Comment by minkles

> Or programs writing to and others reading from a Unix pipe.

write(message), read() -> message

> Or programs talking to each other using HTTP.

request() -> response

> Or Erlang processes communicating concurrently on one machine or across a network.

sendMess(message), waitMess() -> message

> Or different programs sharing one database.

execute(query) -> response

...

I'm a mathematician at heart so I'm staying away from category theory as long as possible.