Web server - AbsoluteAstronomy.com

Web server can refer to either the hardware

Hardware

Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

(the computer) or the software (the computer application) that helps to deliver content that can be accessed through the Internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

.

The most common use of web servers is to host web sites

Website

A website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...

but there are other uses such as data storage or running enterprise applications

Enterprise software

Enterprise software, also known as enterprise application software , is software used in organizations, such as in a business or government, contrary to software chosen by individuals...

Overview

The primary function of a web server is to deliver web pages on the request to client

Client (computing)

A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

s. This means delivery of HTML document

HTML

HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....

s and any additional content that may be included by a document, such as image

Image

An image is an artifact, for example a two-dimensional picture, that has a similar appearance to some subject—usually a physical object or a person.-Characteristics:...

s, style sheets and scripts

JavaScript

JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles....

.

A client, commonly a web browser

Web browser

A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...

or web crawler

Web crawler

A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner or in an orderly fashion. Other terms for Web crawlers are ants, automatic indexers, bots, Web spiders, Web robots, or—especially in the FOAF community—Web scutters.This process is called Web...

, initiates communication by making a request for a specific resource using HTTP and the server responds with the content of that resource or an error message if unable to do so. The resource is typically a real file on the server's secondary memory, but this is not necessarily the case and depends on how the web server is implemented

Implementation

Implementation is the realization of an application, or execution of a plan, idea, model, design, specification, standard, algorithm, or policy.-Computer Science:...

.

While the primary function is to serve content, a full implementation of HTTP also includes ways of receiving content from clients. This feature is used for submitting web form

Form (web)

A webform on a web page allows a user to enter data that is sent to a server for processing. Webforms resemble paper or database forms because internet users fill out the forms using checkboxes, radio buttons, or text fields...

s, including uploading of files.

Many generic web servers also support server-side scripting

Server-side scripting

Server-side scripting is a web server technology in which a user's request is verified by running a script directly on the web server to generate dynamic web pages. It is usually used to provide interactive web sites that interface to databases or other data stores. This is different from...

, e.g., Active Server Pages

Active Server Pages

Active Server Pages , also known as Classic ASP or ASP Classic, was Microsoft's first server-side script engine for dynamically-generated Web pages. Initially released as an add-on to Internet Information Services via the Windows NT 4.0 Option Pack Active Server Pages (ASP), also known as Classic...

(ASP) and PHP

PHP

PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...

. This means that the behaviour of the web server can be scripted

Scripting language

A scripting language, script language, or extension language is a programming language that allows control of one or more applications. "Scripts" are distinct from the core code of the application, as they are usually written in a different language and are often created or at least modified by the...

in separate files, while the actual server software remains unchanged. Usually, this function is used to create HTML documents "on-the-fly" as opposed to returning fixed documents. This is referred to as dynamic

Dynamic web page

A dynamic web page is a kind of web page that has been prepared with fresh information , for each individual viewing. It is not static because it changes with the time , the user , the user interaction , the context A dynamic web page is a kind of web page that has been prepared with fresh...

and static

Static web page

A static web page is a web page that is delivered to the user exactly as stored, in contrast to dynamic web pages which are generated by a web application....

content respectively. The former is primarily used for retrieving and/or modifying information from database

Database

A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

s. The latter is, however, typically much faster and more easily cache

Cache

In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...

d.

Web servers are not always used for serving the world wide web

World Wide Web

The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...

. They can also be found embedded

Embedded system

An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

in devices such as printers, routers, webcam

Webcam

A webcam is a video camera that feeds its images in real time to a computer or computer network, often via USB, ethernet, or Wi-Fi.Their most popular use is the establishment of video links, permitting computers to act as videophones or videoconference stations. This common use as a video camera...

s and serving only a local network

Local area network

A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

. The web server may then be used as a part of a system for monitoring and/or administrating the device in question. This usually means that no additional software has to be installed on the client computer, since only a web browser is required (which now is included with most operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s).

History of web servers

In 1989 Tim Berners-Lee

Tim Berners-Lee

Sir Timothy John "Tim" Berners-Lee, , also known as "TimBL", is a British computer scientist, MIT professor and the inventor of the World Wide Web...

proposed a new project with the goal of easing the exchange of information between scientists by using a hypertext

Hypertext

Hypertext is text displayed on a computer or other electronic device with references to other text that the reader can immediately access, usually by a mouse click or keypress sequence. Apart from running text, hypertext may contain tables, images and other presentational devices. Hypertext is the...

system to his employer CERN

CERN

The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...

. The project resulted in Berners-Lee writing two programs in 1990:

A browser
Web browser
A web browser is a software application for retrieving, presenting, and traversing information resources on the World Wide Web. An information resource is identified by a Uniform Resource Identifier and may be a web page, image, video, or other piece of content...

called WorldWideWeb
WorldWideWeb
WorldWideWeb, later renamed to Nexus to avoid confusion between the software and the World Wide Web, was the first web browser and editor. When it was written, WorldWideWeb was the only way to view the Web....
The world's first web server, later known as CERN httpd
CERN httpd
CERN httpd was a web server daemon originally developed at CERN from 1990 onwards by Tim Berners-Lee, Ari Luotonen and Henrik Frystyk Nielsen...

, which ran on NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...

Between 1991 and 1994, the simplicity and effectiveness of early technologies used to surf and exchange data through the World Wide Web helped to port them to many different operating systems and spread their use among socially diverse groups of people, first in scientific organizations, then in universities and finally in industry.

In 1994 Tim Berners-Lee decided to constitute the World Wide Web Consortium

World Wide Web Consortium

The World Wide Web Consortium is the main international standards organization for the World Wide Web .Founded and headed by Tim Berners-Lee, the consortium is made up of member organizations which maintain full-time staff for the purpose of working together in the development of standards for the...

(W3C) to regulate the further development of the many technologies involved (HTTP, HTML, etc.) through a standardization process.

Common features

Virtual hosting
Virtual hosting
Virtual hosting is a method for hosting multiple domain names on a server using a single IP address. This allows one server to share its resources, such as memory and processor cycles, in order to use its resources more efficiently....

to serve many Web sites using one IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
Large file support
Large file support
Large file support, often abbreviated to LFS, is the term frequently applied to the ability to create files larger than 2 GiB on 32-bit operating systems.- Rationale :...

to be able to serve files whose size is greater than 2 GB on 32 bit OS
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
Bandwidth throttling
Bandwidth throttling
Bandwidth throttling is a reactive measure employed in communication networks to regulate network traffic and minimize bandwidth congestion. Bandwidth throttling can occur at different locations on the network. On a local area network , a sysadmin may employ bandwidth throttling to help limit...

to limit the speed of responses in order to not saturate the network and to be able to serve more clients
Server-side scripting
Server-side scripting
Server-side scripting is a web server technology in which a user's request is verified by running a script directly on the web server to generate dynamic web pages. It is usually used to provide interactive web sites that interface to databases or other data stores. This is different from...

to generate dynamic Web page
Dynamic web page
A dynamic web page is a kind of web page that has been prepared with fresh information , for each individual viewing. It is not static because it changes with the time , the user , the user interaction , the context A dynamic web page is a kind of web page that has been prepared with fresh...

s, still keeping web server and website implementations separate from each other

Path translation

Web servers are able to map the path component of a Uniform Resource Locator

Uniform Resource Locator

In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....

(URL) into:

A local file system resource (for static requests)
An internal or external program name (for dynamic requests)

For a static request the URL path specified by the client is relative to the web server's root directory.

Consider the following URL as it would be requested by a client:

http://www.example.com/path/file.html

The client's user agent

User agent

In computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...

will translate it into a connection to www.example.com with the following HTTP 1.1 request:
GET /path/file.html HTTP/1.1
Host: www.example.com

The web server on www.example.com will append the given path to the path of its root directory. On an Apache server, this is commonly /home/www (On Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

machines, usually /var/www). The result is the local file system resource:

/home/www/path/file.html

The web server then reads the file, if it exists and sends a response to the client's Web browser. The response will describe the content of the file and contain the file itself or an error message will return saying that the file does not exist or is unavailable.

Load limits

A web server (program) has defined load limits, because it can handle only a limited number of concurrent client connections (usually between 2 and 80,000, by default between 500 and 1,000) per IP address

IP address

An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

(and TCP port) and it can serve only a certain maximum number of requests per second depending on:

Its own settings
The HTTP request type
Content origin (static or dynamic)
The fact that the served content is or is not cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...

d
The hardware and software
Computer software
Computer software, or just software, is a collection of computer programs and related data that provide the instructions for telling a computer what to do and how to do it....

limitations of the OS where it is working

When a web server is near to or over its limits, it becomes unresponsive.

Kernel-mode and user-mode web servers

A web server can be either implemented into the OS

Operating system

kernel

Kernel (computing)

In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...

, or in user space

User space

A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...

(like other regular applications).

An in-kernel web server

In-kernel web server

An in-kernel web server is an unlimited HTTP server that runs in kernel space or equivalent. Also called "accelerator".- Benefits :* Performance. The path taken by data from disk to network. Proper asynchronous zero-copy interfaces would make this available from user-space.* Scalability with...

(like TUX

TUX web server

The TUX web server is an in-kernel web server for Linux licensed under the GNU General Public License . It was maintained by Ingo Molnár.It is currently limited to serving static web pages and coordinating between kernelspace modules, userspace modules, and regular userspace web server daemons that...

on GNU/Linux or Microsoft IIS

Internet Information Services

Internet Information Services – formerly called Internet Information Server – is a web server application and set of feature extension modules created by Microsoft for use with Microsoft Windows. It is the most used web server after Apache HTTP Server. IIS 7.5 supports HTTP, HTTPS,...

on Windows) will usually work faster, because, as part of the system, it can directly use all the hardware resources it needs, such as non-paged memory, CPU time-slices, network adapters, or buffers.

Web servers that run in user-mode have to ask the system the permission to use more memory or more CPU resources. Not only do these requests to the kernel take time, but they are not always satisfied because the system reserves resources for its own usage and has the responsibility to share hardware resources with all the other running applications.

Also, applications cannot access the system's internal buffers, which causes useless buffer copies that create another handicap for user-mode web servers. As a consequence, the only way for a user-mode web server to match kernel-mode performance is to raise the quality of its code to much higher standards, similar to that of the code used in web servers that run in the kernel. This is a significant issue under Windows, where the user-mode overhead is about six times greater than that under Linux.

Overload causes

At any time web servers can be overloaded because of:

Too much legitimate web traffic. Thousands or even millions of clients connecting to the web site in a short interval, e.g., Slashdot effect
Slashdot effect
The Slashdot effect, also known as slashdotting, occurs when a popular website links to a smaller site, causing a massive increase in traffic. This overloads the smaller site, causing it to slow down or even temporarily close. The name stems from the huge influx of web traffic that results from...

;
Distributed Denial of Service attacks;
Computer worm
Computer worm
A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...

s that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them);
XSS viruses can cause high traffic because of millions of infected browsers and/or web servers;
Internet bot
Internet bot
Internet bots, also known as web robots, WWW robots or simply bots, are software applications that run automated tasks over the Internet. Typically, bots perform tasks that are both simple and structurally repetitive, at a much higher rate than would be possible for a human alone...

s. Traffic not filtered/limited on large web sites with very few resources (bandwidth, etc.);
Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

(network) slowdowns, so that client requests are served more slowly and the number of connections increases so much that server limits are reached;
Web servers (computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s) partial unavailability. This can happen because of required or urgent maintenance or upgrade, hardware or software failures, back-end (e.g., database
Database
A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

) failures, etc.; in these cases the remaining web servers get too much traffic and become overloaded.

Overload symptoms

The symptoms of an overloaded web server are:

Requests are served with (possibly long) delays (from 1 second to a few hundred seconds).
500, 502, 503, 504 HTTP errors are returned to clients (sometimes also unrelated 404 error
HTTP 404
The 404 or Not Found error message is a HTTP standard response code indicating that the client was able to communicate with the server, but the server could not find what was requested. A 404 error should not be confused with "server not found" or similar errors, in which a connection to the...

or even 408 error may be returned).
TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

connections are refused or reset (interrupted) before any content is sent to clients.
In very rare cases, only partial contents are sent (but this behavior may well be considered a bug
Software bug
A software bug is the common term used to describe an error, flaw, mistake, failure, or fault in a computer program or system that produces an incorrect or unexpected result, or causes it to behave in unintended ways. Most bugs arise from mistakes and errors made by people in either a program's...

, even if it usually depends on unavailable system resources).

Anti-overload techniques

To partially overcome above load limits and to prevent overload, most popular Web sites use common techniques like:

managing network traffic, by using:
- Firewall
  Firewall (computing)
  A firewall is a device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass....
  
  s to block unwanted traffic coming from bad IP sources or having bad patterns;
- HTTP traffic managers to drop, redirect or rewrite requests having bad HTTP patterns;
- Bandwidth management
  Bandwidth management
  Bandwidth management is the process of measuring and controlling the communications on a network link, to avoid filling the link to capacity or overfilling the link, which would result in network congestion and poor performance of the network.- Management :Bandwidth management mechanisms may be...
  
  and traffic shaping
  Traffic shaping
  Traffic shaping is the control of computer network traffic in order to optimize or guarantee performance, improve latency, and/or increase usable bandwidth for some kinds of packets by delaying other kinds of packets that meet certain criteria...
  
  , in order to smooth down peaks in network usage;
deploying Web cache
Web cache
A web cache is a mechanism for the temporary storage of web documents, such as HTML pages and images, to reduce bandwidth usage, server load, and perceived lag...

techniques;
using different domain name
Domain name
A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....

s to serve different (static and dynamic) content by separate web servers, i.e.:
- http://images.example.com
- http://www.example.com
using different domain names and/or computers to separate big files from small and medium sized files; the idea is to be able to fully cache
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...

small and medium sized files and to efficiently serve big or huge (over 10 - 1000 MB) files by using different settings;
using many web servers (programs) per computer, each one bound to its own network card
Network card
A network interface controller is a computer hardware component that connects a computer to a computer network....

and IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

;
using many web servers (computers) that are grouped together so that they act or are seen as one big web server (see also Load balancer);
adding more hardware resources (i.e. RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...

, disks
Disk storage
Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating disks...

) to each computer;
tuning OS parameters for hardware capabilities and usage;
using more efficient computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

s for web servers, etc.;
using other workaround
Workaround
A workaround is a bypass of a recognized problem in a system. A workaround is typically a temporary fix that implies that a genuine solution to the problem is needed...

s, especially if dynamic content is involved.

Market structure

Below is the most recent statistics of the market share of the top web servers on the internet by Netcraft

Netcraft

Netcraft is an Internet services company based in Bath, England.Netcraft provides web server and web hosting market-share analysis, including web server and operating system detection...

survey in March 2011.

Product	Vendor	Web Sites Hosted	Percent
Apache Apache HTTP Server The Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...	Apache Apache Software Foundation The Apache Software Foundation is a non-profit corporation to support Apache software projects, including the Apache HTTP Server. The ASF was formed from the Apache Group and incorporated in Delaware, U.S., in June 1999.The Apache Software Foundation is a decentralized community of developers...	179,720,332	60.31%
IIS Internet Information Services Internet Information Services – formerly called Internet Information Server – is a web server application and set of feature extension modules created by Microsoft for use with Microsoft Windows. It is the most used web server after Apache HTTP Server. IIS 7.5 supports HTTP, HTTPS,...	Microsoft Microsoft Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...	57,644,692	19.34%
nginx Nginx nginx is a Web server and a reverse proxy server for HTTP, SMTP, POP3 and IMAP protocols, with a strong focus on high concurrency, performance and low memory usage. It is licensed under a BSD-like license and it runs on Unix, Linux, BSD variants, Mac OS X, Solaris, and Microsoft Windows.- Overview...	Igor Sysoev Igor Sysoev Igor Vladimirovich Sysoyev , also transliterated as Sysoev, born 5 September 1980 in St. Petersburg, is a professional Russian triathlete and since 2003 he is married with the Russian triathlete and long distance swimmer Irina Abysova.Sysoyev is the winner of the Russian Cup of the year 2009 and...	22,806,060	7.65%
GWS	Google Google Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...	15,161,530	5.09%
lighttpd Lighttpd lighttpd is an open-source web server more optimized for speed-critical environments than common products while remaining standards-compliant, secure and flexible...	lighttpd Lighttpd lighttpd is an open-source web server more optimized for speed-critical environments than common products while remaining standards-compliant, secure and flexible...	1,796,471	0.60%
Sun Java System Web Server	Oracle Oracle In Classical Antiquity, an oracle was a person or agency considered to be a source of wise counsel or prophetic predictions or precognition of the future, inspired by the gods. As such it is a form of divination....

External links

RFC 2616, the Request for Comments
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...

document that defines the HTTP 1.1 protocol.
C64WEB.COM — Commodore 64
Commodore 64
The Commodore 64 is an 8-bit home computer introduced by Commodore International in January 1982.Volume production started in the spring of 1982, with machines being released on to the market in August at a price of US$595...

running as a web server using Contiki
Contiki
Contiki is a small, open source, highly portable multitasking computer operating system developed for use on a number of memory-constrained networked systems ranging from 8-bit computers to embedded systems on microcontrollers, including sensor network motes...

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.