Web server

The term Web server can mean one of two things:

  1. A computer that is responsible for accepting HTTP requests from clients, which are known as Web browsers, and serving them Web pages, which are usually HTML documents.
  2. A computer program that provides the functionality described in the first sense of the term.


Contents

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Common features

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

Every Web server program operates by accepting HTTP requests from the network, and providing an HTTP response to the requester. The HTTP response typically consists of an HTML document, but can also be a raw text file, an image, or some other type of document.

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.

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Origin of returned content

The origin of the content sent by server is called static if it comes from an existing file or dynamic if it is dynamically generated by some other program or script called by Web server. Serving static content is usually much faster than serving dynamic content.

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Path translation

Web servers usually translate the path component of a Uniform Resource Locator (URL) into a local file system resource. 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.

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Concurrency

Web servers often use concurrent programming techniques to increase the responsiveness of the server when providing pages to multiple clients simultaneously.

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Threaded servers

Many web servers are multithreaded. When a user visits a web site, a web server will use a thread to serve the page to that user. If another user visits the site while the previous user is still being served, the web server can serve the second visitor by using a different thread. Thus, the second user does not have to wait for the first visitor to be served. This is very important because not all users have the same speed Internet connection. A slow user should not delay all other visitors from downloading a web page. For better performance, threads used by web servers and other Internet services are typically pooled and reused to eliminate even the small overhead associated with creating a thread.

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Process-based servers

For reliability and security reasons, some web servers using multiple processes (rather than multiple threads within a single process) still remain in production use, such as Apache 1.3. A pool of processes are used, and reused, until a certain threshold of requests have been served by a process before it is replaced by a new one. Because threads share a main process context, a crashing thread may more easily crash the whole application, and a buffer overflow can have more disastrous consequences. Moreover, a memory leak in system libraries which are out of the control of the application programmer cannot be dealt with using threads, but are appropriately dealt with using a pool of processes with a limited life time. Another problem relates to the wide variety of third party libraries which might be used by an application (a PHP extension library for instance) which expects a process context. Using multiple processes also allows to deal with situations which can benefit from privilege separation techniques to achieve better security. This does not prevent multiple threads from being used within the processes of the pool, however, which is technically possible. An application can then leverage the advantages of both processes and threads as necessary.

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Load Limits

A web server can handle a limited number of concurrent client connections (usually between 2 and 60,000) and it can serve only a certain maximum number of requests per second depending on its own settings, the HTTP request type and the hardware and software limits of the OS where it is working.

To partially overcome these limits, most popular Web sites use common techniques like:

  • using different URLs to serve 
 static  content (i.e. http://images.example.com) and
 dynamic content (i.e. http://www.example.com)

by separate Web servers;

  • using many Web servers that are grouped together so that they act or are seen as one big Web server, see also: Load balancer.

The symptoms of an overloaded Web server are:

  • requests are served with noticeably (long) delays (from 1 second to a few hundreds of seconds);
  • 500, 503 HTTP errors are returned to clients (sometimes also unrelated 404 error may be returned);
  • TCP connections are refused or reset before any content is sent to clients.
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Historical note

This NeXTcube used by Berners-Lee at CERN became the first Web server.
This NeXTcube used by Berners-Lee at CERN became the first Web server.

In 1989 Tim Berners-Lee proposed to his employer CERN (European center 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, Berners-Lee wrote two programs: a browser called WorldWideWeb and the world's first Web server, which ran on NeXTSTEP. Today, this machine is on exhibition at CERN's public museum, Microcosm.

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Software

The four top most common Web or HTTP server programs are:

There are thousands of different Web server programs available, many of them are specialized for some uses and can be tailored to satisfy specific needs.

See Category:Web server software for a comprehensive list of HTTP server programs.

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Statistics

The most popular Web servers, used for public Web sites, are tracked by Netcraft Web Server Survey, with details given by Netcraft Web Server Reports.

The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.

Apache has been the most popular Web server on the Internet since April of 1996. The November 2005 Netcraft Web Server Survey found that more than 70% of the Web sites on the Internet are using Apache, thus making it more widely used than all other Web servers combined. The Apache HTTP Server is a project of the Apache Software Foundation

Another site provide statistics is SecuritySpace: [1] and they also provide a detail break down for each version of Web server: [2]

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See also

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External links