Explained at

Web server explained

The term web server can mean one of two things:

  1. A computer program that is responsible for accepting HTTP requests from clients, which are known as web browsers, and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and linked objects (images, etc.).
  2. A computer that runs a computer program as described above.

Common features

Although web server programs differ in detail, they all share some basic common features.

  1. HTTP: every web server program operates by accepting HTTP requests from the client, and providing an HTTP response to the client. The HTTP response usually consists of an HTML document, but can also be a raw file, an image, or some other type of document (defined by MIME-types); if some error is found in client request or while trying to serve the request, a web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.
  2. Logging: usually web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the webmaster to collect statistics by running log analyzers on log files.

In practice many web servers implement the following features also:

  1. Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.
  2. Handling of static content (file content recorded in server's filesystem(s)) and dynamic content by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).
  3. HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.
  4. Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).
  5. Virtual hosting to serve many web sites using one IP address.
  6. Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
  7. Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

Origin of returned content

The origin of the content sent by server is called:

Serving static content is usually much faster (from 2 to 100 times) than serving dynamic content, especially if the latter involves data pulled from a database.

Path translation

Web servers are able to map the path component of a Uniform Resource Locator (URL) into:

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 web browser 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 Unix machines, this is commonly /var/www/htdocs. The result is the local file system resource:

/var/www/htdocs/path/file.html

The web server will then read the file, if it exists, and send a response to the client's web browser. The response will describe the content of the file and contain the file itself.

Although web servers programs differ in detail, they all do share some basic common features.

  1. HTTP: every web server program operates by accepting HTTP requests from the client, and providing an HTTP response to the client. The HTTP response usually consists of an HTML document, but can also be a raw file, an image, or some other type of document (defined by MIME-types); if some error is found in client request or while trying to serve the request, a web server has to send an error response which may include some custom HTML or text messages to better explain the problem to end users.
  2. Logging: usually web servers have also the capability of logging some detailed information, about client requests and server responses, to log files; this allows the webmaster to collect statistics by running log analyzers on log files.

In practice many web servers implement the following features also:

  1. Authentication, optional authorization request (request of user name and password) before allowing access to some or all kind of resources.
  2. Handling of static content (file content recorded in server's filesystem(s)) and dynamic content by supporting one or more related interfaces (SSI, CGI, SCGI, FastCGI, JSP, PHP, ASP, ASP .NET, Server API such as NSAPI, ISAPI, etc.).
  3. HTTPS support (by SSL or TLS) to allow secure (encrypted) connections to the server on the standard port 443 instead of usual port 80.
  4. Content compression (i.e. by gzip encoding) to reduce the size of the responses (to lower bandwidth usage, etc.).
  5. Virtual hosting to serve many web sites using one IP address.
  6. Large file support to be able to serve files whose size is greater than 2 GB on 32 bit OS.
  7. Bandwidth throttling to limit the speed of responses in order to not saturate the network and to be able to serve more clients.

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 60,000, by default between 500 and 1,000) per IP address (and IP port) and it can serve only a certain maximum number of requests per second depending on:

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

Overload causes

At any time web servers can be overloaded because of:

Overload symptoms

The symptoms of an overloaded web server are:

Anti-overload techniques

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

Historical notes

In 1989 Tim Berners-Lee proposed to his employer CERN (European Organization for Nuclear Research) a new project, which had the goal of easing the exchange of information between scientists by using a hypertext system. As a result of the implementation of this project, in 1990 Berners-Lee wrote two programs:

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

The following years are recent history which has seen an exponential growth of the number of web sites and servers.

Market structure

Given below is a list of top Web server software vendors published in a Netcraft survey in December 2007.

VendorProductWeb Sites HostedPercent
ApacheApache76,591,44249.35%
MicrosoftIIS55,502,88635.76%
GoogleGFE8,555,3915.51%
OverseeOversee1,573,7011.01%
lighttpdlighttpd1,521,2500.98%
nginxnginx767,9660.49%
Others-10,690,1016.89%
Total-155,202,737100.00%

There are hundreds of different web server programs available, many of which are specialized for very specific purposes, so the fact that a web server is not very popular does not necessarily mean that it has a lot of bugs or poor performance.

See for a longer list of HTTP server programs.

See also

External links

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Web server".

Everything.Explained.At is © Copyright 2009-2010, A B Cryer, All Rights Reserved.