Undertow offers Channels as a form of async communication to the application. Channels are non blocking and allow Undertow to scale very nicely. However, they are quite fiddly to work with from an application developer's perspective. It is often preferable to use a Publisher/Subscriber system.

Java 9 has this available in the Flow library but integrating this would require all users to migrate to use Java9. Instead, it would be a good idea to use org.reactivestreams.Publisher for retrieving the request body and org.reactivestreams.Subscriber for writing to response bodies.

Implementing this would add a dependency onto at least reactivestreams. It the Undertow doesn't want to roll its own Publisher, it's possible to use rxjava2's Flowable/Observable or reactive's Flux.