Universidad de Chile - Tupper 2007, Santiago - Fono/Fax: (56 2) 698 8427 - Email: hthiemer @...

1Universidad de Chile - Tupper 2007, Santiago - Fono/Fax: (56 2) 698 8427 - Email: hthiemer @ cec.uchile.cl

Módulo ECI - 11: Fundamentos de Redes de Computadores

What happens when many clients want to contact the server at the same time ?

The iterative approach consist in “enqueuing” the requests (this is done automatically by the system) and serving them one by one : Accept the next connection and build the socket Read request Process request (send the response)

This will inevitably mean that if there is a client with a small request (for example, a small file request) will have to wait until the large requests are over

If more than the allowed number of clients at socket queue request a service during this time they are rejected !!!

There may be also some delays in attending clients by waiting some information (client are asked to type in something)

There are many Disk I/O operations in a file serving scenario which are normally “slow”and do not require network traffic or CPU

So there is a lot of resources standing idle with this schema



A sequential (iterative) server attending more than a client

A SERVERA CLIENT

A CLIENT

A CLIENT

4444



During the transfer of the file the server cannot hear at the port 4444 for requests

A SERVERA CLIENT

A CLIENT

A CLIENT

4444



Only after the transmition is over the server can attend another request

A SERVERA CLIENT

A CLIENT

A CLIENT

4444



This may involve some delay (typing, transmitting large files)

A SERVERA CLIENT

A CLIENT

A CLIENT

4444



Sometimes a large waiting time for the client may mean a timeot for the connection

A SERVERA CLIENT

A CLIENT

A CLIENT

4444Timeout



The Concurrent Server A concurrent server can attend many clients at the

same time While transferring a file, it can still keep listening

for requests Every time a request comes, a new parallel line of

statements execution is started for attending this request. After this the server can “hear” again at the server socket for further requests

Different approaches have been developed to implement parallel lines of executions within a program

The operative system plays an important role



In concurrent servers there is a main program which listens for client requests

A SERVER

A CLIENT

A CLIENT

A CLIENT

4444



After a client is contacted, a new line of execution is created which will attend the

request in parallel.

A SERVER

A CLIENT

A CLIENT

A CLIENT

4444



The main program will return to the listening loop while the first client is being attended, so

it can accept another request

A SERVER

A CLIENT

A CLIENT

A CLIENT

4444



… and create another parallel line of execution for attending the client

A SERVER

A CLIENT

A CLIENT

A CLIENT

4444



… so a request of a third client can be immediately be accepted

A SERVER

A CLIENT

A CLIENT

A CLIENT

4444



… thus having the three clients being attended in parallel

A SERVER

A CLIENT

A CLIENT

A CLIENT

4444



The Concurrent Server Algorithm Master program of the server

1. Create a server socket 2. Wait for and accept a client’s request

3. Create a parallel line of execution to attend the requirement of the new client4. Goto 2.

Slave (parallel) process :1. Receive the parameters of the communication established by the master (socket or input/output data streams)2. Attend client’s request (read filename, transfer data)3. Return (disappear !)



Parallel Lines of execution If there is only 1 CPU, why create parallel processes?

It often makes server programming easier. Because there are more CPU involved !!!!!

The concept of parallel processes implemented at the S.O level appeared with UNIX and C.

In C a new process can be created by executing the fork() statement

int i = fork() creates an exact copy of the running process. Both continue executing the same program with the same variables.

For the “father” the value of i will be the id of the created process. For the child process this value will be 0

When programming concurrent servers the father will be the main process accepting requests and the child will process the client’s request



A simplified example of the use of for for implementing concurrent servers

main() { int pid, msock, ssock; msock = passivesock(port, “tcp”, qlen); /* see chapter 10.4 from Internetworking with tcp/ip from Douglas

Commer for the implementation */

while(1) { ssock = accept(msock, &fsin, &alen);

pid = fork(); if (pid == 0) { process client’s request; return; } }



Problems with fork() in UNIX The creation of a new process is a time costly procedure

In some texts it is suggested to create a set of processes at the beginning which will be activated later by the main program. When a client arrives the parameters of the communication are passed vi pipes to child.

The new created process duplicates all the variables !!! It is not easy to manage the processes which ended in an

abnormal way. They keep the resources !!!. Sometimes is preferred to use the “select” statement: it

basically selects from an array of sockets the first one which has available data to read.



In JAVA is better to use Threads A thread is a sequence or a flow of instructions which are

executed in parallel to the main sequence of the program. It has a start and an end.

A thread can only be created and lives inside an already running process. A process can start as many threads as necessary.

Because of this, the main program has a better control of the threads it started. They can be created, started, suspended, or reactivated by the program.

Threads are implemented as methods of a certain class. When this method (normally called run) is activated, it starts to run in parallel to the method that called it.

As any other method, it can define its own set of variables



Using threads to implement concurrent servers

Main (master) program:

Create ServerSocket ss;While(true) {

Socket s = ss.accept();Create new thread with the socket s as parameter;start executing thread;

}

Define a thread class with a parallel method (run) which implements the processing of the clients request



Implementation of Threads One way (perhaps the most clear one) is to define a new

class which is an extension of the Thread class and overwrite its run() method. Thread is an existing class This thread class has a run method (originally with no statements)

which will be executed in parallel when activated. In order to execute this method a program must create an object of

the extended class and apply the start() method to it.

The definition of the new Thread class should look like this: public class MyThread extends Thread {

And somewhere should appear: public void run() {

//instructions to be executed in parallel }



Examplepublic class SimpleThread extends Thread { public SimpleThread(String str) { super(str); }

public void run() { for (int i = 0; i < 10; i++) { System.out.println(i + " " + getName()); try { this.sleep((int)(Math.random() * 1000)); } catch (InterruptedException e) {} } System.out.println("DONE! " + getName()); } } The this.sleep(miliseconds) must appear in a try and catch

block



Use of this new classpublic class TwoThreadsTest { public static void main (String[] args) { SimpleThread t1,t2;

t1 = SimpleThread("Jamaica"); t2 = SimpleThread("Fiji");

t1.start(); t2.start() } } The start() triggers the execution of the run

method of the thread. There are other methods which can be applied to a thread: suspend(), resume(), stop(). These are however deprecated because their use can easily lead to deadlocks



Sometimes you cannot program the server as an extension of a thread

For example, if the server has to implement a graphical interface it should be declared as an extension of a Frame, if it is programmed as an Applet it must extend the Applet class

The we can use the interface Runnable, which means the Server will also be seen as a Runnable class and has to implement the run method run().

To initiate a parallel execution of the run method a Thread object must be created. In the creation a pointer to the server object (this) should be passed as parameter. With this, the Thread object will have access to the run method.

The run method implemented in the server class will be executed by performing start() on the new created Thread object.



Example with interface Runnable

See and execute the program NoSincron.java

Note that the Threads created in this way will have access to all resources of the master.

With the other form of creating threads, resources from the master object can be made available by passing a point to itself on creation of the thread



Critical regions and semaphores in Java

Java provides basically two methods for controlling concurrent access to resourcesYou can declare a whole method as a critical region

(see sincron1)You can use the semaphore of any object (see

sincron2)Another way to use semaphores in java is with wait()

and notify()



Implementing a concurrent file server (as an extension of Thread)

Define a new Class as an extension of the Thread class which has a socket as Object variable.

Program a constructor which receives a socket as parameter. Program the run methods to attend the client connected at the other

extreme of the socket. Program a main method which consists of an infinite while. Inside the

while program the acceptance of a new client, the creation of a new thread with the socket obtained, and the starting of the thread execution.

The client does not change, in fact, it does not know if it is being served concurrently or in parallel

See: MultiArchServidorThread.java &MultiArchServidor.java



Broadcasting a text to many clients

Hello

Hello

Hello

Hello

Hello

Hello



The server: receiving a new client

4444

Client contacts server at 4444



The server: receiving a new client

4444

A new socket is placed and the output channel is opened They are kept in an array



The server: Broadcasting a message

When a message is entered the server will distribute it to the connected clients

BroadcastCliente

BraodcastServerNF

Type a message



It is now easy to extend this to a chat system

Hello

Hello

Hello

Hello

Hello



Conditions for implementing a chat system

Server must be listening to requests of new clients AND to messages which are sent by already connected

Client must be listening to messages from the server AND to the keyboard for messages the user wants to transmit.

There are many approaches for implementing this in TCP/IP

No one is the “absolute correct” solution, all them have their advantages and drawbacks

Normally (like everywhere in computers) “faster” solutions will use more memory



Solution 1 The server:

1. A thread listens to new clients trying to join the chat party

2. When a new clients connects, a PrintWriter and a BufferedReader for that client are created. The print Writer is kept in a vector

3. A thread is created and receives a pointer to the PrintWriters’ vector and the BufferedReader. It reads the input from the client and writes it to all PrintWriters (clients)

The client 1. Graphical interface for reading lines from the keyboard (there is

a thread which triggers the execution of the actionPerformed method) and sends it to the server

2. A thread will read input from the server and display it on a text area

See: Chat1Server.java, Chat1ClientThread.java & Chat1Client


Módulo ECI - 11: Fundamentos de Redes de ComputadoresSolution 1 Schema

Client1

Client2

Client3

New Client

Thread

PrintWriter

BufferedReader



Solution 2 The server:1. The server has only one thread listening at one port for attending

all the requests of the clients.

2. The clients contact the server for registering (reg), sending a message (msg) or logging out (des)

3. The server keeps a PrintWiter vector to keep track about which clients participate in the chat party. Additionally, it keeps a vector with the nicknames of the clients

The client 1. Graphical interface for reading lines from the keyboard (there is

a thread which triggers the execution of the actionPerformed method) and sends it to the server. It also shows the nicknames of the participants

2. A thread will read input from the server. This can be a message (msg) or a refreshment of the login list

See: CICChat.java & CICServer.java


Módulo ECI - 11: Fundamentos de Redes de ComputadoresSolution 2 Schema

Client1

Client2

New Client


Módulo ECI - 11: Fundamentos de Redes de ComputadoresSolution 2

Client1

Client2

Client3



Peer-to-peer solution for TCP/IP

Every program should be client and server at the same time

When a new member wants to join the party, he/she shoud contact anyone of the group and ask for the list of contacts

After retrieving the list of contacts (hostnames or addresses) he/she should open an input and output channel with everyone (including the one who provided the list)



Peer-to-peer solution: a user starts a chat party by listening on a port for others wanting to join



A user wanting to join contacts the initiator. An InputStream and an OutputStream is opened on each



A third user may contact anyone and recover the list of participants, in this case each one has the address of the other

in the list



Then contacts all of them opening Input/Output sreams. All them now have two entries on their participant list



Particularities of this implementation The program is written in the ChatPtP.java file In order to allow testing in one computer, it is necessary to give the

port number at which the program will listen for newcomers java ChatPtP localport starts a new Chat party java ChatPtP remotehost remoteport localport joins an existing

party (on remotehost at remoteport a program is waiting for new members)

The is one thread for listening to newcomers Another listens for input on keyboard and sends it to all connected

participants There is one thread listening for input for each of the other

participants Not very easy, isn’t it ?



Problems of this implementation

What happens if one user leaves the chat party ? What happens if during the second connection

phase (retrieving the list and contacting the participants) another newcomer joins the party by asking a third one for the list ?

This is a very critical problem which has been more or less addressed in some (sometimes hard to understand) papers.



Tarea 1: ftp client & server (completar ArchClientRobust & ArchServerRobust)

El cliente puede bajar o subir archivosPUT filenameGET filenameDEL filename

Pedir la lista de archivos disponiblesDIR

Un plus: cambiar de directorio



Tarea 2: Completar el Chat Peer-to-peer TCP/IP (ChatPtP.java)

El programa debe tener una interfaz gráfica Esta interfaz debe mostrar los participantes

(nicknames) Un participante debe despedirse del grupo

antes de abandonar la sesión (esto no solucionara todos los problemas pero ayuda)



A concurrent web server This will be implemented only for HTML files and classes (like servlets),

but it is easily extensible for attending different types of requirements For each client a new thread is created The process is according to the type of the request

HttpProcessorprocessRequest()

HttpFilebrowser

Httpd (server)

thread

HttpInputStream

HttpOutputStream

HttpClass

HttpException

Echo

HttpClassProcessor



Implementing state in a web server

The server must keep tracking of the different users that contact it

The client must submit this information The Echo2 class keeps track of a

“shopping cart” for different users The client must submit a request like:

user=username&product=productcode&qtty=number



Transmitting Objects via TCP

Transmission: marshalling, delivery & unmarshalling.

The key for this is the Object Serialization: convert the into a representation which can be transmitted across the net (String)

All native Java Objects are serializables. For user defined objects it is necessary to write

implements Serializable in their definition (this does not include static variables or references to local things such as files or sockets)



Transmitting Objects via TCP

The classes which allows the transmit:ObjectInputStream readObjetct()ObjectOutputStream writeObject()

The user can change the “standard” serialization mechanism by declaring implements Externalizable

This means, the user must implement Void writeExternal(ObjectOutputStram o)Void readExternal(ObjectInputStream i);

Universidad de Chile - Tupper 2007, Santiago - Fono/Fax: (56 2) 698 8427 - Email: hthiemer @...

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