Un
SERVER è un computer a disposizione di tutto il suo popolo dei CLIENT,
sempre pronto a soddisfare ogni richiesta per cui è stato predisposto. Esso fornisce informazioni su richiesta, comunicandole in modo rigidamente formalizzato.
In pratica un server permette ad un
client (il vostro browser: Google Chrome, Firefox, Safari, Opera o altro)
di collegarsi mediante la porta virtuale e di
richiedere informazioni seguendo una precisa disciplina.
Immaginate
un funzionario presente all'interno di un ufficio aperto al pubblico.
Immaginate anche che questo funzionario sia espertissimo, solerte,
democratico, ma rigido nelle applicare le regole e completamente ottuso a
ciò che non conosce. ... questo è un SERVER. Se voi avete bisogno di
comunicare con quel funzionario, dovete rispettare le sue regole e i suoi
tempi ... altrimenti non vi capisce e vi manda al diavolo!
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 spiegato tecnicamente 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.
