In computing, a server is a piece of computer hardware or software (computer program) that provides functionality for other programs or devices, called "clients". This architecture is called the client–server model. Servers can provide various functionalities, often called "services", such as sharing data or System resource 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.
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.
In computing, "server" dates at least to RFC 5 (1969), one of the earliest documents describing ARPANET (the predecessor of Internet), 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 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 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 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 messages to the client, rather than the client Pull technology messages from the server as in request-response. Using the HTTP Publish-Subscribe Server, Oracle
|Application server||Hosts (computer programs that run inside a web browser) allowing users in the network to run and use them, without having to install a copy on their own computers. Unlike what the name might imply, these servers do not need to be part of the World Wide Web; any local network would do.||Computers with a web browser|
|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. and are examples of catalog servers.||Any computer program that needs to find something on the network, such a Windows domain 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||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.|
|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.||, accounting software, asset management software or virtually any computer program that consumes well-organized data, especially in large volumes|
|Fax server||Shares one or more over a network, thus eliminating the hassle of physical access||Any fax sender or recipient|
|File server||Shares Computer file and Computer folder, 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 games||Personal computers or 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 (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 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 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. Server virtualization allows for a more efficient infrastructure.||Any networked computer|
|Web server||Hosts . 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|
Since servers are usually accessed over a network, many run unattended without a computer monitor 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), PowerShell, Secure Shell and web browser out-of-band management systems such as Dell's Dell DRAC or HP's iLo.
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.
Specialist server-oriented operating systems have traditionally had features such as:
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 carbon dioxide in a year.