How to have common DTOs or entities across Microservices

Question

Say I have 5 different microservices. Each use one common DTO, that is UserDTO.java or an entity User.java.

If these DTOs are to be accessed by all, I came up with two approaches

1. To place all these DTOs in all the microservices

2. To create a project say Project-commons and keep all the common DTOs there and then add the dependency of this project in all microservices

I know that the sharing common DTOs across microservices would kill the concept/principles of microservices, but I wanted to know is there any other approaches available at all?

Answer 1

Actually, sharing a common code library does not kill the concept/principle of keeping changes isolated to one microservice, as long as you strictly version that library. That way, one microservice team can decide it needs new functionality in the shared library, and can bump the version number of the library and add the new functionality. Existing microservices that use that library won't be affected until they independently decide to start using the newer version of that library.

So this is what we do. We have jar files that we share across our microservices, but we push specific versions of those jar files to Nexus and reference those jar files by version number in each microservice.

Note that this concept is no different than two microservices depending on a third party component, like an Apache Commons library. Just because Apache comes out with a new version of a library doesn't mean anyone's binaries change. Each codebase that depends on that library can decide if and when it moves to the new version independent of what version another codebase might be using or what the most recent version of the library might be.