Editing Process calculus (section)

=== Parallel composition ===
Parallel composition of two processes <math>\mathit{P}</math> and <math>\mathit{Q}</math>, usually written <math>P \vert Q</math>, is the key primitive distinguishing the process calculi from sequential models of computation. Parallel composition allows computation in <math>\mathit{P}</math> and <math>\mathit{Q}</math> to proceed simultaneously and independently. But it also allows interaction, that is synchronisation and flow of information from <math>\mathit{P}</math> to <math>\mathit{Q}</math> (or vice versa) on a channel shared by both. Crucially, an agent or process can be connected to more than one channel at a time.

Channels may be synchronous or asynchronous. In the case of a synchronous channel, the agent sending a message waits until another agent has received the message. Asynchronous channels do not require any such synchronization.  In some process calculi (notably the [[π-calculus]]) channels themselves can be sent in messages through (other) channels, allowing the topology of process interconnections to change. Some process calculi also allow channels to be ''created'' during the execution of a computation.