In computing, a server is a computer program or a
device that provides functionality for other programs
or devices, called "clients". This architecture is called
the client–server model, and a single overall
computation is distributed across multiple processes
or devices. Servers can provide various functionalities,
often called "services", such as sharing data or resources
among multiple clients, or performing computation for a
client. A single server can serve multiple clients, and a
single client can use multiple servers. A client process
may run on the same device or may connect over a network
to a server on a different device. Typical servers are
database servers, file servers, mail servers, print servers,
web servers, game servers, and application servers.
Client–server systems are today most frequently implemented
by (and often identified with) the request–response model:
a client sends a request to the server, which performs some
action and sends a response back to the client, typically
with a result or acknowledgement. Designating a computer as
"server-class hardware" implies that it is specialized for
running servers on it. This often implies that it is more
powerful and reliable than standard personal computers, but
alternatively, large computing clusters may be composed of
many relatively simple, replaceable server components.
History of Server
The use of the word server in computing comes from queuing theory, where it dates to the mid 20th century, being notably used in Kendall (1953) (along with "service"), the paper that introduced Kendall's notation. In earlier papers, such as the Erlang (1909), more concrete terms such as "[telephone] operators" are used. In computing, "server" dates at least to RFC 5 (1969),  one of the earliest documents describing ARPANET (the predecessor of Internet), and is contrasted with " user", distinguishing two types of host: "server-host" and "user-host". The use of "serving" also dates to early documents, such as RFC 4, contrasting "serving-host" with "using-host". The Jargon File defines "server" in the common sense of a process performing service for requests, usually remote, with the 1981 (1.1.0) version reading: SERVER n. A kind of DAEMON which performs a service for the requester, which often runs on a computer other than the one on which the server runs.
Strictly speaking, the term server refers to a computer program or process (running program). Through metonymy, it refers to a device used to (or a device dedicated to) running one or several server programs. On a network, such a device is called a host. In addition to server, the words serve and service (as noun and as verb) are frequently used, though servicer and servant are not.[a] The word service (noun) may refer to either the abstract form of functionality, e.g. Web service. Alternatively, it may refer to a computer program that turns a computer into a server, e.g. Windows service. Originally used as "servers serve users" (and "users use servers"), in the sense of "obey", today one often says that "servers serve data", in the same sense as "give". For instance, web servers "serve [up] web pages to users" or "service their requests". The server is part of the client–server model; in this model, a server serves data for clients. The nature of communication between a client and server is request and response. This is in contrast with peer-to-peer model in which the relationship is on-demand reciprocation. In principle, any computerized process that can be used or called by another process (particularly remotely, particularly to share a resource) is a server, and the calling process or processes is a client. Thus any general purpose computer connected to a network can host servers. For example, if files on a device are shared by some process, that process is a file server. Similarly, web server software can run on any capable computer, and so a laptop or a personal computer can host a web server. While request–response is the most common client–server design, there are others, such as the publish–subscribe pattern. In the publish–subscribe pattern, clients register with a pub–sub server, subscribing to specified types of messages; this initial registration may be done by request–response. Thereafter, the pub–sub server forwards matching messages to the clients without any further requests: the server pushes messages to the client, rather than the client pulling messages from the server as in request–response.