Hot questions for Using ZeroMQ in akka

Question:

The Akka doc talks about a variety of seemingly inter-related Akka technologies without distinguishing much between them:

  • Akka Networking
  • Akka Remoting
  • Akka Clustering
  • The Akka ZeroMQ module

My understanding is that "Akka Networking" is simply a module/lib that gives Akka the ability to speak to remote actor systems over TCP. Akka Remoting is another module/lib (not contained in the core Akka JAR) that gives Akka the use of Gossip protocols. And Akka Clustering is yet another module/lib that then uses these Gossip protocols to allow remote actor systems to cluster together and sharestate changes in a viral/"service discovery"-esque manner. And my understanding of Akka ZeroMQ is that it accomplishes the same thing as Akka Clustering, except using ZeroMQ as the basis of the network connections and protocols (instead of Gossip).

So first, if my understanding of these different modules/libs are incorrect, please begin by correcting me!

Assuming I'm more or less on target here, then my main concern is that I might have Remote Actor System 1 (RAS1) using Akka Clustering (and hence Gossip) trying to communicate with Remote Actor System 2 (RAS2) which uses Akka ZeroMQ. In this case, we're using two completely different clustering technologies and protocols, so does this mean these two remote systems can't speak to each other, or does special care need to be taken so that they are compatible with each other?


Answer:

Akka Remoting is what allows for one actor to speak to another actor on a different machine. For Akka Remoting to work you need to know the specific IP address (or hostname), ActorSystem name, and Actor path of the actor you want to talk to. The ActorSystem name can be different in the 2 machines.

Akka Clustering takes away the problem of having to know the specific machine you are talking to (via Cluster-aware Routing or via a Receptionist that listens for machines joining or leaving the cluster). Cluster-aware Routing also allows for things like having a minimum of X instances of an actor running on any machine in the cluster. Akka Clustering uses the Gossip protocol to maintain the list of cluster members. A cluster-enabled application must know the address of at least one host which must always be running to be able to join the cluster. There might be 2, 3 or more, but the idea is that at least one of them must always be up. Akka Clustering is built on top of Akka Remoting.

Although I haven't worked with Akka ZeroMQ, I assume it works similarly to Akka AMQP. I see it more as an alternative to Remoting, in the sense that it enables actors on different machines to talk to each other, with the advantage that none of the actors need to know any specifics about any other machines where actors are running. However, as with Remoting, you need to manually create the actors that receive the messages, whereas with Clustering the cluster takes care of it (as long as you've configured your Routers correctly).

Regarding your last question. The easiest way I can think about for a Cluster to talk to Akka ZeroMQ would be to have one (or several?) actors in the Cluster that talk to ZeroMQ actors (i.e., you can actually mix and match). Have an actor inside the Cluster that listens on the queue, and have another one that publishes the message to the queue. Sort of an Adapter pattern.