I have strong ideological arguments against process calculi, exactly because of the parallel composition. I think that parallel composition is not realistical, because there is no meaning in the “parallel” unless you have a God view over the distributed computation.
This is a very brute argument, but I can make it detailed (and I did it, here and there in this open notebook).
In my opinion we are still in the process of letting go the old ideas of the industrial revolution. The main idea which we need to exorcise out of every corner of the mind is that there is a benevolent (or not) dictator who organize the process of the world (be it a factory, a government, a ministry, or a school class) in a way which is easy to lead because it has well placed bottlenecks which give a global meaning to the process.
Concretely, the very successful idea of organizing stuff, which comes from the industrial revolution, is that one has to abstract over the individuals, the subjects, then to stream the interactions between them (the individual abstracted into functions) by creating a hierarchy of bottlenecks. The advantage is that structure gives a meaning to what is happening.
A meaning is simply like a hash table.
The power of this system of organization is tremendous. It led to the creation of the modern states, as well as to the creation of economic and ideological systems, like capitalism and communism, which are both alike in the way they treat individuals as abstractions.
This kind of organisation pervades everything, in very concrete and punctual ways, so much so that the material structure which holds together our society (like in particular the server-client structure of the net, as a random example, but less some of the net protocols) has grown in the way it is not only because there are some universal laws and invisible hands which constrain it, but also because this structure is an addition of a myriad of components which have been designed in this way and not in another because of the industrial revolution ideology of control and abstraction.
The power of the industrial revolution main idea is that you can take any naturally occurring process (like apples growing in trees and people culling them and making pies) and structure it in a meaningful way and transform it into a viral process (apple pies making factory). You just have to abstract apples and peoples into resources and synchronize the various parts, to define the inputs and outputs and then optimize your control over it and then you can make 10^9 evaluations of the abstract notion of “apple pie” and put them on the shelves of the supermarket, instead of 10^3 individual apple pies as grandmothers used to make in their kitchens.
Now, in the factory of apple pies, the notion of parallel processes makes perfect sense. Contrary to that, in the real real world with trees and apples and grandmothers with their ovens, P | Q makes sense only in retrospect.
If you were God then you could look from far above at all these grandmas and see lots of P | Q. But the grandmas don’t need the parallel composition to make their delicious apple pies. Moreover, the way of life is that generally there is no need for a centralized control, no need for a meaning. Viruses and cells don’t know they are viruses and cells. They work very well without knowing they do some tasks inside an environment.
The life ozone cell may be in parallel with the life of another from the God’s point of view, but this relation is certainly not a part of, nor a need for these life processes to function.
The big question for me is: how to replicate this by techne? It is clearly possible, as proved by the world we live in. It looks to me very promising to try to work under these self-imposed constraints: no meaning, no parallel composition in particular, no abstraction, no levels. It is surprising that chemlambda works at all already.