My Experience with Message Passing Concurrency

I'm working on a peer-to-peer file synchronization program. It's really concurrent and distributed, and it's forced me to learn a thing or two about the concurrency models that we use as programmers. Over the past few years, I've tried both the shared-state model common to C, C++, Java, C#, Python, etc, and the message-passing model unique to Erlang and Scala. In my experience, the message-passing model (aka Actor Model) is far superior to the shared-state model for writing distributed applications. After having used both extensively, I'd go so far as to say that for distributed programming, shared-state concurrency is upside down and backwards.

Here's my informal proof that message-passing concurrency is necessary in a distributed system:

1. Since the system in distributed, real shared state is impossible.
2. The only way for the distributed components of an application to communicate is by sending a message from one to another.
3. Everything else is an abstraction on top of message passing.

HTTP is an abstraction on top of message-passing. AJAX is an abstraction on top of HTTP on top of message-passing. XML-RPC is an abstraction on top of HTTP on top of message-passing. SOAP is an abstraction on top of an abstraction on top of an abstraction on top of HTTP on top of message-passing. Any RPC mechanism is an abstraction on top of message-passing. SQL queries to a remote database are an abstraction on top of message-passing. CORBA is a nasty, tangled mess on top of a foundation of message-passing.

See a pattern?

Not only are all of these abstractions, but they are leaky abstractions. Just about all RPC (Remote Procedure Call) frameworks try to pretend that remote objects or local. But the facade is impossible to keep from leaking. Calls to a remote object might fail or take arbitrarily long. If you want to make the same call to two different remote nodes, those calls must be made synchronously and sequentially; in order to call them in parallel on the remote nodes, they most be called in parallel on the local node. If a call to a remote node triggers a call back to the local node, which may trigger a call to a third node, you end up with a huge spaghetti mess of calls and threads.

I've been down that road. It wasn't pretty. There is a better way.

I think AJAX has opened our eyes a bit. It's a lot closer to message-passing than it is to RPC. In a REST architecture, you "send" messages by POSTing to a URL and you "receive" messages by GETing a URL. All messages are clearly local copies that were serialized and deserialized from the remote data. There isn't any leaky abstraction of data or classes pretending to be in two places at once. Data, timeouts, and failures are in your face and you have to deal with them. So, AJAX is a lot less leaky.

But AJAX is far from perfect. I can't help but think of the Greenspun's Tenth Rule of Programming with a twist on distributed programming: "Any sufficiently complicated distributed platform contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of a message-passing system (Erlang)". Once you get message-passing in your head, you can't help but think of AJAX in that way.

I've been down a better road. It was much more pleasant.

About six months ago, I rewrote major portions of our application with message-passing concurrency. I took a long time. It was tricky. It hurt my head. But it worked. It's a success. It's capable of things that the shared-state system could never do.

Having done it, I can emphatically say that if I could start all over, I would go with message-passing. Most of the work was building the infrastructure and wrapping my head around a new way of thinking. But now that I've done both of those, I've paid the costs and I'm reaping the rewards. We're moving the application in directions that would have been nearly in possible with the old concurrency model.

In my experience, message-passing concurrency is the best way to write distributed applications. But it isn't an easy road to go down. Support for it just isn't there in most programming languages and environments. So far, Erlang has been the pioneer. I would humbly agree that the way I've implemented message-passing in Python, compared to Erlang, looks like an ad-hoc, bug-ridden half-implementation. But for various reasons, I can't use Erlang. I'm hoping for someone to create Erlang++ or E# or Eython or something that combines the concurrency model of Erlang with a modern programming language. Until then, I'll just keep on cobbling together what I can onto the programming language I happen to be using.

More on that in another article.