30 Linda—Process Communication

Linda is a concept for process communication.

For an introduction and a deeper description, see [Carreiro & Gelernter 89a] or [Carreiro & Gelernter 89b], respectively.

One process is running as a server and one or more processes are running as clients. The processes are communicating with sockets and supports networks.

The server is in principle a blackboard on which the clients can write (out/1), read (rd/1) and remove (in/1) data. If the data is not present on the blackboard, the predicates suspend the process until they are available.

There are some more predicates besides the basic out/1, rd/1 and in/1. The in_noblock/1 and rd_noblock/1 does not suspend if the data is not available—they fail instead. A blocking fetch of a conjunction of data can be done with in/2 or rd/2.

Example: A simple producer-consumer. In client 1:

     producer :-
     produce(X) :- .....

In client 2:

     consumer :-
     consume(A) :- .....

Example: Synchronization

            in(ready),  %Waits here until someone does out(ready)

Example: A critical region

            in(region_free),  % wait for region to be free
            out(region_free), % let next one in

Example: Reading global data

     or, without blocking:
            rd_noblock(data(Data)) ->
            ;     write('Data not available!'),nl

Example: Waiting for one of several events

            in([e(1),e(2),...,e(n)], E),
     %  Here is E instantiated to the first tuple that became available