View source for Fortran implementation of the Actor Model using MPI

A very simple ping-pong program is shown here. There are 2 programs, ''[[ping]]'' and ''[[pong]]'', that shoot messages among them, starting each with a random number of plays. The first program reaching 0 will send a message for the other program to stop and stops itself. File ''[[moduleMPImanagement]]'' is necessary to compile both programs.

If you examine the code there is no single MPI_BARRIER or any other explicit synchronization point. The main subroutine is:

    subroutine main()
        type(T_pingPong), pointer :: pingPong
        integer :: STAT_CALL
        STAT_CALL = UNDEFINED
        pingPong => constructPingPong()
        STAT_CALL = startGame(pingPong)
        if (STAT_CALL .NE. SUCCESS) then
            print*, "STAT_CALL = ", STAT_CALL
            stop "subroutine main, error calling startGame, ERR01"
        end if 
        call loop(pingPong) !This loop reads MPI messages
        call EXIT(SUCCESS)
    end subroutine main

There is a call to an infinite loop. This routine reads the MPI message queue identifying messages to itself (MPI_PROBE). 

    recursive subroutine loop(pingPong)
        type(T_pingPong), pointer   :: pingPong
        integer                     :: STAT_CALL
        integer                     :: STATUS(MPI_STATUS_SIZE)
        STAT_CALL = UNDEFINED
        call MPI_PROBE(MPI_ANY_SOURCE,                                          &
                       MPI_ANY_TAG,                                             &
                       MPI_COMM_WORLD,                                          &
                       STATUS,                                                  &
                       STAT_CALL)
        if (STAT_CALL .NE. SUCCESS) then
            print*, "STAT_CALL = ", STAT_CALL
            stop "subroutine loop, error calling MPI_PROBE, ERR01"
        end if 
        if (STATUS(MPI_SOURCE) .EQ. getOtherMPI_id(pingPong)) then
            if (STATUS(MPI_TAG) .EQ. getMsgPlayBall3Tag()) then
                STAT_CALL = receiveBall(pingPong, STATUS(MPI_TAG))
                if (STAT_CALL .NE. SUCCESS) then
                    print*, "STAT_CALL = ", STAT_CALL
                    stop "subroutine loop, error calling receiveBall, ERR02"
                end if 
            else if (STATUS(MPI_TAG) .EQ. getMsgEndGameTag()) then 
                STAT_CALL = killPingPong(pingPong)
               if (STAT_CALL .NE. SUCCESS) then
                    print*, "STAT_CALL = ", STAT_CALL
                    stop "subroutine main, error calling killPingPong, ERR02"
                end if 
            end if 
        end if 
        if (pingPong%gameON) call loop(pingPong)
    end subroutine loop

In the recursive subroutine loop 2 messages are processed: getMsgPlayBall3Tag and getMsgEndGameTag. As promised by the actor model:
*internal state is encapsulated; 
*messages are processed one at a time; and 
*communication is asynchronous.