Software Engineering I
Sept 25, 2013 Notes

Last time we discussed slots and hooks
We also started the client-server architecture and design of the client and
server.

Concurrency in Client-Server Systems
 - inherently concurrent
 - client has at least the appearance of running at the same time as the
   server
 
  Client does the following concurrently

    - waits for interaction with the end user and responds
    - waits for and responds to messages from the server
    -  implemented as multiple threads

  Server does the following concurrently

    - Waits for interaction with the server user and responds as necessary
    - Waits for clients to connect and establishes connections as necessary
    - For each client connected, waits for and responds to messages from that
      client

    Server generally runs as n + 2 threads, where n is the number of clients

Thin vs fat clients
   Thin clients are as small as possible and do little work
     - reliant upon the server

   Fat clients are delegated as much work as possible

   Proper choice depends upon network resources

Messages in the client-server system are sent via protocol
  - Languages and rules of conversation

Job of the software engineer when developing a client server system
  1. Determine work to be performed by client and server
  2. How will the work be distributed? Thin, intermediate, fat client?
  3. Communications Protocol
  4. Startup, handling connections, send/receive messages, and termination

Technology needed to build a client-server system
  -Java sockets
  -network concepts
  -TCP/IP <--  Transmission Control Protocol / Internet Protocol
   Route messages from one computer to another
  -Computers using IP called hosts
  -IP address in numeric form - v4 - 4 numbers separated by dots
                              - v6 - 6 numbers separated by colons
  -IP address may be in host name form, too.

Servers are identified by a port number from 0 to 65535

  ports 0-1023 are reserved for specific use
    e.g. 80 is the http port, 443 is the https (ssl) port

  The hostname or ip address + port number gives the server address
    e.g. Tomcat and Glassfish web servers run on port 8080 by default
         If run locally, navigate to http://localhost:8080 or 
         http://127.0.0.1:8080 to access those servers

Java Sockets

  java.net is used for many TCP/IP operations

  The Socket class is central to the java.net package

  Both client and server must have a socket to connect and exchange information

  1. Server starts listening on a port 

      ServerSocket ss = new ServerSocket(port);  //The port is an int 
                                                 //such as 5555

  2. Client connects (client socket)

      Socket cs = new Socket(host, port);

  3. Server must have a thread for listening for connections

      Socket scs = ss.accept();

     Waits indefinitely for a client to connect, then creates a Socket to 
     handle the connection.

     IOException results from connection problems.

     Be aware that the connection is symmetric.

There are two streams of information

  - client to server
  - server to client

  InputStream and OutputStream are used from java.io

  output = cs.getOutputStream();
  input = cs.getInputStream();

  It is possible to use PrintWriter and Scanner with both.

  You can use filters, too

    DataOutputStream <-- send primitives
    ObjectOutputStream
    ObjectInputStream <-- send and receive objects

  Must use serialization to transmit objects
  Must implement java.io.Serializable

  Example:

  output = new ObjectOutputStream(cs.getOutputStream());
  output.writeObject(msg);

  input = new ObjectInputStream(clientSocket.getInputStream());
  msg = input.readObject();  //Waits until object is real (received) over
                             //socket. Get I/O error if connection is 
                             //terminated.