A server is a computer program or a machine that waits for requests from other machines or software (clients) and responds to them. A server typically process data.[lower-alpha 1] The purpose of a server is to share data or hardware and software resources among clients. This architecture is called the client–server model. The clients may run on the same computer or may connect to the server over a network. Typical computing servers are database servers, file servers, mail servers, print servers, web servers, game servers, and application servers.
Server machines (which can be either actual or virtual machines) run server programs. In turn, a server program turns the machine on which it runs into a server machine. However, designating a machine as "server-class hardware" implies that it is more powerful and reliable than standard personal computers or is specialized for performing the server's role. Servers may be composed of large clusters of relatively simple, replaceable machines.
The term server is used quite broadly in information technology. In theory, any computerized process that shares a resource to one or more client processes is a server. So, while the existence of files on a machine does not classify it as a server, if it uses some mechanism to share these files then it can be a file server. Similarly, web server software can be run on any capable computer, and so a laptop or personal computer can fulfill the role of a web server.
In the hardware sense, the word server typically designates computer models specialized for their role. In a general sense, a server will perform its role better than a generic personal computer.
The purpose of a server is to share data or resources. 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 server||Hosts web apps (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 machines. Unlike what the name might imply, these servers need not 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. Directory servers and name servers are examples of catalog servers.||Any computer program that needs to find something on the network, such a 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)|
|Computing server||It 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.||Spreadsheets, accounting software, asset management software or virtually an computer program that consumes well-organized data, especially in large volumes|
|Fax server||Shares one or more fax machines over a network, thus eliminating the hassle of physical access||Any fax sender or recipient|
|File server||Shares files and 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 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 media streaming (transmitting content in a way that portions received can be watched or listened as they arrive, as opposed downloading a whole huge file and then using it)||User-attended personal computers equipped with a monitor and a speaker|
|Print server||Shares one of more printers over a network, thus eliminating the hassle of physical access||Computers in need of printing something|
|Sound server||Enables computer programs of a computer to play sound and record sound, individually or cooperatively||Computer programs of the same computer|
|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 includes 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|
|Web server||Hosts web pages. A web server is what makes world wide web possible. Each website has one or more web servers.||Computers with a web browser|
Almost the entire structure of the Internet 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 user requires one or more interactions with one or more server. There are exceptions that do not use dedicated servers; for example peer-to-peer file sharing, some implementations of telephony (e.g. pre-Microsoft Skype).
Hardware requirement for servers vary widely, depending on the server's purpose and its software.
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 include MMC, SSH or a web browser.
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 rates in order to maximize uptime. Uninterruptible power supplies might be incorporated to insure against power failure. Servers typically include hardware redundancy such as dual power supplies, RAID disk systems, and ECC memory, 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. 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.
These types of servers are often housed in dedicated server 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.
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.
On the Internet the dominant operating systems among servers are UNIX-like open source distributions, such as those based on Linux and FreeBSD, with Windows Server also having a very significant share. Proprietary operating systems such as z/OS and Mac OS X are also deployed, but in much smaller numbers.
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 facilities to permit regular and frequent online backups of critical data,
- 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.
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 enabling efficiency.
- By contrast, in the peer-to-peer networking model all computers to act as either a server or client as needed.
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