Editing Server (computing)

{{Short description|Computer to access a central resource or service on a network}}
[[File:Client-server-model.svg|thumb|right|200px|A [[computer network diagram]] of [[Client (computing)|client]] computers communicating with a server computer via the [[Internet]]]]
[[File:Wikimedia Foundation Servers-8055 35.jpg|thumb|[[Wikimedia Foundation]] rackmount servers on racks in a data center]]
[[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.

== 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 [[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>

== 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]]

[[Computer hardware|Hardware]] requirement for servers vary widely, depending on the server's purpose and its software. Servers often are more powerful and expensive than the clients that connect to them.

The name server is used both for the hardware and software pieces. For the hardware servers, it is usually limited to mean the high-end machines although software servers can run on a variety of hardwares.

Since servers are usually accessed over a network, many run unattended without a [[computer monitor]] or input device, audio hardware and [[Universal Serial Bus|USB]] interfaces. Many servers do not have a [[graphical user interface]] (GUI).  They are configured and managed remotely. Remote management can be conducted via various methods including [[Microsoft Management Console]] (MMC), [[PowerShell]], [[Secure Shell|SSH]] and [[web browser|browser-based]] [[out-of-band management]] systems such as Dell's [[Dell DRAC|iDRAC]] or HP's [[HP Integrated Lights-Out|iLo]].

=== 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|author2=Huang|author3=Shen|author4=Chu|year=2010|access-date=2017-01-30}}</ref> along with extensive [[Preboot Execution Environment|pre-boot]] memory testing and verification. Critical components might be [[hot swappable]], allowing technicians to replace them on the running server without shutting it down, and to guard against overheating, servers might have more powerful fans or use [[water cooling]]. They will often be able to be configured, powered up and down, or rebooted remotely, using [[out-of-band management]], typically based on [[Intelligent Platform Management Interface|IPMI]]. Server casings are usually [[rack unit|flat and wide]], and designed to be rack-mounted, either on [[19-inch rack]]s or on [[Open Rack]]s.

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.

{{gallery
|File:DL380sREAR.jpg|A [[server rack]] seen from the rear
|File:Wikimedia Foundation Servers-8055 08.jpg|[[Wikimedia Foundation]] servers as seen from the front
|File:Wikimedia Foundation Servers-8055 23.jpg|[[Wikimedia Foundation]] servers as seen from the rear
|File:Wikimedia Foundation Servers-8055 02.jpg|[[Wikimedia Foundation]] servers as seen from the rear
}}

=== Clusters ===
{{Main|Server farm}}
A ''server farm'' or ''server cluster'' is a collection of computer servers maintained by an organization to supply server functionality far beyond the capability of a single device. Modern [[data center]]s are now often built of very large clusters of much simpler servers,<ref>{{cite web|url=http://www.cnet.com/news/google-uncloaks-once-secret-server-10209580/|title=Google uncloaks once-secret server|publisher=CBS Interactive|work=CNET|access-date=2017-01-30}}</ref> and there is a collaborative effort, [[Open Compute Project]] around this concept.

=== Appliances ===
A class of small specialist servers called [[network appliance]]s are generally at the low end of the scale, often being smaller than common desktop computers.

=== Mobile ===
A mobile server has a portable form factor, e.g. a [[laptop]].<ref>{{cite web|url=http://www.eurocom.com/products/showroom/products_files/mobileserver/mobileserver.cfm|title=Mobile Server, Power to go, EUROCOM Panther 5SE|archive-url=https://web.archive.org/web/20130317041045/http://www.eurocom.com/products/showroom/products_files/mobileserver/mobileserver.cfm|archive-date=2013-03-17}}</ref> In contrast to large [[data center]]s or rack servers, the mobile server is designed for on-the-road or ''ad hoc'' deployment into emergency, disaster or temporary environments where traditional servers are not feasible due to their power requirements, size, and deployment time.<ref>{{cite web|url=http://www.notebookreview.com/default.asp?newsID=6833&news=Getax+X500+rugged+mobile+server+notebook|title=Mobile Server Notebook|date=27 January 2022}}</ref> The main beneficiaries of so-called "server on the go" technology include network managers, software or database developers, training centers, military personnel, law enforcement, forensics, emergency relief groups, and service organizations.<ref name="deskeng1">{{cite web|url=http://www.deskeng.com/articles/aaambz.htm|title=Server-caliber Computer Doubles as a Mobile Workstation|access-date=2020-02-08|archive-date=2016-03-03|archive-url=https://web.archive.org/web/20160303225722/http://www.deskeng.com/articles/aaambz.htm}}</ref> To facilitate portability, features such as the [[Computer keyboard|keyboard]], [[display device|display]], [[Battery (electricity)|battery]] ([[uninterruptible power supply]], to provide power redundancy in case of failure), and mouse are all integrated into the chassis.

== 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]] [[Linux distribution|distributions]], such as those based on [[Linux]] and [[FreeBSD]],<ref name="linuxusage">{{cite web |url= http://w3techs.com/technologies/details/os-linux/all/all|title=Usage statistics and market share of Linux for websites|access-date=18 Jan 2013}}</ref> with [[Windows Server]] also having a significant share. Proprietary operating systems such as [[z/OS]] and [[macOS Server]] are also deployed, but in much smaller numbers. Servers that run Linux are commonly used as Webservers or Databanks. Windows Servers are used for Networks that are made out of Windows Clients.

Specialist server-oriented operating systems have traditionally had features such as:
* [[GUI]] not available or optional
* Ability to [[Reconfigurable computing|reconfigure]] and update both hardware and software to some extent without restart
* Advanced [[backup]] facilities to permit regular and frequent online backups of critical [[data]],
* [[Network transparency|Transparent]] data transfer between different [[Volume (computing)|volumes]] or devices
* Flexible and advanced networking capabilities
* 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}}</ref>

In practice, today many desktop and server operating systems share similar [[code base]]s, differing mostly in configuration.

== Energy consumption ==
In 2010, data centers (servers, cooling, and other electrical infrastructure) were responsible for 1.1–1.5% of electrical energy consumption worldwide and 1.7–2.2% in the United States.<ref>{{cite news |url=https://www.nytimes.com/2011/08/01/technology/data-centers-using-less-power-than-forecast-report-says.html?_r=0 |title=Data Centers Using Less Power Than Forecast, Report Says |date=31 Jul 2011 |newspaper=[[NY Times]] |access-date=18 Jan 2013 |first=John |last=Markoff}}</ref>{{Update inline|date=November 2023}} One estimate is that total energy consumption for information and communications technology saves more than 5 times its [[carbon footprint]]<ref>{{cite web |url=http://www.smart2020.org/_assets/files/02_Smart2020Report.pdf |archive-url=https://web.archive.org/web/20101122051650/http://www.smart2020.org/_assets/files/02_Smart2020Report.pdf  |archive-date=22 November 2010 |title=SMART 2020: Enabling the low carbon economy in the information age |date=6 Oct 2008 |publisher=[[The Climate Group]] |access-date=18 Jan 2013 }}</ref> in the rest of the economy by increasing efficiency.

Global energy consumption is increasing due to the increasing demand of data and bandwidth. [[Natural Resources Defense Council]] (NRDC) states that data centers used 91 billion kilowatt hours (kWh) electrical energy in 2013 which accounts to 3% of global electricity usage.<ref>{{cite web | url=https://lifelinedatacenters.com/data-center/reduce-data-center-carbon-footprints/ | title=How to Reduce Data Center Carbon Footprints &#124; Lifeline Data Centers | date=12 February 2015 }}</ref>{{Update inline|date=November 2023}}

[[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.

== See also ==
* [[Peer-to-peer#Distributed storage and search|Peer-to-peer]]

== Notes ==
{{Notelist}}

== References ==
{{Reflist|30em}}

== Further reading ==
{{Commons category|Servers}}
{{refbegin}}
* {{cite journal | author-link = Agner Krarup Erlang | first = Agner Krarup | last = Erlang
| title = The theory of probabilities and telephone conversations | journal = Nyt Tidsskrift for Matematik B | volume = 20 | pages = 33–39 | archive-url = https://web.archive.org/web/20111001212934/http://oldwww.com.dtu.dk/teletraffic/erlangbook/pps131-137.pdf | archive-date = 2011-10-01 | url = http://oldwww.com.dtu.dk/teletraffic/erlangbook/pps131-137.pdf | year = 1909 }}
* {{Cite journal
| last1 = Kendall | first1 = D. G. | author-link1 = David George Kendall
| title = Stochastic Processes Occurring in the Theory of Queues and their Analysis by the Method of the Imbedded Markov Chain
| doi = 10.1214/aoms/1177728975
| jstor = 2236285
| journal = The Annals of Mathematical Statistics
| volume = 24
| issue = 3
| pages = 338–354 | year = 1953
| doi-access = free
}}
{{refend}}

{{Computer sizes}}
{{Authority control}}

[[Category:Servers (computing)| ]]
[[Category:Server hardware]]