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A server is a that provides information to other computers called "clients" on . This architecture is called the client–server model. Servers can provide various functionalities, often called "services", such as sharing data or among multiple clients or performing 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.

(2024). 9780470901823, John Wiley & Sons. .
Typical servers are , , , , , , and application servers.
(1993). 9780134742229, . .

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 computers, 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 ,
(1992). 9780195070316, Oxford University Press. .
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 "telephone operators" are used.

In computing, "server" dates at least to RFC 5 (1969), one of the earliest documents describing (the predecessor of ), 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 defines server in the common sense of a process performing service for requests, usually remote, server with the 1981 version reading:[2]

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 or process (running program). Through , 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. . Alternatively, it may refer to a computer program that turns a computer into a server, e.g. . 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, "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 clients. The nature of communication between a client and server is request and response. This is in contrast with 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 . Similarly, software can run on any capable computer, and so a 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 messages to the client, rather than the client messages from the server as in request-response. Using the HTTP Publish-Subscribe Server, Oracle


Purpose
The role of a server is to share data as well as to share 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.
Application serverHosts application back ends that user clients (front ends, 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 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. and are examples of catalog servers.Any computer program that needs to find something on the network, such a attempting to log in, an looking for an email address, or a user looking for a file
Communications serverMaintains 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 networkCommunication endpoints (users or devices)
Shares vast amounts of computing resources, especially 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.
Maintains and shares any form of (organized collections of data with predefined properties that may be displayed in a table) over a network., accounting software, asset management software or virtually any computer program that consumes well-organized data, especially in large volumes
Shares one or more over a network, thus eliminating the hassle of physical accessAny fax sender or recipient
Shares and , storage space to hold files and folders, or both, over a networkNetworked computers are the intended clients, even though local programs can be clients
Enables several computers or gaming devices to play multiplayer video gamesPersonal computers or gaming consoles
Makes communication possible in the same way that a makes communication possibleSenders and recipients of email
Shares or over a network through (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
Shares one or more printers over a network, thus eliminating the hassle of physical accessComputers in need of printing something
Enables computer programs to play and record sound, individually or cooperativelyComputer programs of the same computer and network clients.
Acts as an 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 serverShares hardware and software resources with other virtual servers. It exists only as defined within specialized software called . The presents virtual hardware to the server as if it were real physical hardware. Server virtualization allows for a more efficient infrastructure.Any networked computer
Hosts . A web server is what makes the World Wide Web possible. Each has one or more web servers. Also, each server can host multiple websites.Computers with a web browser
Almost the entire structure of the is based upon a client–server model. High-level , 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 user requires one or more interactions with one or more servers. There are exceptions that do not use dedicated servers; for example, and some implementations of (e.g. pre-Microsoft ).


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 or input device, audio hardware and 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), , and out-of-band management systems such as Dell's or HP's iLo.


Large servers
Large traditional single servers would need to be run for long periods without interruption. would have to be very high, making hardware reliability and durability extremely important. enterprise servers would be very and use specialized hardware with low in order to maximize . Uninterruptible power supplies might be incorporated to guard against power failure. Servers typically include hardware redundancy such as dual , systems, and , along with extensive pre-boot memory testing and verification. Critical components might be , 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 . 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 , and designed to be rack-mounted, either on 19-inch racks or on .

These types of servers are often housed in dedicated . 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 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 . In contrast to large 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, , battery (uninterruptible power supply, to provide power redundancy in case of failure), and mouse are all integrated into the chassis.


Operating systems
On the Internet, the dominant among servers are UNIX-like open-source distributions, such as those based on and , with also having a significant share. Proprietary operating systems such as z/OS and 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:

  • not available or optional
  • Ability to reconfigure and update both hardware and software to some extent without restart
  • Advanced facilities to permit regular and frequent online backups of critical ,
  • Transparent data transfer between different volumes or devices
  • Flexible and advanced networking capabilities
  • Automation capabilities such as daemons in UNIX and 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 , 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 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.

Environmental groups have placed focus on the carbon emissions of data centers as it accounts to 200 million metric tons of in a year.


See also
  • Peer-to-peer


Notes

Further reading
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