Source : Free On-Line Dictionary of Computing

message passing
     
        One of the two techniques for communicating between parallel
        processes (the other being {shared memory}).
     
        A common use of message passing is for communication in a
        {parallel computer}.  A process running on one processor may
        send a message to a process running on the same processor or
        another.  The actual transmission of the message is usually
        handled by the {run-time support} of the language in which the
        processes are written, or by the {operating system}.
     
        Message passing scales better than {shared memory}, which is
        generally used in computers with relatively few processors.
        This is because the total communications {bandwidth} usually
        increases with the number of processors.
     
        A message passing system provides primitives for sending and
        receiving messages.  These primitives may by either
        {synchronous} or {asynchronous} or both.  A synchronous send
        will not complete (will not allow the sender to proceed) until
        the receiving process has received the message.  This allows
        the sender to know whether the message was received
        successfully or not (like when you speak to someone on the
        telephone).  An asynchronous send simply queues the message
        for transmission without waiting for it to be received (like
        posting a letter).  A synchronous receive primitive will wait
        until there is a message to read whereas an asynchronous
        receive will return immediately, either with a message or to
        say that no message has arrived.
     
        Messages may be sent to a named process or to a named
        {mailbox} which may be readable by one or many processes.
     
        Transmission involves determining the location of the
        recipient and then choosing a route to reach that location.
        The message may be transmitted in one go or may be split into
        {packet}s which are transmitted independently (e.g. using
        {wormhole routing}) and reassembled at the receiver.  The
        message passing system must ensure that sufficient memory is
        available to buffer the message at its destination and at
        intermediate nodes.
     
        Messages may be typed or untyped at the programming language
        level.  They may have a priority, allowing the receiver to
        read the highest priority messages first.
     
        Some message passing computers are the {MIT J-Machine
        (http://www.ai.mit.edu/projects/cva/cva_j_machine.html)}, the
        {Illinois Concert Project
        (http://www-csag.cs.uiuc.edu/projects/concert.html)} and
        {transputer}-based systems.
     
        {Object-oriented programming} uses message passing between
        {object}s as a metaphor for procedure call.
     
        (1994-11-11)