
A server is a
computer
A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...
that provides information to other computers called "
clients" on a
computer network
A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...
.
This
architecture
Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and construction, constructi ...
is called the
client–server model
The client–server model is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate ov ...
. Servers can provide various functionalities, often called "services", such as sharing data or
resources
''Resource'' refers to all the materials available in our environment which are Technology, technologically accessible, Economics, economically feasible and Culture, culturally Sustainability, sustainable and help us to satisfy our needs and want ...
among multiple clients or performing
computation
A computation is any type of arithmetic or non-arithmetic calculation that is well-defined. Common examples of computation are mathematical equation solving and the execution of computer algorithms.
Mechanical or electronic devices (or, hist ...
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. Typical servers are
database servers,
file server
In computing, a file server (or fileserver) is a computer attached to a network that provides a location for shared disk access, i.e. storage of computer files (such as text, image, sound, video) that can be accessed by workstations within a co ...
s,
mail servers,
print servers,
web server
A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...
s,
game servers, and
application servers.
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
A personal computer, commonly referred to as PC or computer, is a computer designed for individual use. It is typically used for tasks such as Word processor, word processing, web browser, internet browsing, email, multimedia playback, and PC ...
s, but alternatively, large
computing clusters may be composed of many relatively simple, replaceable server components.
History
The use of the word ''server'' in computing comes from
queueing theory
Queueing theory is the mathematical study of waiting lines, or queues. A queueing model is constructed so that queue lengths and waiting time can be predicted. Queueing theory is generally considered a branch of operations research because th ...
, where it dates to the mid 20th century, being notably used in (along with "service"), the paper that introduced
Kendall's notation. In earlier papers, such as the , more concrete terms such as "
elephoneoperators" are used.
In computing, "server" dates at least to RFC 5 (1969),
one of the earliest documents describing
ARPANET
The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first computer networks to implement the TCP/IP protocol suite. Both technologies became the tec ...
(the predecessor of
Internet
The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...
), 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".
The
Jargon File
The Jargon File is a glossary and usage dictionary of slang used by computer programmers. The original Jargon File was a collection of terms from technical cultures such as the MIT Computer Science and Artificial Intelligence Laboratory, MIT AI Lab ...
defines ''server'' in the common sense of a process performing service for requests, usually remote, with the 1981 version reading:
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.
Operation

Strictly speaking, the term ''server'' refers to a
computer program
A computer program is a sequence or set of instructions in a programming language for a computer to Execution (computing), execute. It is one component of software, which also includes software documentation, documentation and other intangibl ...
or
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
* Business process, activities that produce a specific s ...
(running program). Through
metonymy
Metonymy () is a figure of speech in which a concept is referred to by the name of something associated with that thing or concept. For example, the word " suit" may refer to a person from groups commonly wearing business attire, such as sales ...
, 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 verb and as noun respectively) are frequently used, though ''servicer'' and ''servant'' are not. The word ''service'' (noun) may refer to the abstract form of functionality, e.g.
Web service
A web service (WS) is either:
* a service offered by an electronic device to another electronic device, communicating with each other via the Internet, or
* a server running on a computer device, listening for requests at a particular port over a n ...
. Alternatively, it may refer to a computer program that turns a computer into a server, e.g.
Windows service
In Windows NT operating systems, a Windows service is a computer program that operates in the background. It is similar in concept to a Unix daemon. A Windows service must conform to the interface rules and protocols of the Service Control Manag ...
. 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
A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...
s "serve
pweb pages to users" or "service their requests".
The server is part of the
client–server model
The client–server model is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate ov ...
; 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
Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network, forming a peer-to-peer network of Node ...
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
In computing, a file server (or fileserver) is a computer attached to a network that provides a location for shared disk access, i.e. storage of computer files (such as text, image, sound, video) that can be accessed by workstations within a co ...
. Similarly,
web server
A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...
software can ''run'' on any capable computer, and so a
laptop
A laptop computer or notebook computer, also known as a laptop or notebook, is a small, portable personal computer (PC). Laptops typically have a Clamshell design, clamshell form factor (design), form factor with a flat-panel computer scree ...
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.
Purpose
The role of a server is to share data as well as to share
resources
''Resource'' refers to all the materials available in our environment which are Technology, technologically accessible, Economics, economically feasible and Culture, culturally Sustainability, sustainable and help us to satisfy our needs and want ...
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.
Almost the entire structure of the
Internet
The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...
is based upon a
client–server model. High-level
root nameservers,
DNS, and routers direct the traffic on the internet. There are millions of servers connected to the Internet, running continuously throughout the world and virtually every action taken by an ordinary
Internet
The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...
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
Peer-to-peer file sharing is the distribution and sharing of digital media using peer-to-peer (P2P) networking technology. P2P file sharing allows users to access media files such as books, music, movies, and games using a P2P software program th ...
and some implementations of
telephony
Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunications services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is ...
(e.g. pre-Microsoft
Skype
Skype () was a proprietary telecommunications application operated by Skype Technologies, a division of Microsoft, best known for IP-based videotelephony, videoconferencing and voice calls. It also had instant messaging, file transfer, ...
).
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
A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a electronic visual display, visual display, support electronics, power supply, Housing (engineering), housing, electri ...
or input device, audio hardware and
USB
Universal Serial Bus (USB) is an industry standard, developed by USB Implementers Forum (USB-IF), for digital data transmission and power delivery between many types of electronics. It specifies the architecture, in particular the physical ...
interfaces. Many servers do not have a
graphical user interface
A graphical user interface, or GUI, is a form of user interface that allows user (computing), users to human–computer interaction, interact with electronic devices through Graphics, graphical icon (computing), icons and visual indicators such ...
(GUI). They are configured and managed remotely. Remote management can be conducted via various methods including
Microsoft Management Console (MMC),
PowerShell
PowerShell is a shell program developed by Microsoft for task automation and configuration management. As is typical for a shell, it provides a command-line interpreter for interactive use and a script interpreter for automation via a langu ...
,
SSH and
browser-based out-of-band management systems such as Dell's
iDRAC or HP's
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 tolerant and use specialized hardware with low
failure rate
Failure is the social concept of not meeting a desirable or intended objective, and is usually viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. On ...
s in order to maximize
uptime.
Uninterruptible power supplies might be incorporated to guard against power failure. Servers typically include hardware
redundancy such as dual
power supplies,
RAID
RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical Computer data storage, data storage components into one or more logical units for th ...
disk systems, and
ECC memory
Error correction code memory (ECC memory) is a type of computer data storage that uses an error correction code (ECC) to detect and correct ''n''-bit data corruption which occurs in memory.
Typically, ECC memory maintains a memory system immun ...
, along with extensive
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
file:KKP Auslauf.jpg, Cooling tower and water discharge of a nuclear power plant
Water cooling is a method of heat removal from components and industrial equipment. Evaporative cooling using water is often more efficient than air cooling. Water i ...
. They will often be able to be configured, powered up and down, or rebooted remotely, using
out-of-band management, typically based on
IPMI. Server casings are usually
flat and wide, and designed to be rack-mounted, either on
19-inch racks or on
Open Racks.
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.
Clusters
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 centers are now often built of very large clusters of much simpler servers, and there is a collaborative effort,
Open Compute Project around this concept.
Appliances
A class of small specialist servers called
network appliances 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
A laptop computer or notebook computer, also known as a laptop or notebook, is a small, portable personal computer (PC). Laptops typically have a Clamshell design, clamshell form factor (design), form factor with a flat-panel computer scree ...
. In contrast to large
data centers 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. 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.
To facilitate portability, features such as the
keyboard,
display,
battery (
uninterruptible power supply
An uninterruptible power supply (UPS) or uninterruptible power source is a type of continual power system that provides automated backup electric power to a electrical load, load when the input power source or mains electricity, mains power fai ...
, to provide power redundancy in case of failure), and mouse are all integrated into the chassis.
Operating systems

On the Internet, the dominant
operating system
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ...
s among servers are UNIX-like
open-source
Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use and view the source code, design documents, or content of the product. The open source model is a decentrali ...
distributions, such as those based on
Linux
Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...
and
FreeBSD
FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...
,
with
Windows Server also having a significant share. Proprietary operating systems such as
z/OS
z/OS is a 64-bit operating system for IBM z/Architecture mainframes, introduced by IBM in October 2000. It derives from and is the successor to OS/390, which in turn was preceded by a string of MVS versions.Starting with the earliest:
...
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
reconfigure and update both hardware and software to some extent without restart
* Advanced
backup
In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "wikt:back ...
facilities to permit regular and frequent online backups of critical
data
Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...
,
*
Transparent data transfer between different
volumes or devices
* Flexible and advanced networking capabilities
* Automation capabilities such as
daemons in UNIX and
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.
In practice, today many desktop and server operating systems share similar
code bases, 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. One estimate is that total energy consumption for information and communications technology saves more than 5 times its
carbon footprint 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
The Natural Resources Defense Council (NRDC) is a United States–based 501(c)(3) non-profit international environmental advocacy group, with its headquarters in New York City and offices in Washington, D.C., San Francisco, Los Angeles, Chicag ...
(NRDC) states that data centers used 91 billion kilowatt hours (kWh) electrical energy in 2013 which accounts to 3% of global electricity usage.
Environmental groups have placed focus on the carbon emissions of data centers as it accounts to 200 million metric tons of
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
in a year.
See also
*
Peer-to-peer
Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or workloads between peers. Peers are equally privileged, equipotent participants in the network, forming a peer-to-peer network of Node ...
Notes
References
Further reading
*
*
{{Authority control
Server hardware