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[[File:First-server-cern-computer-center.jpg|thumb|The first [[World Wide Web|WWW]] server is located at [[CERN]] with its original sticker that says: "This machine is a server. DO NOT POWER IT DOWN!!"]]
A '''server''' is a [[computer]] that provides information to other computers called "[[Client (computing)|clients]]" on a [[computer network]].<ref name="Cisco Networking Academy x508">{{cite web | title=1.1.2.2 Clients and Servers | website=Cisco Networking Academy | url=http://cisco.num.edu.mn/CCNA_R&S1/course/module1/1.1.2.2/1.1.2.2.html | archive-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| archive-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.
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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>
{{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
== Operation ==
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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 [[File system|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>
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|[[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 as 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]]
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=== Large servers ===
Large traditional single servers would need to be run for long periods without interruption. [[Availability]] would have to be very high, making hardware reliability and durability extremely important. [[Mission-critical]] enterprise servers would be very [[Fault tolerance|fault tolerant]] and use specialized hardware with low [[failure rate]]s in order to maximize [[uptime]]. [[Uninterruptible power supply|Uninterruptible power supplies]] might be incorporated to guard against power failure. Servers typically include hardware [[Redundancy (engineering)|redundancy]] such as dual [[power supply|power supplies]], [[RAID]] [[hard disk|disk]] systems, and [[ECC memory]],<ref>{{cite web|url=http://www.cs.rochester.edu/~kshen/papers/usenix2010-li.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.cs.rochester.edu/~kshen/papers/usenix2010-li.pdf |archive-date=2022-10-09 |url-status=live|title="A Realistic Evaluation of Memory Hardware Errors and Software System Susceptibility". Usenix Annual Tech Conference 2010|author=Li
These types of servers are often housed in dedicated [[data centers]]. These will normally have very stable power and Internet and increased security. Noise is also less of a concern, but power consumption and heat output can be a serious issue. Server rooms are equipped with air conditioning devices.
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== Operating systems ==
[[File:Cobalt Qube 3 Front.jpg|thumb|Sun's [[Cobalt Qube]] 3; a computer [[server appliance]] (2002); running Cobalt Linux (a customized version of Red Hat Linux, using the 2.2 Linux kernel), complete with the Apache web server.]]
On the Internet, the dominant [[operating system]]s among servers are UNIX-like [[open source software|open-source]] [[
Specialist server-oriented operating systems have traditionally had features such as:
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* Automation capabilities such as [[Daemon (computer software)|daemons]] in UNIX and [[Windows service|services]] in Windows
* Tight system security, with advanced user, resource, data, and memory protection.
* Advanced detection and alerting on conditions such as overheating, processor and disk failure.<ref>{{cite web|title=Server Oriented Operating System|url=http://www.ingelec.uns.edu.ar/rts/soos/|access-date=2010-05-25|archive-url=http://web.archive.org/web/20110531174040/http://www.ingelec.uns.edu.ar/rts/soos/|archive-date=31 May 2011}}</ref>
In practice, today many desktop and server operating systems share similar [[code base]]s, differing mostly in configuration.
== Energy consumption ==
In
Global energy consumption is increasing due to the increasing demand of data and bandwidth.
[[Environmental groups]] have placed focus on the carbon emissions of data centers as it accounts to 200 million metric tons of [[carbon dioxide]] in a year.
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