mercoledì 11 giugno 2014

Sei un Server o un Client?








Un SERVER WEB è un servizio (e quindi un’applicazione) in esecuzione su un computer host (server) in attesa di connessioni con altri (client). 

Tale servizio si compie nel fornire una serie di informazioni codificate secondo uno specifico protocollo o linguaggio. 

L'applicazione, essendo un programma, ha bisogno di un computer che, se fosse dedicato, risponderebbe al meglio alla sua funzione. Pertanto, ogni computer in rete (HOST), in base alla funzione svolta in una fase della sua attività, potrebbe essere un SERVER o un CLIENT.

La funzione a cui è chiamato un computer, nell'ottica della logica standard delle reti, assegna la qualifica di SERVER o CLIENT. 

In pratica un server web permette ad un client (il vostro browser: Internet Explorer, Firefox, Safari, Opera o altro) di collegarsi mediante la porta 80 (specifica del protocollo TCP/IP) e di richiedere informazioni secondo un determinato protocollo.

Il server Web, alla richiesta, fornisce le informazioni codificate mediante un determinato linguaggio (HTML, XML, XHTML).

Nella vita quotidiana anche noi umani ci alterniamo nei ruoli di client e server.
Nel mondo dei computer questo modo di fare permette loro di comunicare con disciplina e di utilizzare al meglio le loro risorse.

Le macchine non si stancano, non vanno sotto stress; attendono soltanto che l'usura completi il ciclo di vita.

Per gli umani, esistono le strutture delle relazioni interpersonali, complicate al massimo, che rendono instabile, incerta, precaria, la funzione di ogni singolo individuo nella rete mondiale formata da oltre sette miliardi di host.   

Per gli amanti della lingua inglese (come me), ecco lo stesso argomento in inglese:


Client/server describes the relationship between two computer programs in which one program, the client, makes a service request from another program, the server, which fulfills the request. Although the client/server idea can be used by programs within a single computer, it is a more important idea in a network. In a network, the client/server model provides a convenient way to interconnect programs that are distributed efficiently across different locations. 

Computer transactions using the client/server model are very common. For example, to check your bank account from your computer, a client program in your computer forwards your request to a server program at the bank. That program may in turn forward the request to its own client program that sends a request to a database server at another bank computer to retrieve your account balance. 

The balance is returned back to the bank data client, which in turn serves it back to the client in your personal computer, which displays the information for you.
The client/server model has become one of the central ideas of network computing. Most business applications being written today use the client/server model. So does the Internet's main program, TCP/IP.

In marketing, the term has been used to distinguish distributed computing by smaller dispersed computers from the "monolithic" centralized computing of mainframe computers. But this distinction has largely disappeared as mainframes and their applications have also turned to the client/server model and become part of network computing. 

In the usual client/server model, one server, sometimes called a daemon is activated and awaits client requests. Typically, multiple client programs share the services of a common server program. Both client programs and server programs are often part of a larger program or application. Relative to the Internet, your Web browser is a client program that requests services (the sending of Web pages or files) from a Web server (which technically is called a Hypertext Transport Protocol or HTTP server) in another computer somewhere on the Internet. 

Similarly, your computer with TCP/IP installed allows you to make client requests for files from File Transfer Protocol (FTP) servers in other computers on the Internet.
Other program relationship models included master/slave, with one program being in charge of all other programs, and peer-to-peer, with either of two programs able to initiate a transaction.




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