Sam Newman (2019), Monolith to Microservices, O′Reilly.
We want our new system, initially be supported by, and wrapping, the existing system. The idea is that the old and the new can coexist, giving the new system time to grow and potentially entirely replae th old system.
The key benefit to this pattern is that it supports our goal of allowing for incremental migration to a new system.
Until the call to the moved functionality is redirected, that the new functionality is not technically live. Once your new service implements the same equivalent functionality as your monolith, you could then consider using a pattern like parallel run to give you confidence.