in some cases the processing can take some time (in the observer method). a common trick is to use an ejb + @Asynchronous for the observer-method. with that the event-source can continue and the observer operates in a new thread.
however, that isn't enough if there is a huge amount of such events. for use-cases like that i've prototyped an add-on for apache deltaspike which allows to use disruptor for firing and observing events. it includes a simple qualifier support, however, it doesn't provide an 1:1 implementation of the cdi event rules. it can be used the same way as cdi events:
the add-on creates a disruptor-processor for every observer methods. the method gets invoked on the cdi-bean itself. the cdictrl-module provided by deltaspike is used for starting the cdi request- and session-scope before an observer method gets called (and to stop it afterwards). with that the application-, session- and request-scope (as well as all scopes based on them which start autom.) can be used for such beans.
since the observer methods are invoked on the contextual-instance, cdi features like dependency-injection can be used as expected. the result is really nice. there is a huge difference in view of the pure event-processing performance of the events:
the result might look strange at the first view. however, the effects are quite clear. since the observer-logic in the demo is fast, the benefit of using @Asynchronous is lost. here the pure invocation of observers in ejbs is slower (than observers in a cdi-bean), because the container has to handle additional ejb features like transactions. the cdi bean as well as the stateful ejb is application-scoped and therefore only one (cached) instance is needed.
in the charts above it's hard to see, but the numbers show that tomee is faster in case of the deltaspike/disruptor add-on (due to the highly optimized proxies of openwebbeans 1.2.x) and as7 is a bit faster with dispatching standard cdi events.
currently the add-on works with tomee and as7 (tested with v7.2) and is available here. the repository also contains several tests, which illustrate the supported use-cases.