Checking if remote Akka actor is available

Question

How can I check if a remote actor, for which I have obtained an actorRef via actorFor, is alive? Any reference to documentation would be appreciated. I am using Akka from Scala.

I've seen reference to supervisors and deathwatch, but don't really feel my use-case needs such heavy machinery. I just want to have my client check if the master is up using a known path, and if it is send a message introducing itself. If the master is not up, then it should wait for a bit then retry.

Update 2: Suggestions are that I just use a ping-pong ask test to see if it's alive. I understand this to be something like

implicit val timeout = Timeout(5 seconds)
val future = actor ? AreYouAlive
try{
    Await.result(future, timeout.duration)
}catch{
    case e:AskTimeoutException => println("It's not there: "+e)
}

I think I've been confused by the presence of exceptions in the logs, which remain there now. E.g.

• Error: java.net.ConnectException:Connection refused
• Error: java.nio.channels.ClosedChannelException:null

Perhaps this is just how it works and I must accept the errors/warning in the logs rather than try to protect against them?

Answer 1

Just send it messages. Its machine could become unreachable the nanosecond after you sent your message anyway. IF you don't get any reply, it is most likely dead. There's a large chapter on this in the docs: http://doc.akka.io/docs/akka/2.0.1/general/message-send-semantics.html

Answer 2

You should never assume that the network is available. Our architect here always says that there are two key concepts that come into play in distributed system design.

They are:

• Timeout
• Retry

Messages should 'timeout' if they don't make it after x period of time and then you can retry the message. With timeout you don't have to worry about the specific error - only that message response has failed. For high levels of availability you may want to consider using tools such as zookeeper to handle clustering/availability monitoring. See leader election here for example: http://zookeeper.apache.org/doc/trunk/recipes.html