Since RxJava 2.x, Observables are no longer aware of backpressure. Grails RxJava plugin uses Observables in both branches (1.x and 2.x) which might lead do different behaviour depending on the use case.

http://www.nurkiewicz.com/2017/08/1x-to-2x-migration-observable-vs.html

It would be interesting if in the 2.x branch, Flowable can be an extra option living side by side with the current Observable solution, so one can choose which of both would better fit in it's particular needs. I believe that this can be a real benefit for the plugin and in some scenarios around this request/response approach.