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Client/Server Middleware

RPC, Messaging, Peer to Peer

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RPC, Messaging, Peer to Peer

How do clients and servers talk to each

other

How are the request and response

synchronized

How are dissimilar data representation on

different computer handled

How if one of the parties are not available

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Peer to Peer communication

Early client server application were implementedusing low level, conversational, peer to peerprotocols

- sockets

- TLI

- CPIC/APPC

- Net Bios

- Named Pipes

P2P protocols are hard to code and maintain

P2P means that 2 sides of communication link usethe same protocol interface to conductconversation

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Peer to peer communication

Not popular - solution

Provide higher level of abstraction

RPC, MOMs, ORBs

Use by very demanding user app/system-levelsystem

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What is a socket?

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Original idea came from UNIX

The network is just like a file

system

Read and write a stream of data tothe network through socket

A socket is bound to aport numberso

that the TCP layer can identify the

correct application for the data

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What is a socket?

A little background

Socket provides the TCP/IP communication protocol

Introduced in 1981- UNIX BSD 4.2

Later Sun build RPC and NFS over the socket

Is supported by all OS

WinSocks for Windows 3.1

Built into Windows 95, and NT For other OS it is not new

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Communication between processes :

using port and socket

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message

agreed portany port

socketsocket

Internet address = 138.37.88.249Internet address = 138.37.94.248

other portsclient server

Port: The destination of a message

Socket: The final point for processes communication

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ports in TCP are also represented by a number in the

range 1 65 535

Ports below 1024 are restricted to use by well-known

services. For example, Telnet (port 23)

SMTP (port 25)

HTTP (port 80)

POP3 (port 110)

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9

Sockets

Figure A

Figure B

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Socket Operations

TCP sockets can perform a variety of

operations:

Establish a connection to a remote host

Send data to a remote host

Receive data from a remote host

Close a connection

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Socket Operations

There is a special type of socket that provides

a service that will bind to a specific port

number. Normally used only in servers, this

socket can perform the following operations:

Bind to a local port

Accept incoming connections from remote hosts

Unbind from a local port

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TCP and the Client/Server Paradigm

In network programming, applications that

use sockets are divided into clients and

servers.

A client is software that initiates a connection

and sends requests.

A server is software that listens for

connections and processes requests.

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Network Clients

Network clients initiate connections and controlnetwork transactions.

The server fulfills the requests of the client but

not the other way round. The network client speaks to the server using a

network protocol. E.g an HTTP clientcommunicates with an HTTP server using HTTP.

Port numbers are used to enable clients to locateserver applications. E.g. a web server uses port80.

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Network Servers

The role of the network server is to bind to a specific

port and to listen for new connections.

Unlike the client, the server must run continually in

the hope that some client will want its services. The server runs indefinitely. Normally, it is

automatically started when the host computer of the

server is started.

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Network Servers

Some servers can handle only one connection at a

time, while others can handle many connections

concurrently, through the use ofthreads.

Some protocols (e.g. HTTP/1.0) normally allow onlyone request per connection. Others, like POP3,

support a sequence of requests.

Servers answer the client request by sending either a

response or an error message.

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Socket Types

In Java, there are 4 main socket types:

ServerSocket

Socket

DatagramSocket

MulticastSocket

Accessible by importing java.net.*;

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TCP Sockets and Java

Java provide the following classes for TCPsockets:

java.net.Socket

java.net.ServerSocket

The Socket class should be used whenwriting client software.

The ServerSocket class should be usedwhen writing server software.

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Socket (client) Textbook page 273

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Socket Class

Socket objects represent client sockets, and

is a communication channel between two TCP

communications ports belonging to one or

two machines.

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Socket Class

There are several constructors for the Socket

class.

The easiest way to create a socket is shown

below:Socket mySocket;

try {

mySocket = new Socket("www.aol.com", 80);

} catch (Exception e) {

}

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http://www.aol.com/http://www.aol.com/

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Reading from and Writing to TCP Sockets

In Java, once a socket is created, it is

connected and ready to read/write by using

the socket's input and output streams. Use the methods getInputStream() and

getOutputStream() to access those streams.

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Socket socket;InputStreamReader isr;

BufferedReader br;

PrintStream ps;

try {

socket = new Socket("www.aol.com",80);

isr = new InputStreamReader(socket.getInputStream())

br = new BufferedReader(isr);

ps = new PrintStream(socket.getOutputStream());


}

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ServerSockets(server) Textbook page 311


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Server Socket

Server Socket runs on the server and listens forincoming TCP connections.

Server socket is bound to a certain port on the servermachine.

When it is successfully bound to a port, it immediatelylistens to any attempt of incoming connection

When a server detects an attempt of incomingconnection, it negotiates the connection between client

and server. This creates/opens a regular Socket which handles

communication between the client and server

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ServerSocket Class

java.net.ServerSocket represents the serversocket

ServerSocket object is created on a local port

and calls method accept() to listen toincoming connection

accept() will block until a connection is

detected. Then it returns a Socket object thathandles communication with a client

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The easiest way to create a socket to listen at

a certain port is shown below:ServerSocket mySocket;

try {

mySocket = new ServerSocket(80);


}

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Accepting and Processing Requests from

TCP Clients

The most important function of a server socket is to

accept client sockets. Once a client socket is obtained,

the server can perform all the "real work" of server

programming, which involves reading from andwriting to the socket to implement a network

protocol.

Example: a mail server that provides access to stored

messages would listen to commands and send backmessage contents.

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ServerSocket server;

BufferedReader reader;

PrintWriter writer;

server = new ServerSocket(13);

while (true) {

Socket client = server.accept();

reader = new BufferedReader(new InputStreamReader(

client.getInputStream()));

writer = new PrintWriter(

new OutputStreamWriter(

client.getOutputStream()));

}

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Take a break!


What are threads?

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Ch03 Socket