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Transcript
Practicum: - Client-Server Computing in Java 15-211 Fundamental Data Structures and Algorithms Margaret Reid-Miller 13 April 2004 Reminders HW7 is out! due on Wednesday, April 28, 11:59pm Read: Chapter 10 2 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 … 4 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 5 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 6 Network layers Application Application Protocol Protocol Hardware Hardware network protocol data flow 7 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. 8 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 9 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 10 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 11 Network layers Application Application Protocol Protocol Hardware Hardware network protocol data flow 12 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) 13 Socket communication Sockets provide a bi-directional communication channel from one process to another. process write read Host process network socket read write socket Host Messages are queued • at sending socket until transmitted across the network and • at receiving socket until received by the receiving process. 14 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() 15 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 16 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 17 Client-server interaction Server listenSocket = ServerSocket(port) connectionSocket = listenSocket.accept() read request(s) from connectionSocket write reply(s) to connectionSocket Client TCP session clientSocket = Socket(hostid,port) send request(s) to clientSocket read reply(s) from clientSocket connectionSocket.close() clientSocket.close() 18 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. 19 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. 20 Open Socket Server side: ServerSocket listenSocket = new ServerSocket(portNumber); while (true) { Socket connectionSocket = listenSocket.accept(); … } Client side: Same port number Socket clientSocket = new Socket(hostId, portNumber); 21 Create I/O streams Client and server: E.g., character streams Buffering wrapper for efficiency BufferedReader in = new BufferedReader( new InputStreamReader( theSocket.getInputStream())); Automatically PrintWriter out = flush with println new PrintWriter( theSocket.getOutputStream(), true); 22 Client-Server Communication Client side: out.println(request); reply = in.readLine(); Server side: request = in.readLine(); … reply = … out.println(reply); 23 Cleanup Client and Server: out.close(); in.close(); theSocket.close(); 24 Example: A Pig Latin Server Client-server interaction Server listenSocket = ServerSocket(port) connectionSocket = listenSocket.accept() read request(s) from connectionSocket write reply(s) to connectionSocket Client TCP session clientSocket = Socket(hostid,port) send request(s) to clientSocket read reply(s) from clientSocket connectionSocket.close() clientSocket.close() 26 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 socket output stream PrintWriter out = new PrintWriter( clientSocket.getOutputStream(),true); … 27 Java client, cont’d create socket 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); in.close(); out.close(); clientSocket.close(); 28 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 socket listenSocket.accept(); input stream BufferedReader in = new BufferedReader( new InputStreamReader( connectionSocket.getInputStream())); 29 Java server, cont’d create socket output stream read request from client service the request PrintWriter out = new PrintWriter( connectionSocket.getOutputStream(),true); clientSentence = in.readLine(); String pigLatin = pigTranslate(clientSentence); out.println(pigLatin); send reply to client in.close(); out.close(); connectionSocket.close(); } end of while loop; go back and wait for another request 30 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 31 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) { … } } 32 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 33 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) { … } start main server loop thread for it this.start(); } … 34 Multithreaded server, cont’d run() is invoked by start() accept a connection create a new thread to serve client … public void run() { try { while (true) { Socket connectionSocket = listenSocket.accept(); Connection c = Connection(connectionSocket); … 35