Overlooking Axon Frameworks architecture, you can notice that in general systems using the framework are "Message Driven", "Responsive", "Resilient" and "Elastic". According to Reactive Manifesto, the same holds for Reactive Systems in general.
Although we can state that Axon Framework is a type of reactive system, we can't say that it is fully reactive.
Reactive programming is an approach to writing software that embraces asynchronous I/O. Asynchronous I/O is a small idea that portends big changes for software. The idea is simple: alleviate inefficient resource utilization by reclaiming resources that would otherwise be idle, as they waited for I/O activity. Asynchronous I/O inverts the normal design I/O processing: the clients are notified of new data instead of asking for it, which frees the client to do other work while waiting for these notifications.
By their nature, a reactive API and Axon are a great fit, as most of framework's operations are async and non-blocking. Providing a dedicated extension for this was thus a logical step to take. To that end, we chose to use Pivotal’s Project Reactor to build this extension. Reactor builds on top of the Reactive Streams specification and is the de-facto standard for Java enterprise and Spring applications. As such, we feel it to be a great fit to provide an extension in, making Axon more reactive.
Not all Axon components offer a reactive API, yet. We will incrementally introduce more "reactiveness" to this extension, giving priority to components where users can benefit the most.