Acomputer network diagram ofclient computers communicating with a server computer via theInternetWikimedia Foundation rackmount servers on racks in a data centerThe firstWWW server is located atCERN with its original sticker that says: "This machine is a server. DO NOT POWER IT DOWN!!"
Client–server systems are most frequently implemented by (and often identified with) therequest–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 standardpersonal computers, but alternatively, largecomputing clusters may be composed of many relatively simple, replaceable server components.
The use of the wordserver in computing comes fromqueueing theory,[4] where it dates to the mid 20th century, being notably used inKendall (1953) (along with "service"), the paper that introducedKendall's notation. In earlier papers, such as theErlang (1909), more concrete terms such as "[telephone] operators" are used.
In computing, "server" dates at least to RFC 5 (1969),[5] one of the earliest documents describingARPANET (the predecessor ofInternet), and is contrasted with "user", distinguishing two types ofhost: "server-host" and "user-host". The use of "serving" also dates to early documents, such as RFC 4,[6] contrasting "serving-host" with "using-host".
TheJargon File definesserver in the common sense of a process performing service for requests, usually remote,[7] with the 1981 version reading:[8]
SERVER n. A kind ofDAEMON 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 the number of servers needed.[9]
A network based on theclient–server model where multiple individualclients request services and resources from centralized servers
Strictly speaking, the termserver refers to acomputer program orprocess (running program). Throughmetonymy, 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 ahost. In addition toserver, the wordsserve andservice (as verb and as noun respectively) are frequently used, thoughservicer andservant are not.[a] The wordservice (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 servers "serve [up] web pages to users" or "service their requests".
The server is part of theclient–server model; in this model, a server serves data forclients. The nature of communication between a client and server isrequest and response. This is in contrast withpeer-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, iffiles on a device are shared by some process, that process is afile server. Similarly,web server software canrun on any capable computer, and so alaptop or a personal computer can host a web server.
While request–response is the most common client-server design, there are others, such as thepublish–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 clientswithout any further requests: the serverpushes messages to the client, rather than the clientpulling messages from the server as in request-response.[10]
The role of a server is to share data as well as to shareresources and distribute work. A server computer can serve its own computer programs as well; depending on the scenario, this could be part of aquid pro quo transaction, or simply a technical possibility. The following table shows several scenarios in which a server is used.
Hosts applicationback ends that user clients (front ends,web apps or locally installed applications) in the network connect to and use. These servers do not need to be part of theWorld Wide Web; anylocal network would do.
Clients with a browser or a local front end, or a web 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 andname servers are examples of catalog servers.
Any computer program that needs to find something on the network, such as aDomain member attempting to log in, anemail client looking for an email address, or a user looking for a file
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
Shares vast amounts of computing resources, especiallyCPU andrandom-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.
Sharesdigital video ordigital audio over a network throughmedia 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
Acts as anintermediary 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.
Shares hardware and software resources with other virtual servers. It exists only as defined within specialized software calledhypervisor. Thehypervisor presents virtual hardware to the server as if it were real physical hardware.[11] Server virtualization allows for a more efficient infrastructure.[12]
Hostsweb pages. A web server is what makes theWorld Wide Web possible. Eachwebsite has one or more web servers. Also, each server can host multiple websites.
Computers with a web browser
Almost the entire structure of theInternet is based upon aclient–server model. High-levelroot nameservers,DNS, and routers direct the traffic on the internet. There are millions of servers connected to the Internet, running continuously throughout the world[13] and virtually every action taken by an ordinaryInternet user requires one or more interactions with one or more servers. There are exceptions that do not use dedicated servers; for example,peer-to-peer file sharing and some implementations oftelephony (e.g. pre-MicrosoftSkype).
Arack-mountable server with the top cover removed to reveal internal components
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.
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 veryfault tolerant and use specialized hardware with lowfailure rates in order to maximizeuptime.Uninterruptible power supplies might be incorporated to guard against power failure. Servers typically include hardwareredundancy such as dualpower supplies,RAIDdisk systems, andECC memory,[14] along with extensivepre-boot memory testing and verification. Critical components might behot 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 usewater cooling. They will often be able to be configured, powered up and down, or rebooted remotely, usingout-of-band management, typically based onIPMI. Server casings are usuallyflat and wide, and designed to be rack-mounted, either on19-inch racks or onOpen Racks.
These types of servers are often housed in dedicateddata 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.
Aserver farm orserver cluster is a collection of computer servers maintained by an organization to supply server functionality far beyond the capability of a single device. Moderndata centers are now often built of very large clusters of much simpler servers,[15] and there is a collaborative effort,Open Compute Project around this concept.
A class of small specialist servers callednetwork appliances are generally at the low end of the scale, often being smaller than common desktop computers.
A mobile server has a portable form factor, e.g. alaptop.[16] In contrast to largedata centers or rack servers, the mobile server is designed for on-the-road orad hoc deployment into emergency, disaster or temporary environments where traditional servers are not feasible due to their power requirements, size, and deployment time.[17] 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.[18] To facilitate portability, features such as thekeyboard,display,battery (uninterruptible power supply, to provide power redundancy in case of failure), and mouse are all integrated into the chassis.
Sun'sCobalt Qube 3; a computerserver 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 dominantoperating systems among servers are UNIX-likeopen-sourcedistributions, such as those based onLinux andFreeBSD,[19] withWindows Server also having a significant share. Proprietary operating systems such asz/OS andmacOS 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:
In 2024, data centers (servers, cooling, and other electrical infrastructure) consumed 415terawatt-hours of electrical energy, and were responsible for roughly 1.5% of electrical energy consumption worldwide,[21] and for 4.4% in the United States.[22] One estimate is that total energy consumption for information and communications technology saves more than 5 times itscarbon footprint[23] in the rest of the economy by increasing efficiency.
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 ofcarbon dioxide in a year.
^"1.1.2.2 Clients and Servers".Cisco Networking Academy. Archived fromthe original on 2024-04-07. Retrieved2024-04-07.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.
^Richard A. Henle; Boris W. Kuvshinoff; C. M. Kuvshinoff (1992).Desktop computers: in perspective. Oxford University Press. p. 417.ISBN978-0-19-507031-6.Server is a fairly recent computer networking term derived from queuing theory.
