I typically use one of two patterns to modernize a monolith to a microservice architecture, depending on the overall requirements and project needs. I have best results with the "strangler pattern" that allows me to migrate functionality without changing existing systems. It also allows monolith systems that cannot be migrated to remain while microservices handle the rest of the load. In situations where a deep amount of technical debt prevents the client systems from easily switching to a new API endpoint, the "Parallel Run" pattern allows us to keep the monolith up and working while newer, and newly retooled systems use microservices, This pattern also allows us to compare results between microservices and monolith.