CS 696 Emerging Technologies: Distributed Objects

1. Definitions
2. Basic Mechanics of Client-Server
3. Issues
   1. Concurrency
   2. Protocol
   3. Error Checking
   4. Logging
   5. Stateful vs Stateless Servers

Doc 3, Basic Client-Server Slide # 2

class foo
   {
   bar server = new bar();
   
   public void makeRequest()
      {
      server.request();
      }
   }

class bar
   {
   public String request()
      {
      return "The answer is no";
      }
   }

Doc 3, Basic Client-Server Slide # 3

A program on machine A can be a server to program on other machine

Sometimes the machines running the server (client) program is called the server (client)

In distributed objects system an object on one machine can be a server to an object on another machine

Doc 3, Basic Client-Server Slide # 4

the server program must already running

the server program must be listening to a port for incoming connections

the client must know the server host machine and the port the server using

Doc 3, Basic Client-Server Slide # 5

import java.net.*;
import java.io.*;
import java.util.*;

class JustADemoServer
   {
   public static void main(String[] args ) throws Exception
      {
      ServerSocket acceptor = new ServerSocket( 0 );
      System.out.println("Using port " + acceptor.getLocalPort());
      
      while (true)
         {
         Socket client = acceptor.accept();
         
         InputStream rawInput = client.getInputStream();
         BufferedReader input = new BufferedReader(
                           new InputStreamReader( rawInput) );
         
         OutputStream rawOutput =  client.getOutputStream();
         PrintWriter      output = new PrintWriter( rawOutput );

         String inputLine = input.readLine().toLowerCase();
         
         if ( inputLine.startsWith("date") )
            {
            Date now = new Date();
            output.println(now.toString());
            output.flush();
            }
            
         input.close();
         output.close();  
         }
      }
   }
Doc 3, Basic Client-Server Slide # 6

import java.net.*;
import java.io.*;
import sdsu.util.ProgramProperties;

public class GetDate
   {
   public static void main(String[] args) throws Exception
      {
      ProgramProperties commandLine = 
         new ProgramProperties( args);

      String host = commandLine.getString( "h" );
      int port = commandLine.getInt( "p" );

      Socket   server = new Socket( host, port);

      InputStream rawInput = server.getInputStream();
      BufferedReader input = new BufferedReader(
                           new InputStreamReader( rawInput) );
      
      OutputStream rawOutput =  server.getOutputStream();
      PrintWriter      output = new PrintWriter( rawOutput );

      output.println( "Date" );
      output.flush();

      System.out.println(inputLine.read());

      output.close();
      input.close();
      }
   }
Doc 3, Basic Client-Server Slide # 7

telnet host port

java GetDate -h=rohan.sdsu.edu -p=45232

Doc 3, Basic Client-Server Slide # 8

A server "long time" to compute the answer for the client

Many clients connect to the server in a short time

Trivial to implement.

Easy to serialize accesses to a central database.

Server is locked while dealing with a request. If the request does take longer than allowed, no other clients can get service.

Doc 3, Basic Client-Server Slide # 9

Individual client requests can be of any complexity and duration

Complexity inherent in concurrent processing.

Doc 3, Basic Client-Server Slide # 10

Set of rules and conventions used by communicating participants

Well defined ( Protocol can not be vague )

Complete ( All situation must be addressed )

Computer program must be able to parse protocol

Extendible

Doc 3, Basic Client-Server Slide # 11

import java.net.*;
import java.io.*;
import sdsu.util.ProgramProperties;

public class GetDate
   {
   public static void main(String[] args) throws Exception
      {
      ProgramProperties commandLine = 
         new ProgramProperties( args);

      String host = commandLine.getString( "h" );
      int port = commandLine.getInt( "p" );

      Socket   server = new Socket( host, port);

      InputStream rawInput = server.getInputStream();
      
      OutputStream rawOutput =  server.getOutputStream();

      rawInput .close();
      rawOutput .close();
      }
   }

Doc 3, Basic Client-Server Slide # 12

Doc 3, Basic Client-Server Slide # 13

Client connects to the server

Client makes one request

Server handles request

Client and server drop connection

Client connects to the server

Client makes one request

Server handles request

Client makes another request

Server handles request

etc.

Client and server drop connection when done

Server needs to remember information about the client while the client is connected - the state of the connection

Stateful servers are much more complicated than stateless servers

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