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Grails 3.3 introduces a new Events API that replaces the previous implementation that was based on Reactor 2.x (which is no longer maintained and deprecated).

In Grails 3.3 and above a new link:{api}grails/events/bus/EventBus.html[EventBus] abstraction has been introduced. Like the `PromiseFactory` notion, there are implementations of the `EventBus` interface for common asynchronous frameworks like GPars and RxJava.

To use the Grails Events abstraction you should add a dependency on the `events` plugin to your `build.gradle` file:

[source,groovy,subs="attributes"]
.build.gradle
----
implementation 'org.apache.grails:grails-events'
----

If no asynchronous framework is present on the classpath then by default Grails creates an EventBus based off of the currently active `PromiseFactory`. The default implementation is link:{api}org/grails/async/factory/future/CachedThreadPoolPromiseFactory.html[CachedThreadPoolPromiseFactory] which uses a thread pool that will create threads as needed (the same as `java.util.concurrent.Executors.newCachedThreadPool()`).

If you wish to use a popular async framework such as RxJava as the `EventBus` implementation then you will need to add the appropriate dependency. For example for RxJava 1.x:

[source,groovy,subs="attributes"]
.build.gradle
----
runtimeOnly 'org.apache.grails.events:grails-events-rxjava3'
----

The following table summarizes async framework support and the necessary dependency:

.EventBus Implementations
|===
|Framework | Dependency | Implementation Class

|GPars 1.2.x
|`grails-events-gpars`
|`org.grails.events.gpars.ActorEventBus`

|RxJava 1.2.x
|`grails-events-rxjava`
|`org.grails.events.rxjava.RxEventBus`

|RxJava 2.x
|`grails-events-rxjava2`
|`org.grails.events.rxjava2.RxEventBus`

|RxJava 3.x
|`grails-events-rxjava3`
|`org.grails.events.rxjava3.RxEventBus`

|===
