Web server

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The inside and front of a Dell PowerEdge web server, a computer designed for rack mounting
Multiple web servers may be used for a high traffic website, here four Sun Fire X4200 servers are installed together

The term web server can refer to either the hardware (the computer) or the software (the computer application) that helps to deliver web content that can be accessed through the Internet.[1]

The most common use of web servers is to host websites, but there are other uses such as gaming, data storage or running enterprise applications.

Overview

The primary function of a web server is to deliver web pages to clients. 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 text content.

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 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 forms, including uploading of files.

Many generic web servers also support server-side scripting using Active Server Pages (ASP), PHP, 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 create HTML documents dynamically ("on-the-fly") as opposed to returning static documents. The former is primarily used for retrieving and/or modifying information from databases. The latter is typically much faster and more easily cached but cannot deliver dynamic content.

Web servers are not always used for serving the World Wide Web. They can also 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 and/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!!"

In 1989 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. The project resulted in Berners-Lee writing two programs in 1990:

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 then to industry.

In 1994 Tim 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.

Common features

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:

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 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 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 implemented into the OS kernel, or in user space (like other regular applications).

An in-kernel web server (like Microsoft IIS on Windows or TUX on GNU/Linux) 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 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 handicap 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 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 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;
  • 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 viruses can cause high traffic because of millions of infected browsers and/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, 503, 504, 408, or even 404, which is inappropriate for an overload condition.
  • 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 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 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 and IP address
  • Using many web 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
Market share of major web servers

Below is the latest statistics of the market share of the top web servers on the internet by Netcraft Survey May, June, July 2013.

Product Vendor May 2013 Percent
Apache Apache 359,441,468 53.42%
IIS Microsoft 112,303,412 16.69%
nginx NGINX, Inc. 104,411,087 15.52%
GWS Google 23,029,260 3.42%
Product June 2013 Percent July 2013 Percent Change
Apache 358,974,045 53.34% 364,696,792 52.19% -1.15%
IIS 115,920,681 17.22% 137,351,211 19.65% +2.43%
nginx 97,991,191 14.56% 95,017,255 13.60% -0.96%
GWS 26,036,616 3.87% 27,406,059 3.92% +0.05%


See also

References

  1. ↑ "What is web server?'". webdevelopersnotes. 2010-11-23. Retrieved 2010-11-23. 

External links

Website management
Concepts
Web hosting control panels (comparison)
Domain name managers and registrars
Web content management system
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