Client-Server Network Programming Session 2: Programming with Java and project

1. Client-Server Network ProgrammingSession 2: Programming with Java and project A workshop by Dr. Junaid Ahmed Zubairi Department of Computer Science State University of New York at Fredonia

2. Workshop Outline • Client-Server Mechanisms • How the Applications work • Introduction to sockets • Socket types • Programming with sockets • Concurrent Processing • Programming project

3. Lab 2 Exercise 1 • Take the client source code given earlier and compile and run it • Try using the web server http://www.cia.gov and using the command GET /cia/publications/factbook/index.html HTTP/1.0

4. Lab 2 Exercise 2 • Modify the given client source code to conduct an SMTP dialogue with an email server such as “linus.cs.fredonia.edu”

5. Lab 2 Exercise 3 • Modify the given client source code to conduct communications with a time of day server and get and print the correct time from the server (NOTE: Use TCP port 13 and send a dummy message)

6. Network Programming with Java Ref[4] • You can In Java, it is relatively easier to create a socket and connect it to a server • Make sure you import java.net.* • Socket class is available for TCP sockets and DatagramSocket for UDP sockets • You can define objects that belong to pre-defined classes

7. Java UDP Server • Step 1: Create a datagram socket on a specific port • Step 2: Prepare buffer for receive • Step 3: Prepare datagram packet to receive data from client • Step 4: Receive datagram from client • Step 5: Extract client IP address and port number from the datagram • Step 6: Prepare the response string • Step 7: Prepare the response packet • Step 8: Send out the response • (Compile and run UDPEchoServer.java [Ref 4] file as on the next slides)

8. UDP Server: Initializing vars • import java.io.*; • import java.net.*; • public class UDPEchoServer • { • private static final int PORT = 1234; • private static DatagramSocket dgramSocket; • private static DatagramPacket inPacket, outPacket; • private static byte[] buffer;

9. Open Socket and call run( ) • public static void main(String[] args) • { • System.out.println("Opening port...\n"); • try • { • dgramSocket = new DatagramSocket(PORT);//Step 1. • } • catch(SocketException e) • { • System.out.println("Unable to attach to port!"); • System.exit(1); • } • run(); • }

10. Step 2 through 5 • private static void run() • { • try • { • String messageIn,messageOut; • int numMessages = 0; • do • { • buffer = new byte[256]; //Step 2. • inPacket = new DatagramPacket( • buffer, buffer.length); //Step 3. • dgramSocket.receive(inPacket); //Step 4. • InetAddress clientAddress = • inPacket.getAddress(); //Step 5.

11. Step 5 through 8 • int clientPort =inPacket.getPort(); //Step 5. • messageIn = new String(inPacket.getData(),0,inPacket.getLength());//Step 6. • System.out.println("Message received."); • numMessages++; • messageOut = ("Message " + numMessages+ ": " + messageIn); • outPacket = new DatagramPacket ( • messageOut.getBytes(), messageOut.length(), • clientAddress, clientPort); //Step 7. • dgramSocket.send(outPacket); //Step 8. • }while (true); • }

12. Catch Exceptions • catch(IOException e) • { • e.printStackTrace(); • } • finally //If exception thrown, close connection. • { • System.out.println("\n* Closing connection... *"); • dgramSocket.close(); //Step 9. • } • } • }

13. Java UDP Client • Step 1: Create a new datagram socket • Step 2: Prepare new datagram packet • Step 3: Send out the datagram to specified host and port • Step 4: Prepare buffer for response • Step 5: Place the buffer in a packet • Step 6: Use the packet to receive the response from server • Step 7: Close the socket • (Obtain, compile and run UDPEchoClient.java [Ref 4] as on the next slides)

14. Java UDP Client • import java.io.*; • import java.net.*; • public class UDPEchoClient • { • private static InetAddress host; • private static final int PORT = 1234; • private static DatagramSocket dgramSocket; • private static DatagramPacket inPacket, outPacket; • private static byte[] buffer;

15. Simple main( ); calls run( ) • public static void main(String[] args) • { • try • { • host = InetAddress.getLocalHost(); • } • catch(UnknownHostException e) • { • System.out.println("Host ID not found!"); • System.exit(1); • } • run(); • }

16. Create a Socket and Try • private static void run() • { • try • { • dgramSocket = new DatagramSocket(); //Step 1. • //Set up stream for keyboard entry... • BufferedReader userEntry = • new BufferedReader • (new InputStreamReader(System.in)); • String message="", response="";

17. If not the last one; form the out packet • do • { • System.out.print("Enter message: "); • message = userEntry.readLine(); • if (!message.equals("***CLOSE***")) • { • outPacket = new DatagramPacket( • message.getBytes(), • message.length(), • host,PORT);

18. Steps 3 through 7 • dgramSocket.send(outPacket); //Step 3. • buffer = new byte[256]; //Step 4. • inPacket = new DatagramPacket( • buffer, buffer.length); //Step 5. • dgramSocket.receive(inPacket); //Step 6. • response = new String(inPacket.getData(), • 0, inPacket.getLength()); //Step 7. • System.out.println( • "\nSERVER> " + response); • } • }while (!message.equals("***CLOSE***")); • }

19. Exception Handling • catch(IOException e) • { • e.printStackTrace(); • } • finally • { • System.out.println("\n* Closing connection... *"); • dgramSocket.close(); //Step 8. • } • } • }

20. Lab3: Java Client and Server • Compile and run Java client and server. Obtain the Jcreator from • http://www.jcreator.com/Download.htm • Try to implement TCP client and server in Java

21. Concurrent Processing • We should have some clear understanding of the terms processes and concurrency control • A process is a fundamental unit of computation that has an address space and at least one thread of execution

22. Concurrent Processing • Concurrent processing means real or apparent simultaneous computing (by time sharing) • On a shared network, many applications running on different machines may concurrently exchange messages. The network enforces bandwidth sharing • A terminal server may offer concurrent connections to many clients otherwise its use is severely limited

23. Concurrent Process Creation Example • #include <stdlib.h> • #include <stdio.h> • int sum; • main() { • int i; • sum=0; • fork(); • for (i=1; i<=5; i++) { • printf(“the value of i is %d\n”,i); • fflush(stdout); • sum+=i; • } • printf(“the sum is %d\n”, sum); • exit(0); • }

24. Explanation • When the program reaches the line with statement fork(), the system duplicates the process and allows both the original and duplicate processes to execute forward • The original process is called “parent” and the duplicate process is called “child”

25. Parent-Child Identification • It is easy to identify who is parent and who is child. The value returned by fork() is examined. If it is zero, it is the child else it is the parent • Consider the same program with the identification of parent and child

26. Parent-Child Identification • #include <stdlib.h> • #include <stdio.h> • int sum; • main() { • int i; • sum=0; • if (fork()) printf("This is parent\n"); • else printf("This is child\n"); • for (i=1; i<=5; i++) { • printf(“the value of i is %d\n”,i); • fflush(stdout); • sum+=i; • } • printf(“the sum is %d\n”, sum); • exit(0); • }

27. How to change the C server into concurrent program Ref [3] • Let the server accept an incoming connection and call a function to handle the request. Test your program to make sure it works • Modify the code so that instead of calling the function, the server will spawn a new process with fork() or a new thread using pthreads. The new process or thread would handle the connection and exit. The server should stay in the following infinite loop: • Wait for a connection request • On receiving a new request, spawn a new process or thread to handle the request • Go back to step 1

28. How to write concurrent Java server? Ref[4] • Use a support class called ClientHandler that extends class Thread • Create a new object of this class and pass it the client socket as follows: • ClientHandler Handler = new ClientHandler(client) • Handler.start(); • Obtain and run MultiEchoServer.java and several copies of MultiEchoClient.java

29. HW2: Programming Project • (Note: Design client and server completely either in C or Java. Clients and server may run on the same machine. Submit all the source code.). • UDP client sends a test message containing dummy text and message serial number followed by client ID to the server repeatedly. The number of times this message is sent is specified by the user. The concurrent UDP server receives the message and in response sends out its own small ack. message to the client copying the client ID and client message serial number. The client checks its time before sending the message and after receiving ack. message from the server. Once all messages have been processed, the client reports the AVERAGE approx. round trip time taken over all messages received and the total NUMBER of messages lost. Server must display the received message and client ID from the client AFTER ack. is transmitted. All source code must be submitted. Full test session data should be enclosed that involves at least two clients and one server.

30. Help for C Programming • Sockets Programming Presentation: http://www.cs.fredonia.edu/~zubairi/spring2000/cs435/sockets.pdf • Sockets programming sample code: http://www.cs.fredonia.edu/~zubairi/spring2000/cs435/inettime.c • Sockets programming sample code: http://www.cs.fredonia.edu/~zubairi/spring2000/cs435/simple.zip or http://www.cs.fredonia.edu/~zubairi/spring2000/cs435/simple.tar. • Other help: http://www.cs.fredonia.edu/~arnavut/classes/cs437/socket.doc • You must give explanation for all the major steps in the code using comments..