title: Use Keycloak`s users in own application
body: <p>We have several applications where we use custom implemented security and user management. Now we would probably switch to keycloak as it is a better solution.</p>
  
  <p>In every app we use our User model at several places, like Product (f.e. creator_id = 1) a.s.o.
  Similar question was asked here <a href="https://stackoverflow.com/questions/42962828/keycloak-and-spring-boot-rest-api-user-specific-data-stragegy">Keycloak and spring boot rest api - user specific data stragegy</a></p>
  
  <p>My questions are:</p>
  
  <p>How can I keep our User models but use keycloak for authentication and user management without migrating our users to keycloak? And how to sync the process of user registration (with Keycloak)? An useful example would be extremely helpful.</p>