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'''Server''' is a [[computer]] that provides information to other computers called "[[Client (computing)|clients]]" on [[computer network]].<ref name="Cisco Networking Academy x508">{{cite web | title=1.1.2.2 Clients and Servers | website=Cisco Networking Academy | url=https://web.archive.org/web/20240407111300/http://cisco.num.edu.mn/CCNA_R&S1/course/module1/1.1.2.2/1.1.2.2.html | quote=Servers are hosts that have software installed that enable them to provide information...Clients are computer hosts that have software installed that enable them to request and display the information obtained from the server. | access-date=2024-04-07}}</ref> This [[Systems architecture|architecture]] is called the [[client–server model]]. Servers can provide various functionalities, often called "services", such as sharing data or [[System resource|resources]] among multiple clients or performing [[computation]]s 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.<ref>{{cite book |title = Windows Server Administration Fundamentals |url = https://archive.org/details/windowsserveradm00cour |url-access = limited |publisher = [[John Wiley & Sons]] |___location = Hoboken, NJ |series = Microsoft Official Academic Course |year = 2011 |pages = [https://archive.org/details/windowsserveradm00cour/page/n21 2]–3 |isbn = 978-0-470-90182-3}}</ref> Typical servers are [[database server]]s, [[file server]]s, [[mail server]]s, [[print server]]s, [[web server]]s, [[game server]]s, and [[application server]]s.<ref>{{cite book|last1 = Comer |first1 = Douglas E. |last2 = Stevens |first2 = David L |title = Vol III: Client-Server Programming and Applications |publisher = [[Prentice Hall]] |___location = West Lafayette, IN |series = Internetworking with TCP/IP |year = 1993 |pages = 11d |isbn = 978-0-13-474222-9 |url-access = registration |url = https://archive.org/details/internetworkingw00come_0}}</ref>
Client–server systems are usually 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 acknowledgment. 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 computer]]s, but alternatively, large [[computing cluster]]s may be composed of many relatively simple, replaceable server components.
== History ==
The use of the word ''server'' in computing comes from [[queueing theory]],<ref>{{cite book |title=Desktop computers: in perspective |author=Richard A. Henle |author2=Boris W. Kuvshinoff |author3=C. M. Kuvshinoff |publisher=Oxford University Press |year=1992 |url=https://books.google.com/books?id=g4krAAAAYAAJ&q=server+%22queuing+theory%22 |page=417 |isbn=978-0-19-507031-6 |quote=Server is a fairly recent computer networking term derived from queuing theory.}}</ref> where it dates to the mid 20th century, being notably used in {{Harvtxt|Kendall|1953}} (along with "service"), the paper that introduced [[Kendall's notation]]. In earlier papers, such as the {{Harvtxt|Erlang|1909}}, more concrete terms such as "[telephone] operators" are used.
In computing, "server" dates at least to RFC 5 (1969),<ref name="rulifson">{{cite IETF |title=DEL |rfc=5 |last=Rulifson |first=Jeff |author-link=Jeff Rulifson |date=June 1969 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=30 November 2013}}</ref> one of the earliest documents describing [[ARPANET]] (the predecessor of [[Internet]]), and is contrasted with "user", distinguishing two types of [[Host (network)|host]]: "server-host" and "user-host". The use of "serving" also dates to early documents, such as RFC 4,<ref>{{cite IETF |title=Network Timetable |rfc=4 |last=Shapiro |first=Elmer B. |date=March 1969 |publisher=[[Internet Engineering Task Force|IETF]] |access-date=30 November 2013}}</ref> contrasting "serving-host" with "using-host".
The [[Jargon File]] defines ''server'' in the common sense of a process performing service for requests, usually remote,<ref>[http://www.catb.org/jargon/html/S/server.html server]</ref> with the 1981 version reading:<ref>[http://www.catb.org/jargon/oldversions/jarg110.txt]</ref>
{{quote |SERVER n. A kind of [[Daemon (computing)|DAEMON]] which performs a service for the requester, which often runs on a computer other than the one on which the server runs.}} The average utilization of a server in the early 2000s was 5 to 15%, but with the adoption of virtualization this figure started to increase to reduce the number of servers needed.<ref>{{cite web | url=https://semiengineering.com/chip-aging-accelerates/ | title=Chip Aging Accelerates | date=14 February 2018 }}</ref>
== Operation ==
[[File:Server-based-network.svg|thumb|A network based on the '''[[client–server model]]''' where multiple individual ''clients'' request services and resources from centralized servers]]
Strictly speaking, the term ''server'' refers to a [[computer program]] or [[process (computing)|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 (network)|host]]''. In addition to ''server'', the words ''serve'' and ''service'' (as verb and as noun respectively) are frequently used, though ''servicer'' and ''servant'' are not.{{efn|A [[servant (CORBA)|CORBA servant]] is a server-side [[Object (computer science)|object]] to which [[method call]]s from [[remote method invocation]] are [[Forwarding (object-oriented programming)|forwarded]], but this is an uncommon usage.}} The word ''service'' (noun) may refer to 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 server]]s "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 [[Client (computing)|clients]]. The nature of communication between a client and server is [[Request–response|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 ''[[Push technology|pushes]]'' messages to the client, rather than the client ''[[Pull technology|pulling]]'' messages from the server as in request-response.<ref>[https://docs.oracle.com/cd/E24329_01/web.1211/e21049/pubsub.htm Using the HTTP Publish-Subscribe Server], Oracle</ref>
== Purpose ==
{{main category|Servers (computing)}}
The role of a server is to share data as well as to share [[System resource|resources]] and distribute work. A server computer can serve its own computer programs as well; depending on the scenario, this could be part of a ''[[quid pro quo]]'' transaction, or simply a technical possibility. The following table shows several scenarios in which a server is used.
{| class="wikitable"
|-
!Server type
!Purpose
!Clients
|-
|[[Application server]]
|Hosts application [[Frontend and backend|back ends]] that user clients (front ends, [[web app]]s or locally installed applications) in the network connect to and use. These servers do not need to be part of the [[World Wide Web]]; any [[local network]] would do.
|Clients with a browser or a local front end, or a web server
|-
|[[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 server]]s and [[name server]]s are examples of catalog servers.
|Any computer program that needs to find something on the network, such a [[Windows ___domain|Domain member]] attempting to log in, an [[email client]] looking for an email address, or a user looking for a file
|-
|[[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
|Communication endpoints (users or devices)
|-
|[[Supercomputer|Computing server]]
|Shares vast amounts of computing resources, especially [[Central processing unit|CPU]] and [[random-access memory]], over a network.
|Any computer program that needs more CPU power and RAM than a personal computer can probably afford. The client must be a networked computer; otherwise, there would be no client-server model.
|-
|[[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.
|[[Spreadsheet]]s, [[accounting software]], [[Digital asset management|asset management software]] or virtually any computer program that consumes well-organized data, especially in large volumes
|-
|[[Fax server]]
|Shares one or more [[fax machine]]s over a network, thus eliminating the hassle of physical access
|Any fax sender or recipient
|-
|[[File server]]
|Shares [[Computer file|files]] and [[Computer folder|folders]], storage space to hold files and folders, or both, over a network
|Networked computers are the intended clients, even though local programs can be clients
|-
|[[Game server]]
|Enables several computers or gaming devices to play [[multiplayer video game]]s
|Personal computers or [[Video game console|gaming consoles]]
|-
|[[Mail server]]
|Makes [[email]] communication possible in the same way that a [[post office]] makes [[snail mail]] communication possible
|Senders and recipients of email
|-
|[[Media server]]
|Shares [[digital video]] or [[digital audio]] over a network through [[Streaming media|media streaming]] (transmitting content in a way that portions received can be watched or listened to as they arrive, as opposed to downloading an entire file and then using it)
|User-attended personal computers equipped with a monitor and a speaker
|-
|[[Print server]]
|Shares one or more [[Printer (computing)|printers]] over a network, thus eliminating the hassle of physical access
|Computers in need of printing something
|-
|[[Sound server]]
|Enables computer programs to play and record sound, individually or cooperatively
|Computer programs of the same computer and network clients.
|-
|[[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 include content control and filtering, improving traffic performance, preventing unauthorized network access or simply routing the traffic over a large and complex network.
|Any networked computer
|-
|[[Virtual private server|Virtual server]]
|Shares hardware and software resources with other virtual servers. It exists only as defined within specialized software called [[hypervisor]]. The [[hypervisor]] presents virtual hardware to the server as if it were real physical hardware.<ref>{{cite web |last1=IT Explained |title=Server - Definition and Details |url=https://www.paessler.com/it-explained/server |website=www.paessler.com |language=en}}</ref> Server virtualization allows for a more efficient infrastructure.<ref>{{cite web |last1=IT Explained |title=DNS Server Not Responding |url=https://dnsservernotresponding.org/ |website=www.dnsservernotresponding.org |language=en |access-date=2020-02-11 |archive-date=2020-09-26 |archive-url=https://web.archive.org/web/20200926091559/https://dnsservernotresponding.org/ |url-status=dead }}</ref>
|Any networked computer
|-
|[[Web server]]
|Hosts [[web page]]s. A web server is what makes the [[World Wide Web]] possible. Each [[website]] has one or more web servers. Also, each server can host multiple websites.
|Computers with a web browser
|}
Almost the entire structure of the [[Internet]] is based upon a [[client–server]] model. High-level [[root nameserver]]s, [[Domain Name System|DNS]], and routers direct the traffic on the internet. There are millions of servers connected to the Internet, running continuously throughout the world<ref>{{cite web|title=Web Servers|url=http://www.serverwatch.com/stypes/index.php/V2Vi|publisher=IT Business Edge|access-date=July 31, 2013}}</ref> and virtually every action taken by an ordinary [[Internet]] user requires one or more interactions with one or more servers. There are exceptions that do not use dedicated servers; for example, [[File sharing|peer-to-peer file sharing]] and some implementations of [[telephony]] (e.g. pre-Microsoft [[Skype]]).
== Hardware ==
[[File:Inside and Rear of Webserver.jpg|thumb|A [[19-inch rack|rack-mountable]] server with the top cover removed to reveal internal components]]
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