Com S 541 Lecture -*- Outline -*-

* Simple message protocols (5.3)

  Q: What's a protocol?
 
  def: a *protocol* is a way to realize some information exchange 
  between two or more parties using a specified
  sequence of message exchanges

** RMI (5.3.1)

------------------------------------------
      RMI (= RPC)

% Example: file RMI.oz

proc {ServerProc Msg}
   case Msg
   of calc(X Y) then
      Y=X*X+2.0*X+2.0
   end
end

Server = {NewPortObject2 ServerProc}

proc {ClientProc Msg}
   case Msg
   of work(Y) then Y1 Y2 in
      {Send Server calc(10.0 Y1)}
      {Wait Y1}
      {Send Server calc(20.0 Y2)}
      {Wait Y2}
      Y=Y1+Y2
   end
end

Client = {NewPortObject2 ClientProc}

{Browse {Send Client work($)}}
------------------------------------------

        Q: Are these port objects symmetric?
        No, the server doesn't know about the client.

        Q: Are message executed concurrently by the server?
        No, sequentially.
            They recommend this as simplest to program and reason about
            (esp. if there is internal state)

** Asynchronous RMI (5.3.2)

   same as RMI, but client continues after send without waiting.

   Q: How would you make the sends asynchronous?
   Just don't wait, the send is already asynchronous.

** Callbacks (5.3.3-5)

   Q: What's a callback?
   When the server calls the client (to get more info)

   Q: How would you implement callbacks?

   2 techniques:
     - client doesn't wait for the reply, uses a thread to wait
         (end to end synchronization through dataflow variables,
         see RMIcallbackthread.oz)
     - pass a continuation (record or closure) to server
         saves on threads, see 5.3.4-5

   Q: What if the client's callback has to call the server again?
   Then neither can wait

** Exceptions (5.3.6)

   Q: What would you do if the server can encounter exceptions?
   Pass result back a tagged record, indicating success or exception,
   client can raise exception if finds it.