Systems on the network can
usually be described as one of the following:
Server - A system that provides services to other systems in its network. There
are file servers, boot servers, database servers, license servers, print
servers, installation servers, and even servers for particular applications.
This chapter uses the term server to mean a system that provides file systems
and installation software for other systems on the network.
Client - A system that uses remote services from a server. Some clients have
limited disk storage capacity, or perhaps none at all, and they have to rely on
remote file systems from a server to function.
Other clients might use remote services (such as installation software) from a
server, but they don't rely on a server to function. A standalone system, which
has its own hard disk containing the root (/), /usr, and /export/home file
systems and swap space, is a good example of this type of client.
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.
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.