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If a module needs to have a completely independent lifecycle (meaning the code-commit-to-production flow), then it should be a microservice.
It should have its own code repository, CI/CD pipeline, and so on.
A smaller scope makes it far easier to test a microservice.
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You have a legacy application.
Microservices are more difficult to manage (in some ways).
Microservices require a larger technology stack.
Microservices make networking essential.
Microservices are slower to develop.