[jedberg]

That's why you do it slowly. You take a small part of the monolith and make a service that does the same thing. Then you replace the code in the monolith with a call to the service, while keeping track of how often it is called in the monolith.

As you keep moving along, some things that depend on that first service will start calling the new service directly, and some will still call it in the monolith. But your tracking will tell you how often and who is doing that, so you can find out why.

In the meantime, nothing will break, because the monolith is still a pass through proxy to your service.