Message protocol vs. Function call API

[Mihai Balea]

This came up on a different thread, but I believe it is an interesting  
subject on its own.

On Sep 30, 2009, at 9:36 PM, Robert Virding wrote:

> P.S. One long, never-resolved discussion Joe and I had for many  
> years was whether you should have an API like is done in gen_XXX or  
> to explicitly specify the message protocol. Having the message  
> protocol would allow you to increase concurrency.

I've been mulling this question myself lately. Besides increased  
concurrency, a message protocol would also allow for other nifty  
things, like versioning. Function calls make synchronous APIs easier.  
So I was wondering what folks on this list think... besides the  
obvious "it depends on the situation".

Mihai