Sorry for the late response but I am currently working on some grails projects!
Indeed the evolution of Grails in a realtime/event loop framework is not straightforward but it will bring a new area for Java/Groovy developers seeking for a more efficient way to scale their web apps. The are many benefits considering the fact that a multi-threaded environment will merge with server realtime events/event loop. I have recently reviewed a Node.js framework called Sails.js and literally, if you wouldn't have to code in JavaScript it would be exactly like Grails (the name is obvious). I really liked the fact that in Sails.js you could write the business logic inside the controllers as always but the client/server communication was handled with Socket.IO. The server code is the same like handling HTTP requests but in client you use Socket.IO. Sails.js automatically understands that it is a Socket.IO request and calls the relevant controller/action. "It just makes HTTP and Socket.IO interoperable, making it easier than ever to add realtime/pubsub/comet functionality to your app". I think that kind of functionality would be absolutely great for Grails and not having to add extra code/plugins(events/atmosphere plugin) or write the logic inside Services. Maybe the switching from HTTP/Socket.IO request could be configurable on Controller/action level through annotations or as always globally (Config.groovy or UrlMappings.groovy). Of cource Socket.IO reguires both client/side implementations, so there will be a need for a relevant plugin that can be used inside gsp. Another issue is the server. Do you considering Netty for this matter? Tomcat 8 defaults to NIO, with no extra configuration, so we could leverage that!
Well thank you for giving me the opportunity to discuss with you these matters and I'm looking forward for your response!
Christopher Dimitriadis