Event Handler¶

There is an Event Handler which takes the place of a logging system in Akka:

akka.event.EventHandler

You can configure which event handlers should be registered at boot time. That is done using the ‘event-handlers’ element in akka.conf. Here you can also define the log level.

akka {
  # event handlers to register at boot time (EventHandler$DefaultListener logs to STDOUT)
  event-handlers = ["akka.event.EventHandler$DefaultListener"]
  event-handler-level = "DEBUG" # Options: ERROR, WARNING, INFO, DEBUG
}

The default one logs to STDOUT and is registered by default. It is not intended to be used for production. There is also an SLF4J event handler available in the ‘akka-slf4j’ module.

Example of creating a listener from Scala (from Java you just have to create an ‘UntypedActor’ and create a handler for these messages):

val errorHandlerEventListener = Actor.actorOf(new Actor {
  self.dispatcher = EventHandler.EventHandlerDispatcher

  def receive = {
    case EventHandler.Error(cause, instance, message) => ...
    case EventHandler.Warning(instance, message) => ...
    case EventHandler.Info(instance, message) => ...
    case EventHandler.Debug(instance, message) => ...
    case genericEvent => ...
  }
})

To add the listener:

EventHandler.addListener(errorHandlerEventListener)

To remove the listener:

EventHandler.removeListener(errorHandlerEventListener)

To log an event:

EventHandler.notify(EventHandler.Error(exception, this, message))

EventHandler.notify(EventHandler.Warning(this, message))

EventHandler.notify(EventHandler.Info(this, message))

EventHandler.notify(EventHandler.Debug(this, message))

EventHandler.notify(object)

You can also use one of the direct methods (for a bit better performance):

EventHandler.error(exception, this, message)

EventHandler.error(this, message)

EventHandler.warning(this, message)

EventHandler.info(this, message)

EventHandler.debug(this, message)

The event handler allows you to send an arbitrary object to the handler which you can handle in your event handler listener. The default listener prints it’s toString String out to STDOUT.

EventHandler.notify(anyRef)

The methods take a call-by-name parameter for the message to avoid object allocation and execution if level is disabled. The following formatting function will not be evaluated if level is INFO, WARNING, or ERROR.

EventHandler.debug(this, "Processing took %s ms".format(duration))

From Java you need to nest the call in an if statement to achieve the same thing.

if (EventHandler.isDebugEnabled()) {
  EventHandler.debug(this, String.format("Processing took %s ms", duration));
}