[dragonwriter]

> I think you very much can say that "at the boundaries, applications are procedural" (i.e., they do side-effecting things sequentially).

Well, side-effecting, yes, because that's literally how we define boundaries.

Sequentially? Not so much; concurrency (whether asynchrony or true parallelism) is important largely because simple sequential behavior doesn't capture what happens naturally at the boundaries well. (I suppose on an individual boundary, defined in the right way, there is likely to be a sequencing constraint, but not in aggregate across the boundaries of the system.)

[cryptonector]

You can parallelize as much as you want, but remember, you can't go faster than the slowest serial code. There will be some serial code. Again, I'm talking about order of steps in processing events that themselves may not arrive in any particular order.