls when the session is replayed? How are the outgoing requests indexed and evaluated for sameness when replayed?

It definitely seems possible to implement something like this as a series of high-level one-off integrations, for example, a middleware package for Django apps. But if the goal is a universal and automatic downstream service mocking system, it's not clear to me where this sort of middleware would even sit in the application stack. It seems like it would need to be pretty low-level, but not too low-level because the needs to be decrypted first.

Anyway, if you guys have figured out a good way to do this, I'm definitely interested in hearing how you managed it. I'm in no way a great BE engineer, so there could be something elegant or obvious that I missed.