UML does not have something like class models for symmetric interactions ("symmetric" in Frankel's sense), for communicating processes.
Java does not have a typed high-level abstraction for communication, each of its API is tied to low-level transport.
Why it has been so hard?
Because communication looks bottomless. Well it is bottomless. When it comes to sequential computation you have a bottom --- the machine code. So you feel safe. That is a hardcore of computing to which you can always return to. Somewhat paradoxically this gives a good basis when we try to have high-level abstractions for programming: they have a basis to build on, a home to come back to, a hard kernel against which you can bang your abstraction, and so you are sure that what you are doing with your abstraction is not imagination.
But where is communication? Where can we find it? Is it in routers? Is it in the (hard-wired) algorithm of Ethernet, listening, talking and waiting? Is it in algorithms for switch? Is it in the protocol stacks in each endpoint? Where is its rockbottom? Where can we start from and go back to and test your abstraction? It looks too complex. It looks that what makes up communication is completely ad hoc and a hodgepodge of heterogeneous machineries involving so much interplay among many kinds of software and many kinds of hardware. There can be no general theory here and there can be no general abstraction here not to speak of such basic things as, for example, "signature".
So is it too bold (or even ridiculous) to consider a general modelling/programming language for communication? Not quite: since after all we are not going to be bold at all. We wish to make something useful. And it looks we do need something --- something flexible and (at least modestly) comprehensive and comprehensible: something in fact simple and which serves our purpose. We take this stance, a utilitarian, practical stance. Foundationally we have the pi-calculus. But let's take that story as the undercurrent for some time, and continue our practical thread of thought.
As such the next post will treat examples (and inter alia illustrate the events on the sixth day). à bientôt!
