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A server is a computer dedicated to running a server application. A server application is a computer program that accepts network connections in order to service requests by sending back responses. Examples of server applications include mail servers, file servers, and proxy servers.
Term usage in information technology
A server is simply a computer that provides services or resources to other computers.
Server is an adjective in the term server operating system. A server operating system is intended or better enabled to run server applications. The differences between the server version and the workstation version of an operating system vary. Sometimes the difference can be the removal of an arbitrary limit due to licensing, as in the case of Windows 2000, or it can be the addition of bundled applications as in Mac OS X Server. Some server applications (e.g. Microsoft IIS ) impose arbitrary limits on the number of HTTP connections they will accept, depending on whether they are running under a server operating system or not.
A server is a computer system that has been designated for running a specific server application. A computer that is designated for only one server application is often named for that application. For example, when the software Apache HTTP Server is used as the web server for a company's website, the computer running Apache is also called the web server. Server applications can be divided among server computers over an extreme range, depending upon the workload. Every server application can run concurrently on a single computer under light loading, but multiple server computers may be required for each application under a heavy load. Under medium loading, it is common to use one server computer per server application, in order to limit the amount of damage caused by failure of any single server computer or security breach of any single server application. Any server computer can also be used as a workstation, but it is avoided in practice, again to contain risk.
Server is also a designation for computer models intended for use in running server applications, often under heavy workloads, unattended, for an extended period of time. While any workstation computer can run server operating systems and server applications, a server computer usually has special features intended to make it more suitable. These features can include a faster CPU, faster and more plentiful RAM, and larger hard drives, but these traits are shared with high-end desktops. More obvious distinctions include redundancy in power supplies, network connections, and storage devices as well as modular design. Blade servers are an example of the latter and are often used in server farms.
The name server or server appliance also applies to network-connected computer appliances or "appliance hardware" that provide specific services for other clients on the network. Though the appliance is a server computer, loaded with a server operating system and running a server application, the user need not configure any of it — it is a black box that does a specific job. The simplest servers are most often sold as appliances, like switches, routers, gateways, and print servers.
Server hardware
Hardware requirements for servers vary, depending on the server application. Absolute CPU speed is not as critical to a server as it is to a desktop. Servers' duties to provide service to many users over a network lead to different requirements like fast network connections and high I/O throughput. Since servers are typically accessed over a network, servers emphasize function over form, without regard to aesthetics like appearance and noise level, because users may never lay eyes on the machine itself. Servers may accordingly run in headless mode without a monitor in order to free up processing cycles for other tasks. In general, a server becomes more specialized and therefore more efficient as unnecessary and unused services are eliminated. For this reason, many servers lack a graphical user interface, or GUI, because it consumes resources that could be allocated elsewhere. Similarly, servers often lack audio and USB interfaces.
By definition, servers provide services, but it is not always possible to predict when users will need those services. For this reason, servers are often online for weeks or months without interruption, making hardware durability extremely important. Although servers can be built from commodity computer parts, mission-critical servers use specialized hardware with low failure rates in order to maximize uptime. For example, servers may incorporate faster, higher-capacity hard drives, larger computer fans or water cooling to help remove heat, and uninterruptible power supplies that ensure the servers continue to function in the event of a power failure. These components offer higher performance and reliability at a correspondingly higher price. The dominant paradigm in servers is parallel computing, and thus high-performance servers are often placed in rack-mounted configurations to save space inside server rooms or "closets." These special rooms help mute the large amount of noise produced and also restrict physical access to the system administrators for security purposes.
Servers have a unique property in that, the more powerful and complex the system, the longer it takes for the hardware to turn on and begin loading the operating system. Servers often do extensive preboot memory testing and verification and start up of remote management services. The hard drive controllers then start up banks of drives sequentially, rather than all at once, so as not to overload the power supply, and afterwards they initiate RAID system prechecks for correct operation of redundancy. It is not uncommon for a machine to take several minutes to turn on and yet not require a restart for the next calendar year.
Server operating system
Some popular operating systems for servers — such as FreeBSD, Solaris, and Linux — are derived from or are similar to UNIX. UNIX was originally a minicomputer operating system, and as servers gradually replaced traditional minicomputers, UNIX was a logical and efficient choice of operating system for the servers. UNIX-based systems, many of which are free in both senses, are the most popular.
Server-oriented operating systems tend to have certain features in common that make them more suitable for the server environment, such as
an optional or absent GUI,
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ability to reconfigure both hardware and software to some extent without restart,
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advanced backup facilities to permit online backups of critical data at regular and frequent intervals,
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transparent data transfer between different volumes or devices,
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flexible and advanced networking capabilities,
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automation capabilities like daemons in UNIX and services in Windows, and
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tight system security, with advanced user, resource, data, and memory protection.
Server-oriented operating systems can in many cases interact with hardware sensors to detect conditions such as overheating, processor and disk failure, and consequently alert an operator and/or take remedial measures itself.
Because the requirements of servers are, in some cases, almost diametrically opposed to those of desktop computers, it is extremely difficult to design an operating system that handles both environments well; thus, operating systems that are well suited to the desktop may not be ideal for servers and vice versa. Regardless of OS vendor, system configurations that are ideal for servers may be unsatisfactory for desktop use, and configurations that perform well on the desktop may leave much to be desired on servers. This results in many operating systems' release in both server and desktop versions. Nevertheless, the desktop versions of the Windows and Mac OS X operating systems are deployed on a minority of servers, as are some proprietary mainframe operating systems, such as z/OS. The dominant operating systems among servers are UNIX-based and open source kernel distributions.
The rise of the microprocessor -based server was facilitated by the development of Unix to run on the x86 microprocessor architecture. The Microsoft Windows family of operating systems also runs on x86 hardware, and versions beginning with Windows NT have incorporated features that making them suitable for use as server operating systems.
While the role of server and desktop operating systems remains distinct, improvements in the reliability of both hardware and operating systems have blurred the distinction between the two classes. Today, many desktop and server operating systems share similar code bases, differing mostly in configuration. The shift towards web applications and middleware platforms has also lessened the demand for specialist application servers.
Servers on the Internet
Almost the entire structure of the Internet is based upon a client-server model. High-level root nameservers, DNS servers, and routers direct the traffic on the internet. There are millions of servers connected to the Internet, running continuously throughout the world.
Among the many services provided by Internet servers are:
Virtually every action taken by an ordinary Internet user requires one or more interactions with one or more servers.
There are also technologies that operate on an inter-server level.
Servers in daily life
Any computer or device serving out applications or services can technically be called a server. In an office or enterprise environment, the network server is easy to identify. A DSL / Cable modem router qualifies as a server because it provides a computer with application services like IP address assignment (via DHCP ) and NAT, which is the firewall that helps protect a computer from harm. iTunes software implements a music server to stream music between computers. Many home users create shared folders and printers. There are also many private servers for such hosting online games such as Everquest, World of Warcraft, and Counter-strike.
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