When it's necessary to create a custom spring boot starter rather than create a new microservice?

I've a microservice architecture, and need some common logic.
When it's necessary to create a custom spring boot starter rather than create a new microservice?

In my experience, creating a new microservice from the ground up is generally due to preventing any monoliths occurring. Microservices should generally have one job and then do it well. You don't want to muddy up the implementation and purpose of your microservice by adding unrelated operations.

There are many design patterns for the "types" you could be creating but I won't go into too much detail there. Overall, based on what business purpose you are solving you can select your design and begin development. Different designs should be separated and not combined into monolithic styles. Here is a good article showcasing design options: https://www.openlegacy.com/blog/microservices-architecture-patterns/

If you find your self having to re-create multiple microservice serving different use cases you can always utilize a tool such as yeoman to speed up creating these new projects. You can build a generator that will give you a working template so you don't have to spend the time re developing from the ground up each time you need a different service.

Here is a guide that I wrote recently on creating your own yeoman generator: https://medium.com/@dylanlamott/building-a-yeoman-generator-line-by-line-6966debb39a3