The Apache HTTP Server, frequently referred to simply as Apache, is a web server notable for playing a key role in the initial growth of the World Wide Web. Apache was the first viable alternative to the Netscape Communications Corporation web server (currently known as Sun Java System Web Server), and has since evolved to rival other Unix-based web servers in terms of functionality and performance.

The project's name was chosen for two reasons: out of respect for the Native American Indian tribe of Apache (Indé), well-known for their endurance and their skills in warfare, and due to the project's roots as a set of patches to the codebase of NCSA HTTPd 1.3 - making it "a patchy" server.

Apache is developed and maintained by an open community of developers under the auspices of the Apache Software Foundation. The application is available for a wide variety of operating systems, including Unix, FreeBSD, Linux, Solaris, Novell NetWare, Mac OS X, and Microsoft Windows. Released under the Apache License, Apache is characterized as free software and open source software.

Since April 1996 Apache has been the most popular HTTP server on the World Wide Web; since March 2006 however it has experienced a steady decline of its market share, lost mostly against Microsoft Internet Information Services and the .NET platform used by some large blog providers. As of January 2008 Apache served 50.61% of all websites.

History

The first version of the Apache web server was created by Robert McCool, who was heavily involved with the National Center for Supercomputing Applications web server, known simply as NCSA HTTPd. When Rob left NCSA in mid-1994, the development of httpd stalled, leaving a variety of patches for improvements circulating through e-mails.

Rob McCool was not alone in his efforts. Several other developers helped form the original "Apache Group": Brian Behlendorf, Roy T. Fielding, Rob Hartill, David Robinson, Cliff Skolnick, Randy Terbush, Robert S. Thau, Andrew Wilson, Eric Hagberg, Frank Peters, and Nicolas Pioch.

Version 2 of the Apache server was a substantial re-write of much of the Apache 1.x code, with a strong centre on further modularization and the development of a portability layer, the Apache Portable Runtime. The Apache 2.x core has several major enrichments over Apache 1.x. These consist of UNIX threading, better support for non-Unix platforms (such as Microsoft Windows), a new Apache API, and IPv6 support. The first alpha release of Apache 2 was in March 2000, with the first general availability release on 6 April 2002.

Version 2.2 introduced a new authorization API that allows for more flexibility. It also features improved cache modules and proxy modules.

Features

Apache supports a variety of features, many implemented as compiled modules which extend the core functionality. These can range from server-side programming language support to authentication schemes. Some common language interfaces support mod_perl, mod_python, Tcl, and PHP. Popular authentication modules include mod_access, mod_auth, and mod_digest. A sample of other features include SSL and TLS support (mod_ssl), a proxy module, a useful URL rewriter (also known as a rewrite engine, implemented under mod_rewrite), custom log files (mod_log_config), and filtering support (mod_include and mod_ext_filter).

Popular compression methods on Apache include the external extension module, mod_gzip, implemented to help with reduction of the size (weight) of web pages served over HTTP. Apache logs can be analyzed through a web browser using free scripts such as AWStats/W3Perl or Visitors.

Virtual hosting allows one Apache installation to serve many different actual websites. For example, one machine, with one Apache installation could simultaneously serve www.example.com, www.test.com, test47.test-server.test.com, etc.

Apache features configurable error messages, DBMS-based authentication databases, and content negotiation. It is also supported by several graphical user interfaces (GUIs) which permit easier, more intuitive configuration of the server.

Usage

Apache is primarily used to serve both static content and dynamic Web pages on the World Wide Web. Many web applications are designed expecting the environment and features that Apache provides.

Apache is the web server component of the popular LAMP web server application stack, alongside MySQL, and the PHP/Perl/Python programming languages.

Apache is redistributed as part of various proprietary software packages including the Oracle Database or the IBM WebSphere application server. Mac OS X integrates Apache as its built-in web server and as support for its WebObjects application server. It is also supported in some way by Borland in the Kylix and Delphi development tools. Apache is included with Novell NetWare 6.5, where it is the default web server.

Apache is used for many other tasks where content needs to be made available in a secure and reliable way. One example is sharing files from a personal computer over the Internet. A user who has Apache installed on their desktop can put arbitrary files in the Apache's document root that can then be shared.

Programmers developing web applications often use a locally installed version of Apache in order to preview and test code as it is being developed.

Microsoft Internet Information Services (IIS) is the main competitor to Apache, trailed by Sun Microsystems' Sun Java System Web Server and a host of other applications such as Zeus Web Server. Some of the biggest web sites in the world are run using Apache. Google's search engine front end is based on a modified version of Apache, named Google Web Server (GWS). Wikimedia projects, including Wikipedia are also run on Apache servers.

License

Main article: Apache License

The software license under which software from the Apache Foundation is distributed is a distinctive part of the Apache HTTP Server's history and presence in the open source software community. The Apache License allows for the distribution of both open and closed source derivations of the source code.

The Free Software Foundation does not consider the Apache License to be compatible with version 2 of the GNU General Public License (GPL) in that software licensed under the Apache License can't be integrated with software that is distributed under the GPL:

This is a free software license but it is incompatible with the GPL. The Apache Software License is incompatible with the GPL because it has a specific requirement that is not in the GPL: it has certain patent termination cases that the GPL does not require. We do not think those patent termination cases are inherently a bad idea, but nonetheless they are incompatible with the GNU GPL. - http://www.gnu.org/philosophy/license-list.html

However, version 3 of the GPL includes a provision (Section 7e) which allows it to be compatible with licenses that have patent retaliation clauses, including the Apache License.

The name Apache is a registered trademark and may only be used with the trademark holder's express permission.