Why use this instead of using a specific subject to subscribe? How to check what are the types of eventstream available - ie. if i want to subscribe and dont know what are the available streams?
This topic is based on a conversation between an anonymous user of Akka and our “Ask AI about Akka” bot in the documentation.
What is this context.system.eventStream.subscribe?
The context.system.eventStream.subscribe is a method used in Akka to subscribe an actor to a particular type of message on the event stream. The event stream is a mechanism in Akka for publishing messages to multiple subscribers. It is often used for system messages like logging and dead letters, but can also be used for user-defined messages.
Here is an example of how to use it:
val listener = system.actorOf(Props[DeadLetterListener]())
system.eventStream.subscribe(listener, classOf[DeadLetter])
In this example, an actor of type DeadLetterListener is created and then subscribed to DeadLetter messages on the event stream. This means that whenever a DeadLetter message is published on the event stream, the listener actor will receive it.
It’s also worth noting that the event stream uses a classification system, which means you can subscribe to a group of related messages by subscribing to their common superclass.
Why use this instead of using a specific subject to subscribe?
This is different from subscribing to a specific subject or actor because the event stream is a publish-subscribe model that allows messages to be broadcast to multiple subscribers.
One of the main reasons to use the event stream is to decouple the producers of messages from the consumers. This means that the producer of a message doesn’t need to know about the specific actors that are interested in the message. Instead, it just publishes the message to the event stream, and any actor that has subscribed to that type of message will receive it.
This can be particularly useful for system-wide notifications, such as logging or dead letters, where multiple parts of the system might be interested in these events. It can also be used for user-defined messages where you want to broadcast a message to multiple actors without having to keep track of them individually.
How to check what are the types of eventstream available
In Akka, the event stream is not limited to specific types of events. It can carry any type of event that extends AnyRef (or Object in Java). This includes system messages like log messages and dead letters, as well as any user-defined messages.
Here’s an excerpt from the Akka documentation:
The event stream is always there and ready to be used, you can publish your own events (it accepts
AnyRefObject) and subscribe listeners to the corresponding JVM classes.
This means that the types of events available on the event stream are not predefined, but rather depend on what types of messages you choose to publish on the stream.
If you want to subscribe to a specific type of event, you can do so by specifying the class of the event when you subscribe.
if i want to subscribe and dont know what are the available streams
In Akka, the event stream is not categorized into different streams. Instead, it’s a single stream that can carry any type of event. The types of events that can be published to the event stream are not predefined, but rather depend on what types of messages you or the system choose to publish.
When you subscribe to the event stream, you specify the type of event you’re interested in.
If you don’t know what types of events are being published, you might need to look at the code that’s publishing events to the stream. If you’re working with a large or unfamiliar codebase, this could potentially be challenging. You might need to search for calls to system.eventStream.publish or similar methods.
Alternatively, you could potentially subscribe to all events by subscribing to the Object class, which is the superclass of all classes in Java. However, this could result in receiving a large number of events, many of which you might not be interested in.
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