Section 5: More Parallel Algorithms Michelle Kuttel [email protected].
1 CSC111H Client-Server: An Introduction Dennis Burford [email protected].
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Transcript of 1 CSC111H Client-Server: An Introduction Dennis Burford [email protected].
2
Internet Communication
• Communication on the Internet is the transfer of data from one machine to another.– downloading of web pages – email – chat rooms.
• Transfer of this information is unreliable – it can get lost along the way.
• So, data to be transferred is broken into pieces called packets (Don’t put all your eggs in one basket!)
3
Internet Communication
• If a computer is expecting a packet, and it doesn’t arrive, it asks for just that packet to be re-sent (rather than the whole message).
• Also, a packet may be corrupted by noise on the network: We need a way of checking its contents (a checksum).
• A packet may pass through many other machines before it reaches its destination (like the many stops of an airmail letter).
4
Identifying a Machine
• Telling the packets where to go…
• Normal Mail is given a destination in the form of a postal address.
• Similarly, every machine on the Internet is uniquely identified by its IP (Internet Protocol) address.
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Identifying a Machine• IP addresses exist in 2 forms:
– DNS (Domain Name Service) form, e.g.casper2.cs.uct.ac.za
marsite.cs.uct.ac.za (my machine)
scilab.uct.ac.za (the scilab server).
– “Dotted Quad” form.Four numbers separated by dots, e.g.
casper2-> 137.158.128.58
marsite-> 137.158.133.27
scilab-> 137.158.128.57
• Use “ping” on DOS prompt
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TCP and Sockets
• TCP/IP (Transmission Control Protocol/Internet Protocol): a reliable direct connection between 2 programs on different machines.
• A TCP connection between 2 programs is like a phone connection:– data can move reliably in both directions simultaneously– before communication can take place, a connection has to be made
(one must call the other)– both parties need something besides the connection (a telephone: a
device to hear and speak).
• In TCP connections, the “telephone” is called a socket.
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TCP and Sockets
• A socket object is needed to allow programs to communicate across a connection
• In Java, there is an existing class called Socket, defined in java.net.
• The constructor takes 2 arguments: – an IP address (...of the machine being contacted)– a port number (...of the program we want to talk to)
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Port Numbers
• Network programs offering a service can “listen” on a particular port number on a computer. The port identifies which service is involved.
• Some default port numbers:– FTP (data connection) 20– telnet 23– http (WWW protocol) 80
• The combination of a port number and IP address identifies the program (like the number of a room in a building).
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Client-Server
• The client-server model is a particular approach to communicating, used in Internet applications.
• In this model an application consists of 2 programs running:– One program runs as a server (providing a service).
– The other program runs as a client (requesting the service).
• The client requests a service as follows:– Creates a Socket object, with the machine address and
appropriate port number of the server
– Sends a message to the server
– Waits for a response from the server
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Protocols
• Each client-server combination has its own rules on how the conversation should work - a protocol.
• A Protocol can be thought of as the rules of conduct and the format of the communication.
• Real world example:– Ordering a McDonald’s meal (in English)
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Protocols
• Web clients and servers – Protocol called HTTP (Hyper-Text Transport Protocol).
• A client program (such as Internet Explorer) requests an html file from a web server (such as Apache). The command is GET <filename>
• A blank line indicates the end of the request.• The server then sends information about the server and the file (such
as when it was last modified), followed by a blank line.• The server then sends the HTML code to the client, which displays it.• The server breaks the connection when it has sent all the HTML.
• Email clients and servers – Protocol called SMTP (Simple Mail Transfer Protocol).
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Chat Program
Fred
-:
Sally
-:
John
-:
Chat ServerClient Client
Client
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Chat Program
Fred
-:
Sally
-:
John
-:
Chat Server
connect connect
connect
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Chat Program
Fred
-:
Sally
-:
John
-:
Chat Server
15
Chat Program
Fred
-: Hello everyone
Sally
-:
John
-:
Chat Server
16
Chat Program
Fred
-: Hello everyone
Sally
-:
John
-:
Chat Server
17
Chat Program
Fred
-: Hello everyone
Sally
-:
John
-:
Chat Server
18
Chat Program
Fred
-: Hello everyone
Fred> Hello everyone
Fred> Hello everyone
Fred> Hello everyone
Sally
-:
John
-:
Chat Server
19
Chat Program
Fred
-:
Fred> Hello everyone
Fred> Hello everyone
Fred> Hello everyone
Sally
-:
John
-:
Chat Server
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Understanding the Files for the Project
• Most of the java files which you have been given are interfaces.• For example: The ChatProgram interface has one method
specified, called acceptMessage. • Your program will implement ChatProgram, because it has to
be able to accept messages from the server.
public interface ChatProgram
{
public boolean acceptMessage(String msg);
}
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The ChatServer
• The server part of the program has been written for you (ChatServer.java).
• ChatServer implements ChatServerInterface. This means that it has methods called login, logout and putMessage.
• To connect to the server, we use the ConnectionToServer class, which implements ChatClientInterface.
• You are required to write the client-side of the chat program.
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ChatServer Interfacepublic interface ChatServerInterface extends java.rmi.Remote
{
boolean login(String id, ChatClientInterface client)
throws java.rmi.RemoteException;
boolean logout(ChatClientInterface client)
throws java.rmi.RemoteException;
boolean putMessage(String msg, ChatClientInterface client)
throws java.rmi.RemoteException;
}
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ChatClientInterface
public interface ChatClientInterface extends java.rmi.Remote
{
boolean putBroadCastMessage(String msg)
throws java.rmi.RemoteException;
public void addPerson(String msg)
throws java.rmi.RemoteException;
public void removePerson(String msg)
throws java.rmi.RemoteException;
}
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The Client-Side• The client which you write must react to 2 events. Both use a
ConnectionToServer object.• How to create a ConnectionToServer object:
– The constructor takes 3 parameters:hostName (IP number of computer you want to connect to)nickName (name to be used for chatting)program (an instance of ChatProgram which you have written).
• From the server to the client:• The ConnectionToServer object ensures that acceptMessage of
ChatProgram is called whenever something arrives at the server
• From the client to the server:– Invoke the sendMessage method of ConnectionToServer
