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Practicum: - Client-Server Computing in Java 15-211 Fundamental Data Structures and Algorithms Peter Lee April 8, 2004 Reminders HW7 is out! due on Wednesday, April 28, 11:59pm Read: Chapter 10 Next time: Guest lecture by Andreas Nowatzyk (Deep Thought) Recap: Learning to Play Games Game playing strategies Last time we were introduced to minimax search as an approach to two-player games For interesting games, it is very important to prune the search tree We saw the alpha/beta pruning technique could be very useful AI But the quality of the evaluation function and the search order is also of critical importance So this leaves us with the question: How do we come up with a good evaluation function and a good search order? These are fundamental questions in Artificial Intelligence Learning We can, of course, be as thoughtful and clever as possible, analyze the game carefully, and design an evaluation function accordingly But this is often difficult Another approach is to arrange for our game playing program to learn Many approaches… Indeed, machine learning is a very big and important topic in CS take 15-381, 15-681, etc many approaches, some of them highly mathematical or statistical Samuels checkers program An early 60’s checkers program by Samuels learned by playing games against good human players It updated its evaluation function whenever it won a game, and ignored losing games In time, it became the best checkers player in major tournaments Learning from humans or computers? While Samuel’s program was very successful, it also had some drawbacks It would actually get worse when playing poor players Humans play very slowly, and so this limited the pace of learning State-of-the-art: Backgammon Gerald Tesauro (IBM) Wrote a program which became “overnight” the best player in the world Not easy! State-of-the-art: Backgammon Learned the evaluation function by playing 1,500,000 games against itself Temporal credit assignment using reinforcement learning Used Neural Network to learn the evaluation function c 8 a 9 d 2 5 b 7 State-of-the-art: Go Average branching factor 360 Regular search methods go bust ! People use higher level strategies Systems use vast knowledge bases of rules… some hope, but still play poorly $2,000,000 for first program to defeat a top-level player Today’s Topic: Intro to Distributed Computing Concepts 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 … 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 interacts with many clients concurrently 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 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 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 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 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) 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() 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 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 Client-server interaction Server Client listenSocket = ServerSocket(port) connectionSocket = listenSocket.accept() read request(s) from connectionSocket write reply(s) to connectionSocket clientSocket = Socket(hostid,port) send request(s) to clientSocket read reply(s) from clientSocket connectionSocket.close() clientSocket.close() Example: A Pig Latin Server Example: Java client import java.io.*; import java.net.* create user input stream connect to the server public class Client { public static void main (String argv[]) throws Exception { BufferedReader user = new BufferedReader(…); Socket clientSocket = new Socket(“foo.cs.cmu.edu”, 6789); create output stream PrintWriter out = new PrintWriter( clientSocket.getOutputStream(),true); … Java client, cont’d create input stream send request to the server read reply from server release the connection … 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); clientSocket.close(); Example: Java server on host foo.cs.cmu.edu: import java.io.*; import java.net.* public class Server { create listening public static void main (String argv[]) throws Exception { socket ServerSocket listenSocket = new ServerSocket(6789); wait for client contact while (true) { Socket connectionSocket = create input listenSocket.accept(); stream BufferedReader in = new BufferedReader( new InputStreamReader( connectionSocket.getInputStream())); Java server, cont’d create output stream PrintWriter out = new PrintWriter( read request connectionSocket.getOutputStream(),true); from client service the request clientSentence = in.readLine(); String pigLatin = pigTranslate(clientSentence); send reply to client out.println(pigLatin); connectionSocket.close(); } end of while loop; go back and wait for another request 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 Exception handling this is always executed, no matter what … 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) { … } } Practical issue: concurrency The 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 Example: multithreaded server on host foo.cs.cmu.edu: import java.io.*; import java.net.* create listening socket public class Server extends Thread { ServerSocket listenSocket; public Server () { try { listenSocket = new ServerSocket(6789); } catch (IOException e) { … } when client contacts us, start a server loop thread for it this.start(); } … Multithreaded server, cont’d run() is invoked by start(), after a new thread is created … public void run() { try { while (true) { Socket connectionSocket = listenSocket.accept(); …