In my experience a new micro-service considered the default option for anything which could be done as a micro-service nowadays. Library API design is the lost art (almost). And the choose is not always obvious for non-technical reasons. Consider a following example: you have 3 micro-services X, Y, Z which need to interact with a platform G but this interaction requires non-trivial chunk of code. It can be done as a library or as a new adapter micro-service A which will encapsulate knowledge about platform G and will interact with X, Y, Z the way it would be easier for X, Y, Z developers to integrate. Micro-service will add network latency but will allow to make/deploy all changes only to service A. With a library one would have to test and release a new version and then ask X, Y, Z maintainers to switch to this new version. In some organizations it will be a very slow process because X, Y, Z can put an update request (from the library team) at the bottom of the backlog. With a micro-service A teams X, Y, Z would have much less power to stop/slow development.