When to use local vs remote actors?

Question

When should I use Actors vs. Remote Actors in Akka?

I understand that both can scale a machine up, but only remote actors can scale out, so is there any practical production use of the normal Actor?

If a remote actor only has a minor initial setup overhead and does not have any other major overhead to that of a normal Actor, then I would think that using a Remote Actor would be the standard, since it can scale up and out with ease. Even if there is never a need to scale production code out, it would be nice to have the option (if it doesn't come with baggage).

Any insight on when to use an Actor vs. Remote Actor would be much appreciated.

Answer 1

Remote Actors cannot scale up, they are only remote references to a local actor on another machine.

For Akka 2.0 we will introduce clustered actors, which will allow you to write an Akka application and scale it up only using config.

Answer 2

The question asks "If a remote actor only has a minor initial setup overhead and does not have any other major overhead then I would think that using a Remote Actor would be the standard". Yet the Fallacies of distributed computing make the point that it is a design error to assume that remoting with any technology has no overhead. You have the overhead of copying the messages to bytes and transmitting it across the network interface. You also have all the complexity of different processes being up, down, stalled or unreachable and of the network having hiccups leading to lost, duplicated or reordered messages.

This great article has real world examples of weird network errors which make remoting hard to make bullet proof. The Akka project lead Roland Kuln in his free video course about akka says that in his experience for every 1T of network messages being sent he sees a corruption. Notes on Distributed Systems for Young Bloods says "distributed systems tend to need actual, not simulated, distribution to flush out their bugs" so even good unit tests wont make for a perfect system. There is lots of advice that remoting is not "free" but hard work to get perfect.

If you need to use remoting for availability, or to move to huge scale, then note that akka does at-least-once delivery with possible duplication. So you must ensure that duplicated messages don't create bad results.

The moment you start to use remoting you have a distributed system which creates challenges which are discussed in Distributed systems for fun and profit. Unless you are doing very simply things like stateless calculators that are idempotent to duplicated messages things get tricky. One of assignments on that akka video course at the link above is to make a replicated key-value store which can deal with lost messages by writing the logic yourself. Its far from being an easy assignment. State distributed across different processes gets very hard, actors encapsulate state, therefore distributing actors can get very hard, depending on the consistency and availability requirements of the system you are building.

This all implies that if you can avoid remoting and achieve what you need to achieve then you would be wise to avoid it. If you do need remoting then Akka makes it easy due to its location transparency. So whilst its a great toolbox to take with you on the job; you should double check if the job needs all the tools or only the simplest ones in the box.

Answer 3

Regular Actors can be used in sending out messages in local project. As for the Remote Actors, you can used it in sending out messages to dependent projects that are connected to the project sending out the message.

Please refer here for the Remote Akka Actors

http://doc.akka.io/docs/akka/snapshot/scala/remoting.html