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A web server is a software application that is used to accept requests for web pages and return the appropriate page over the Internet. The Apache HTTP Server is a commercial grade, open source web server available for most operating systems including Unix, Linux, Mac OS X and Microsoft Windows.
Being an open source application, it is available for anyone to download and use for free. This means it can be used by large companies, small companies, colleges and universities, and home users. The Apache Software Foundation feels that it is important that online publishing tools are free for all users, regardless of whether they can afford commercial software.
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The first version of Apache was based on the web server designed by the National Center for Supercomputing Applications called NCSA httpd 1.3. The first official version of Apache was released in 1995. The core contributors for the new web server were: Brian Behlendorf, Roy T. Fielding, Rob Hartill, David Robinson, Cliff Skolnick, Randy Terbush, Robert S. Thau and Andrew Wilson. Additional contributors were Eric Hagberg, Frank Peters and Nicolas Pioch. The Apache Software Foundation was officially created in 1999.
The alpha version of Apache Version 2 was released in March 2000 with the general release in April 2002. This version focused on the development of the Apache Portable Runtime (a portability layer) and further modularization. It also included such enhancements as support for non-Unix platforms, threading, IPv6 support and a new API.
In March 2005, Apache Version 2.2 was released. This version added production ready caching, SQL database support, access to files based on the owner of the file, changes to regular expression APIs, and a protocol handling for output filters.
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Apache is a modular web server, which means users can add only the functionality that they need. Functions include, programming language support, authentication modules, SSL and TSL support, proxy modules, custom log files, compression modules, security modules and filtering support. Supported languages include PHP, Perl, Python and TCL.
Apache also supports virtual hosting through the Virtual Host directive. This means that a single instance of Apache can serve multiple websites. For this reason it is used by many web hosting companies. Virtual hosts can also be useful to keep testing sites separate from the actual live website.
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The Apache HTTP Server is the leading web server for many different reasons. One of the most obvious reasons is the cost but that is not the only reason. If a product is not good, you can't give it away. As of January, 2010, it was used on 111,000,000 sites. The closest contender is Microsoft's IIS at 49,000,000. Its security modules, support for the different programming languages, and virtual hosting are just three of the reasons it is so loved.