pekko/akka-docs/src/main/paradox/event-bus.md

Classic Event Bus

Originally conceived as a way to send messages to groups of actors, the EventBus has been generalized into a set of @scala[composable traits] @java[abstract base classes] implementing a simple interface:

Scala

@@snip EventBus.scala { #event-bus-api }

Java

@@snip EventBusDocTest.java { #event-bus-api }

@@@ note

Please note that the EventBus does not preserve the sender of the published messages. If you need a reference to the original sender you have to provide it inside the message.

@@@

This mechanism is used in different places within Akka, e.g. the @ref:Event Stream. Implementations can make use of the specific building blocks presented below.

An event bus must define the following three @scala[abstract types]@java[type parameters]:

• Event is the type of all events published on that bus
• Subscriber is the type of subscribers allowed to register on that event bus
• Classifier defines the classifier to be used in selecting subscribers for dispatching events

The traits below are still generic in these types, but they need to be defined for any concrete implementation.

Classifiers

The classifiers presented here are part of the Akka distribution, but rolling your own in case you do not find a perfect match is not difficult, check the implementation of the existing ones on @extrefgithub

Lookup Classification

The simplest classification is just to extract an arbitrary classifier from each event and maintaining a set of subscribers for each possible classifier. This can be compared to tuning in on a radio station. The trait LookupClassification is still generic in that it abstracts over how to compare subscribers and how exactly to classify them.

The necessary methods to be implemented are illustrated with the following example:

Scala

@@snip EventBusDocSpec.scala { #lookup-bus }

Java

@@snip EventBusDocTest.java { #lookup-bus }

A test for this implementation may look like this:

Scala

@@snip EventBusDocSpec.scala { #lookup-bus-test }

Java

@@snip EventBusDocTest.java { #lookup-bus-test }

This classifier is efficient in case no subscribers exist for a particular event.

Subchannel Classification

If classifiers form a hierarchy and it is desired that subscription be possible not only at the leaf nodes, this classification may be just the right one. It can be compared to tuning in on (possibly multiple) radio channels by genre. This classification has been developed for the case where the classifier is just the JVM class of the event and subscribers may be interested in subscribing to all subclasses of a certain class, but it may be used with any classifier hierarchy.

The necessary methods to be implemented are illustrated with the following example:

Scala

@@snip EventBusDocSpec.scala { #subchannel-bus }

Java

@@snip EventBusDocTest.java { #subchannel-bus }

A test for this implementation may look like this:

Scala

@@snip EventBusDocSpec.scala { #subchannel-bus-test }

Java

@@snip EventBusDocTest.java { #subchannel-bus-test }

This classifier is also efficient in case no subscribers are found for an event, but it uses conventional locking to synchronize an internal classifier cache, hence it is not well-suited to use cases in which subscriptions change with very high frequency (keep in mind that “opening” a classifier by sending the first message will also have to re-check all previous subscriptions).

Scanning Classification

The previous classifier was built for multi-classifier subscriptions which are strictly hierarchical, this classifier is useful if there are overlapping classifiers which cover various parts of the event space without forming a hierarchy. It can be compared to tuning in on (possibly multiple) radio stations by geographical reachability (for old-school radio-wave transmission).

The necessary methods to be implemented are illustrated with the following example:

Scala

@@snip EventBusDocSpec.scala { #scanning-bus }

Java

@@snip EventBusDocTest.java { #scanning-bus }

A test for this implementation may look like this:

Scala

@@snip EventBusDocSpec.scala { #scanning-bus-test }

Java

@@snip EventBusDocTest.java { #scanning-bus-test }

This classifier takes always a time which is proportional to the number of subscriptions, independent of how many actually match.

Actor Classification

This classification was originally developed specifically for implementing @ref:DeathWatch: subscribers as well as classifiers are of type ActorRef.

This classification requires an ActorSystem in order to perform book-keeping operations related to the subscribers being Actors, which can terminate without first unsubscribing from the EventBus. ManagedActorClassification maintains a system Actor which takes care of unsubscribing terminated actors automatically.

The necessary methods to be implemented are illustrated with the following example:

Scala

@@snip EventBusDocSpec.scala { #actor-bus }

Java

@@snip EventBusDocTest.java { #actor-bus }

A test for this implementation may look like this:

Scala

@@snip EventBusDocSpec.scala { #actor-bus-test }

Java

@@snip EventBusDocTest.java { #actor-bus-test }

This classifier is still is generic in the event type, and it is efficient for all use cases.

Event Stream

The event stream is the main event bus of each actor system: it is used for carrying @ref:log messages and @ref:Dead Letters and may be used by the user code for other purposes as well. It uses @ref:Subchannel Classification which enables registering to related sets of channels (as is used for RemotingLifecycleEvent). The following example demonstrates how a simple subscription works. Given a simple actor:

@@@ div { .group-scala }

@@snip LoggingDocSpec.scala { #deadletters }

@@@

@@@ div { .group-java }

@@snip LoggingDocTest.java { #imports-deadletter }

@@snip LoggingDocTest.java { #deadletter-actor }

it can be subscribed like this:

@@snip LoggingDocTest.java { #deadletters }

@@@

It is also worth pointing out that thanks to the way the subchannel classification is implemented in the event stream, it is possible to subscribe to a group of events, by subscribing to their common superclass as demonstrated in the following example:

Scala

@@snip LoggingDocSpec.scala { #superclass-subscription-eventstream }

Java

@@snip LoggingDocTest.java { #superclass-subscription-eventstream }

Similarly to @ref:Actor Classification, EventStream will automatically remove subscribers when they terminate.

@@@ note

The event stream is a local facility, meaning that it will not distribute events to other nodes in a clustered environment (unless you subscribe a Remote Actor to the stream explicitly). If you need to broadcast events in an Akka cluster, without knowing your recipients explicitly (i.e. obtaining their ActorRefs), you may want to look into: @ref:Distributed Publish Subscribe in Cluster.

@@@

Default Handlers

Upon start-up the actor system creates and subscribes actors to the event stream for logging: these are the handlers which are configured for example in application.conf:

akka {
  loggers = ["akka.event.Logging$DefaultLogger"]
}

The handlers listed here by fully-qualified class name will be subscribed to all log event classes with priority higher than or equal to the configured log-level and their subscriptions are kept in sync when changing the log-level at runtime:

Scala

@@@vars

system.eventStream.setLogLevel(Logging.DebugLevel)

@@@

Java

@@@vars

system.eventStream.setLogLevel(Logging.DebugLevel());

@@@

This means that log events for a level which will not be logged are typically not dispatched at all (unless manual subscriptions to the respective event class have been done)

Dead Letters

As described at @ref:Stopping actors, messages queued when an actor terminates or sent after its death are re-routed to the dead letter mailbox, which by default will publish the messages wrapped in DeadLetter. This wrapper holds the original sender, receiver and message of the envelope which was redirected.

Some internal messages (marked with the DeadLetterSuppression trait) will not end up as dead letters like normal messages. These are by design safe and expected to sometimes arrive at a terminated actor and since they are nothing to worry about, they are suppressed from the default dead letters logging mechanism.

However, in case you find yourself in need of debugging these kinds of low level suppressed dead letters, it's still possible to subscribe to them explicitly:

Scala

@@snip LoggingDocSpec.scala { #suppressed-deadletters }

Java

@@snip LoggingDocTest.java { #suppressed-deadletters }

or all dead letters (including the suppressed ones):

Scala

@@snip LoggingDocSpec.scala { #all-deadletters }

Java

@@snip LoggingDocTest.java { #all-deadletters }

Other Uses

The event stream is always there and ready to be used, you can publish your own events (it accepts @scala[AnyRef]@java[Object]) and subscribe listeners to the corresponding JVM classes.