Practicum:- Client-Server Computing in Java

15-211 Fundamental Data Structures and Algorithms

Margaret Reid-Miller

13 April 2004

2

Reminders

HW7 is out!

due on Wednesday, April 28, 11:59pm

Read:

Chapter 10

Intro toDistributed Computing

Concepts

4

Distributed computing

Many applications involve coordinated computation by multiple host computers

World-Wide WebInternet Chess ClubAndrew File SystemE-Mail servicesX Windows…

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The client-server paradigm

Almost all modern distributed computing applications are organized around the client-server paradigm

Client initiates communicationsends requests and receives responses interacts with one (or a small number) of

servers at a time

Serverwaits and listens for incoming requests receives requests and sends responses is “always” running interacts with many clients concurrently

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Network communication

Network communication is organized into layershardware 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

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Network layers

Application

Protocol

Hardware

Application

Protocol

Hardware

network

protocol

data flow

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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.

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Hardware layer

Typically Ethernet-based

Data is transmitted in small packets typically less than 1500 bytespackets 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

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Protocol layer

There are two main protocols usedboth provide Internet addressing/routing

TCPTransmission Control Protocolconnection (“session”) orientedprovides long data messages (via

fragmentation and reassembly of packets)provides reliable communication

UDPUnreliable Datagram Protocolnot connection orientedno transmission guarantees, but very

lightweight and efficient

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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

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Network layers

Application

Protocol

Hardware

Application

Protocol

Hardware

network

protocol

data flow

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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)

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Socket communication

process

writeread socket

process

read writesocket

network

Host 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.

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Sockets in Java

Java provides very good support for sockets in the java.net.* package

java.net.Socketcreate: constructor methods (to create sockets) I/O: getOutputStream(), getInputStream()destroy: close()

java.net.ServerSocketcreate: constructor methodswait and listen: accept()destroy: close()

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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

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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

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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

TCPsession

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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.

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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.

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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

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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

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Client-Server Communication

Client side:

out.println(request);

reply = in.readLine();

Server side:

request = in.readLine();…reply = …out.println(reply);

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Cleanup

Client and Server:

out.close(); in.close(); theSocket.close();

Example:A Pig Latin Server

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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

TCPsession

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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

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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

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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:

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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

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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

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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

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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

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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:

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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