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.
The use of the word server in computing comes from queueing theory, where it dates to the mid 20th century, being notably used in Kendall (1953) (along with service), the paper that introduced Kendalls 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.
Strictly speaking, the term server refers to a computer program or process (running program). Through metonymy, it refers to a device used for (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.
Application server : Hosts web apps (computer programs that run inside a web browser) allowing users in the network to run and use them, without having to install a copy on their own computers. Unlike what the name might imply, these servers need not be part of the world wide web; any local network would do.
Catalog server: Maintains an index or table of contents of information that can be found across a large distributed network, such as computers, users, files shared on file servers, and web apps. Directory servers and name servers are examples of catalog servers.
Communications server: Maintains an environment needed for one communication endpoint (user or devices) to find other endpoints and communicate with them. It may or may not include a directory of communication endpoints and a presence detection service, depending on the openness and security parameters of the network
Computing server:Shares vast amounts of computing resources, especially CPU and random-access memory, over a network.
Database server:Maintains and shares any form of database (organized collections of data with predefined properties that may be displayed in a table) over a network.
Fax server: Shares one or more fax machines over a network, thus eliminating the hassle of physical access
File server: Shares files and folder, storage space to hold files and folders, or both, over a network
Game server: Enables several computers or gaming devices to play multiplayer games
Mail server:Makes email communication possible in the same way that a post office makes snail mail communication possible
Media server: Shares digital video or digital audio over a network through media streaming (transmitting content in a way that portions received can be watched or listened as they arrive, as opposed downloading a whole huge file and then using it)
Print server: Shares one or more printers over a network, thus eliminating the hassle of physical access
Sound server:Enables computer programs of a computer to play sound and record sound, individually or cooperatively
Proxy server: Acts as an intermediary between a client and a server, accepting incoming traffic from the client and sending it to the server. Reasons for doing so includes content control and filtering, improving traffic performance, preventing unauthorized network access or simply routing the traffic over a large and complex network.
Web server: Hosts web pages. A web server is what makes world wide web possible. Each website has one or more web servers.
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