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With modern hardware featuring multiple cores, many programming languages have been adding asynchronous, parallel programming APIs, Groovy being no exception.

Popular asynchronous libraries include:

* RxJava - https://reactivex.io
* GPars - http://gpars.org
* Reactor - https://projectreactor.io

By building on top of these various libraries the Async features of Grails aim to simplify concurrent programming within the framework, include the concept of Promises, and a unified event model.

In general, since the Reactive programming model is an evolving space, Grails tries to provide generic support for integrating a range of asynchronous libraries and doesn't recommend any single library as they all have various advantages and disadvantages.
