We have a moderately sized Grails web application using GORM/Hibernate over PostgreSQL and GSPs serving HTML, and also a few REST APIs. We are standardising on Scala, and would like to migrate this application to Play or Spray, with Slick to access the existing database.
Nimble is currently used for authentication/authorisation and user/role/etc. management.
What are the approaches we can take in order to do the migration step by step, avoiding a big bang migration?
They are both JVM languages, is there a way to avoid treating them as separate web apps running on separate ports at arms length?