Message Passing Models II
=========================

UNIX sockets
============

Message passing on the operating system level
Introduced in Unix BSD 4.2
Based on Internet Protocol (TCP/IP, UDP/IP)

Client Server Model
- client process creates socket at one end of communication link
- server process responds through a socket at the other side of link

Relationship not symmetrical
- application must know whether it is server or client

SLIDE ptx sockets manual, Figure 1-1

Basic Strategy
--------------

SLIDE ptx sockets manual, Figure 4-1

1. server creates socket
   process internal data structure

2. bind() assigns address to socket
   server can advertise location of its service

3. server issues listen() to indicate
   that it is ready to receive connection requests

4. server issues accept() to handle
   each connection request

5. client creates socket

6. client issues connect() to establish connection with server

7. client issues write() to send request to server

8. server issues read() to accept request from client

9. server issues write() to send reply to client

Multithreading
--------------

8. server starts thread and passes socket to thread

   - detached thread receives and replies to request

9. server goes to step 4

Example Programs
----------------

SLIDE ptx sockets manual, page 4.5
- TCP server
- TCP client

Remote Procedure Calls (RPC)
============================

"Synchronous message passing"

Communication by procedure calls
- creates a request and blocks until reply is received
- lower layers care for data conversions and communication
- interface language required to provide semantics of
  parameter passing mechanisms 

SLIDE ptx/NFS RPC programming guide, Figure 1-1
SLIDE ptx/NFS RPC programming guide, page 1-3

RPC program

SLIDE ptx/NFS RPC programming guide, page 1-5
SLIDE ptx/NFS RPC programming guide, page 1-6
SLIDE ptx/NFS RPC programming guide, page 1-7

rpcgen: a protocol compiler

Generation of RPC programs from sequential code
- takes remote program interface definition in RPC language
- produces C output which includes
  - stub versions of client routines
  - server skeleton
  - filter routines for parameters

Developer writes server procedures and links them with server skeleton
=> executable server program

User writes main program that makess local procedure calls
to client stubs produced by rpcgen
=> executable client program

SLIDE ptx/RPC rpcgen programming guide, page 2-3
SLIDE ptx/RPC rpcgen programming guide, page 4
SLIDE ptx/RPC rpcgen programming guide, page 5
SLIDE ptx/RPC rpcgen programming guide, page 6-8

World Wide Web (WWW)
====================

Networks of servers providing hypertext documents
Client browsers request document from server
Server provides document
Client browser displays document

Request may trigger any action on server
- start of program
- access of database
- ....
=> result document may be generated on the fly

Accessing any server functionality over the WWW

Java
====

C++ like programming language developed by SUN
Compiler generates portable byte code
WWW browsers incorportate interpreter for byte code

HTML document may contain references to byte code "Applets"
Applets are loaded from server and executed by browser

"Executable documents"
=> documents become applications

"Network computers"
=> operating system becomes browser/document interpreter

Examples
--------

Implementing a (multithreaded) Server

SLIDE Java in a Nutshell, page 145-146

(Object oriented parallel programming, see later)

Implementing a client

SLIDE Java in a Nutshell, page 147-148

Implementing an applet client

SLIDE Java in a Nutshell, page 149-150

Future
------

Extensions: server-side applets "servlets"
- client may return code to server for execution

Extensions: mobile agents
- programs "wandering" through the network