1. Practicum: - Client-Server Computing in Java 15-211 Fundamental Data Structures and Algorithms Margaret Reid-Miller 13 April 2004

2. 2 Reminders  HW7 is out!  due on Wednesday, April 28, 11:59pm  Read:  Chapter 10

3. Intro to Distributed Computing Concepts

4. 4 Distributed computing  Many applications involve coordinated computation by multiple host computers World-Wide Web Internet Chess Club Andrew File System E-Mail services X Windows …

5. 5 The client-server paradigm  Almost all modern distributed computing applications are organized around the client-server paradigm  Client  initiates communication  sends requests and receives responses  interacts with one (or a small number) of servers at a time  Server  waits and listens for incoming requests  receives requests and sends responses  is “always” running  interacts with many clients concurrently

6. 6 Network communication  Network communication is organized into layers  hardware layer  network interface device, connected to a local area network, which in turn is connected to a (packet switched) Internet  protocol layer(s)  basic data transport mechanisms, providing addressing (eg, IP addresses), fragmention/reassembly, reliable transmission  application layer  client and server functionality  Unconcerned with data movement across network

7. 7 Network layers Application Protocol Hardware Application Protocol Hardware network protocol data flow

8. 8 Network layering  Decomposes network communication into simpler tasks.  Each layer provides information hiding from layers above and below.  The data flow is from the application level down the hierarchy, across the network, and back up the hierarchy to the peer application.  Appears as though each layer communicates with its peer at the same layer.

9. 9 Hardware layer  Typically Ethernet-based  Data is transmitted in small packets  typically less than 1500 bytes  packets are easily lost, and often arrive in unpredictable order  Each packet contains routing information  A network device watches for packets that are addressed to itself, ignores the rest  Routers look for packets that are not addressed to local hosts, and forwards them to a non-local network router

10. 10 Protocol layer  There are two main protocols used  both provide Internet addressing/routing  TCP  Transmission Control Protocol  connection (“session”) oriented  provides long data messages (via fragmentation and reassembly of packets)  provides reliable communication  UDP  Unreliable Datagram Protocol  not connection oriented  no transmission guarantees, but very lightweight and efficient

11. 11 Application layer  The hardware and protocol layers are studied in 15-441  Here, we will focus on the application level  Most networking applications use a particular data structure, the socket  A socket provides a high-level abstract interface to a lower-level network protocol service  In Java, sockets are similar in some respects to input/output streams

12. 12 Network layers Application Protocol Hardware Application Protocol Hardware network protocol data flow

13. 13 Sockets  Sockets have a long history in network and operating system design  first appeared in BSD 4.1 Unix in 1981  Socket characteristics  applications explicitly create, use, and destroy sockets  distinction between client sockets and server sockets  different kinds of sockets, depending on the transport protocol (TCP vs UDP)

14. 14 Socket communication process write read socket process read write socket network Host Sockets provide a bi-directional communication channel from one process to another. Messages are queued at sending socket until transmitted across the network and at receiving socket until received by the receiving process.

15. 15 Sockets in Java  Java provides very good support for sockets in the java.net.* package  java.net.Socket  create: constructor methods (to create sockets)  I/O: getOutputStream(), getInputStream()  destroy: close()  java.net.ServerSocket  create: constructor methods  wait and listen: accept()  destroy: close()

16. 16 Socket programming with TCP  Client must contact the server  server must first be waiting and listening  server must thus have created a socket that accepts client connection request  Client contacts server by:  creating its own client TCP socket  uses IP address and port number of server

17. 17 Socket programming, cont’d  When client creates its socket, a TCP session with the server’s TCP is established  On the server side:  when contacted by the client, the server TCP creates a new socket for communication with the client  thus, each client session gets its own private socket on the server

18. 18 Client-server interaction listenSocket = ServerSocket(port) connectionSocket = listenSocket.accept() read request(s) from connectionSocket write reply(s) to connectionSocket connectionSocket.close() clientSocket = Socket(hostid,port) send request(s) to clientSocket read reply(s) from clientSocket clientSocket.close() Server Client TCP session

19. 19 Client Steps 1.Open a socket. 2.Create input and output streams. 3.Send request(s) and receive reply(s) according to the server application protocol. 4.Close streams and socket.

20. 20 Server steps 1.Create a server socket to listen for connection requests. 2.Open a socket to communicate with client. 3.Open input and output streams. 4.Accept requests and send replies. 5.Close streams and socket.

21. 21 Open Socket  Server side: ServerSocket listenSocket = new ServerSocket(portNumber); while (true) { Socket connectionSocket = listenSocket.accept(); … }  Client side: Socket clientSocket = new Socket(hostId, portNumber); Same port number

22. 22 Create I/O streams  Client and server: E.g., character streams BufferedReader in = new BufferedReader( new InputStreamReader( theSocket.getInputStream())); PrintWriter out = new PrintWriter( theSocket.getOutputStream(), true); Buffering wrapper for efficiency Automatically flush with println

23. 23 Client-Server Communication  Client side: out.println(request); reply = in.readLine();  Server side: request = in.readLine(); … reply = … out.println(reply);

24. 24 Cleanup  Client and Server: out.close(); in.close(); theSocket.close();

25. Example: A Pig Latin Server

26. 26 Client-server interaction listenSocket = ServerSocket(port) connectionSocket = listenSocket.accept() read request(s) from connectionSocket write reply(s) to connectionSocket connectionSocket.close() clientSocket = Socket(hostid,port) send request(s) to clientSocket read reply(s) from clientSocket clientSocket.close() Server Client TCP session

27. 27 Example: Java client import java.io.*; import java.net.* public class Client { public static void main (String argv[]) throws Exception { BufferedReader user = new BufferedReader(…); Socket clientSocket = new Socket(“foo.cs.cmu.edu”, 6789); PrintWriter out = new PrintWriter( clientSocket.getOutputStream(),true); … create user input stream connect to the server create socket output stream

28. 28 Java client, cont’d … BufferedReader in = new BufferedReader( new InputStreamReader( clientSocket.getInputStream())); String sentence = user.readLine(); out.println(sentence); String pigLatin = in.readLine(); System.out.println(“Server says:” + pigLatin); in.close(); out.close(); clientSocket.close(); create socket input stream send request to the server read reply from server release the connection

29. 29 Example: Java server import java.io.*; import java.net.* public class Server { public static void main (String argv[]) throws Exception { ServerSocket listenSocket = new ServerSocket(6789); while (true) { Socket connectionSocket = listenSocket.accept(); BufferedReader in = new BufferedReader( new InputStreamReader( connectionSocket.getInputStream())); create listening socket wait for client contact create socket input stream on host foo.cs.cmu.edu:

30. 30 Java server, cont’d PrintWriter out = new PrintWriter( connectionSocket.getOutputStream(),true); clientSentence = in.readLine(); String pigLatin = pigTranslate(clientSentence); out.println(pigLatin); in.close(); out.close(); connectionSocket.close(); } create socket output stream read request from client service the request send reply to client end of while loop; go back and wait for another request

31. 31 Practical issue: cleaning up  Closing connections  It is important to close connections  usually a strict limit on the number of open connections  This means it is very important to handle exceptions, in case the socket creation or I/O fail  exception handler should close any open connections

32. 32 Exception handling … try { Socket clientSocket = new Socket(“foo.cs.cmu.edu”, 6789); … } catch (UnknownHostException e) { System.err.println( “Couldn’t find the server host!”); } catch (IOException e) { System.err.println( “I/O error!”); } finally { try { if (clientSocket != null) { out.close(); in.close(); clientSocket.close(); } } catch (IOException e) { … } } this is always executed, no matter what

33. 33 Practical issue: concurrency  A TCP server usually must be designed to handle multiple clients concurrently  This means that the server should be set up so that the multiple copies of the main server loop can be running at the same time  Java provides a mechanism for such separate “threads” of control, in java.lang.Thread

34. 34 Example: multithreaded server import java.io.*; import java.net.* public class Server extends Thread { ServerSocket listenSocket; public Server () { try { listenSocket = new ServerSocket(6789); } catch (IOException e) { … } this.start(); } … create listening socket start main server loop thread for it on host foo.cs.cmu.edu:

35. 35 Multithreaded server, cont’d … public void run() { try { while (true) { Socket connectionSocket = listenSocket.accept(); Connection c = Connection(connectionSocket); … run() is invoked by start() accept a connection create a new thread to serve client