Synchronous channels for dataflow

Synchronous communication gives extra guarantees over asynchronous ones:

1. Fast producer's speed is auto-adjusted to the slow consumer
2. Writer can make assumptions about the reader, since the reader must have reached the read() command before the writer returns from write()
3. it makes it much easier to reason about the behaviour of a network of processes if we have the guaranteed that a process cannot get far ahead of its neighbours as can happen in asynchronous systems

Can be also extended to 1:n or n:m as well as network channels