Web server

A web server is server software, or hardware dedicated to running this software, that can satisfy client requests on the World Wide Web. A web server can, in general, contain one or more websites. A web server processes incoming network requests over HTTP and several other related protocols.

The inside and front of a Dell PowerEdge server, a computer designed to be mounted in a rack mount environment.

The primary function of a web server is to store, process and deliver web pages to clients.[1] The communication between client and server takes place using the Hypertext Transfer Protocol (HTTP). Pages delivered are most frequently HTML documents, which may include images, style sheets and scripts in addition to the text content.

Multiple web servers may be used for a high traffic website; here, Dell servers are installed together being used for the Wikimedia Foundation.

A user agent, commonly a web browser or web crawler, 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 storage, but this is not necessarily the case and depends on how the web server is implemented.

While the major 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 forms, including uploading of files.

Many generic web servers also support server-side scripting using Active Server Pages (ASP), PHP (Hypertext Preprocessor), or other scripting languages. This means that the behaviour of the web server can be scripted in separate files, while the actual server software remains unchanged. Usually, this function is used to generate HTML documents dynamically ("on-the-fly") as opposed to returning static documents. The former is primarily used for retrieving or modifying information from databases. The latter is typically much faster and more easily cached but cannot deliver dynamic content.

Web servers can frequently be found embedded in devices such as printers, routers, webcams and serving only a local network. The web server may then be used as a part of a system for monitoring or administering 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 systems).

History
The world's first web server, a NeXT Computer workstation with Ethernet, 1990. The case label reads: "This machine is a server. DO NOT POWER IT DOWN!!"
Sun's Cobalt Qube 3 – a computer server appliance (2002, discontinued)

In March 1989 Sir Tim Berners-Lee proposed a new project to his employer CERN, with the goal of easing the exchange of information between scientists by using a hypertext system.[2][3] The project resulted in Berners-Lee writing two programs in 1990:

  • A Web browser called WorldWideWeb[4]
  • The world's first web server, later known as CERN httpd, which ran on NeXTSTEP

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 scientific organizations and universities, and subsequently to the industry.

In 1994 Berners-Lee decided to constitute the World Wide Web Consortium (W3C) to regulate the further development of the many technologies involved (HTTP, HTML, etc.) through a standardization process.

Path translation

Web servers are able to map the path component of a Uniform Resource Locator (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 over HTTP:

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

The client's user agent will translate it into a connection to www.example.com with the following HTTP/2 request:

GET /path/file.html HTTP/2
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 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.

Kernel-mode and user-mode web servers

A web server can be either incorporated into the OS kernel, or in user space (like other regular applications).

Web servers that run in user-mode have to ask the system for 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. Executing in user mode can also mean useless buffer copies which are another limitation for user-mode web servers.

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 (and TCP port) and it can serve only a certain maximum number of requests per second (RPS, also known as queries per second or QPS) depending on:

  • its own settings,
  • the HTTP request type,
  • whether the content is static or dynamic,
  • whether the content is cached, and
  • the hardware and software limitations of the OS of the computer on which the web server runs.

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

Causes of overload

At any time web servers can be overloaded due to:

  • Excess legitimate web traffic. Thousands or even millions of clients connecting to the web site in a short interval, e.g., Slashdot effect;
  • Distributed Denial of Service attacks. A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer or network resource unavailable to its intended users;
  • Computer worms that sometimes cause abnormal traffic because of millions of infected computers (not coordinated among them)
  • XSS worms can cause high traffic because of millions of infected browsers or web servers;
  • Internet bots Traffic not filtered/limited on large web sites with very few resources (bandwidth, etc.);
  • Internet (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 (computers) partial unavailability. This can happen because of required or urgent maintenance or upgrade, hardware or software failures, back-end (e.g., database) failures, etc.; in these cases the remaining web servers get too much traffic and become overloaded.

Symptoms of overload

The symptoms of an overloaded web server are:

  • Requests are served with (possibly long) delays (from 1 second to a few hundred seconds).
  • The web server returns an HTTP error code, such as 500, 502,[5] 503,[6] 504,[7] 408, or even 404, which is inappropriate for an overload condition.[8]
  • The web server refuses or resets (interrupts) TCP connections before it returns any content.
  • In very rare cases, the web server returns only a part of the requested content. This behavior can be considered a bug, even if it usually arises as a symptom of overload.

Anti-overload techniques

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

  • Managing network traffic, by using:
    • Firewalls 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 and traffic shaping, in order to smooth down peaks in network usage
  • Deploying web cache techniques
  • Using different domain names or IP addresses to serve different (static and dynamic) content by separate web servers, e.g.:
    • http://images.example.com
    • http://example.com
  • Using different domain names or computers to separate big files from small and medium-sized files; the idea is to be able to fully cache small and medium-sized files and to efficiently serve big or huge (over 10 – 1000 MB) files by using different settings
  • Using many internet servers (programs) per computer, each one bound to its own network card and IP address
  • Using many internet servers (computers) that are grouped together behind a load balancer so that they act or are seen as one big web server
  • Adding more hardware resources (i.e. RAM, disks) to each computer
  • Tuning OS parameters for hardware capabilities and usage
  • Using more efficient computer programs for web servers, etc.
  • Using other workarounds, especially if dynamic content is involved

Market share
The LAMP (software bundle) (here additionally with Squid), composed entirely of free and open-source software, is a high performance and high-availability heavy duty solution for a hostile environment
Chart:
Market share of all sites of major web servers 2005–2018

February 2019

Below are the latest statistics of the market share of all sites of the top web servers on the Internet by W3Techs Usage of Web Servers for Websites.

ProductVendorPercent
ApacheApache44.3%
nginxNGINX, Inc.41.0%
IISMicrosoft8.9%
LiteSpeed Web ServerLiteSpeed Technologies3.9%
GWSGoogle0.9%

All other web servers are used by less than 1% of the websites.

July 2018

Below are the latest statistics of the market share of all sites of the top web servers on the Internet by W3Techs Usage of Web Servers for Websites.

ProductVendorPercent
ApacheApache45.9%
nginxNGINX, Inc.39.0%
IISMicrosoft9.5%
LiteSpeed Web ServerLiteSpeed Technologies3.4%
GWSGoogle1.0%

All other web servers are used by less than 1% of the websites.

February 2017

Below are the latest statistics of the market share of all sites of the top web servers on the Internet by Netcraft February 2017 Web Server Survey.

ProductVendorJanuary 2017PercentFebruary 2017PercentChangeChart color
IISMicrosoft821,905,28345.66%773,552,45443.16%−2.50red
ApacheApache387,211,50321.51%374,297,08020.89%−0.63black
nginxNGINX, Inc.317,398,31717.63%348,025,78819.42%1.79green
GWSGoogle17,933,7621.00%18,438,7021.03%0.03blue

February 2016

Below are the latest statistics of the market share of all sites of the top web servers on the Internet by Netcraft February 2016 Web Server Survey.

ProductVendorJanuary 2016PercentFebruary 2016PercentChangeChart color
ApacheApache304,271,06133.56%306,292,55732.80%0.76black
IISMicrosoft262,471,88628.95%278,593,04129.83%0.88red
nginxNGINX, Inc.141,443,63015.60%137,459,39116.61%−0.88green
GWSGoogle20,799,0872.29%20,640,0582.21%−0.08blue

Apache, IIS and Nginx are the most used web servers on the World Wide Web.[9][10]

See also
  • Server (computing)
  • Application server
  • Comparison of web server software
  • HTTP compression
  • Open source web application
  • Server Side Includes, Common Gateway Interface, Simple Common Gateway Interface, FastCGI, PHP, Java Servlet, JavaServer Pages, Active Server Pages, ASP.NET, and Server Application Programming Interface
  • Variant object
  • Virtual hosting
  • Web hosting service
  • Web container
  • Web proxy
  • Web service

References
  1. Patrick, Killelea (2002). Web performance tuning (2nd ed.). Beijing: O'Reilly. p. 264. ISBN 059600172X. OCLC 49502686.
  2. Zolfagharifard, Ellie (24 November 2018). "'Father of the web' Sir Tim Berners-Lee on his plan to fight fake news". The Telegraph. ISSN 0307-1235. Retrieved 1 February 2019.
  3. "History of Computers and Computing, Internet, Birth, The World Wide Web of Tim Berners-Lee". history-computer.com. Retrieved 1 February 2019.
  4. Macaulay, Tom. "What are the best open source web servers?". ComputerworldUK. Retrieved 1 February 2019.
  5. Fisher, Tim; Lifewire. "Getting a 502 Bad Gateway Error? Here's What to Do". Lifewire. Retrieved 1 February 2019.
  6. Fisher, Tim; Lifewire. "Getting a 503 Service Unavailable Error? Here's What to Do". Lifewire. Retrieved 1 February 2019.
  7. "What is a 502 bad gateway and how do you fix it?". IT PRO. Retrieved 1 February 2019.
  8. Handbook of digital forensics and investigation. Casey, Eoghan., Altheide, Cory. Burlington, Mass.: Academic Press. 2010. p. 451. ISBN 9780080921471. OCLC 649907705.CS1 maint: others (link)
  9. Vaughan-Nichols, Steven J. "Apache and IIS' Web server rival NGINX is growing fast". ZDNet. Retrieved 1 February 2019.
  10. Hadi, Nahari (2011). Web commerce security: design and development. Krutz, Ronald L. Indianapolis: Wiley Pub. ISBN 9781118098899. OCLC 757394142.
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