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 October 2004 Netcraft Web Server Survey found that more than 67% 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.

Here are some key features of "Apache2":

Core Enhancements

Authn/Authz

- The bundled authentication and authorization modules have been refactored. There is a new authentication backend provider scheme which greatly eases the construction of new authentication backends.

Caching

- mod_cache, mod_disk_cache, and mod_mem_cache have undergone a lot of changes are now considered production-quality. htcacheclean has been introduced to cleanup mod_disk_cache setups.

Proxying

 The new mod_proxy_balancer module provides load balancing services for mod_proxy. The new mod_proxy_ajp module adds support for the Apache JServ Protocol version 1.3 used by Apache Tomcat.

Regular Expression Library Updated

- Version 5.0 of the Perl Compatible Regular Expression Library (PCRE) is now included.

Smart Filtering

- mod_filter introduces dynamic configuration to the output filter chain. It enables filters to be conditionally inserted, based on any Request or Response header or environment variable, and dispenses with the more problematic dependencies and ordering problems in the 2.0 architecture.

Large File Support

- httpd is now built with support for files larger than 2Gb on modern 32-bit Unix systems. Support for handling >2Gb request bodies has also been added.

Event MPM

- The event MPM uses a seperate thread to handle Keep Alive requests and accepting connections. Keep Alive requests have traditionally required httpd to dedicate a worker to handle it. This dedicated worker could not be used again until the Keep Alive timeout was reached.

SQL Database Support

- mod_dbd, together with the apr_dbd framework, brings direct SQL support to modules that need it. Supports connection pooling in threaded MPMs.

Module Enhancements

mod_authnz_ldap

- This module is a port of the 2.0 mod_auth_ldap module to the 2.2 Authn/Authz framework. New features include using LDAP attribute values and complicated search filters in the Require directive.

mod_info

- Added a new ?config argument which will show the configuration directives as parsed by Apache, including their file name and line number. The module also shows the order of all request hooks and additional build information, similar to httpd -V.

mod_ssl

- Added a support for RFC 2817, which allows connections to upgrade from clear text to TLS encryption.

Program Enhancements

httpd

- A new command line option -M has been added that lists all modules that are loaded based on the current configuration. Unlike the -l option, this list includes DSOs loaded via mod_so.

Module Developer Changes

APR 1.0 API

- Apache 2.2 uses the APR 1.0 API. All deprecated functions and symbols have been removed from APR and APR-Util. For details, see the APR Website.

Connection Error Logging

- A new function, ap_log_cerror has been added to log errors that occure with the client's connection. When logged, the message includes the client IP address.

Test Configuration Hook Added

- A new hook, test_config has been added to aid modules that want to execute special code only when the user passes -t to httpd.

Set Threaded MPM's Stacksize

- A new directive, ThreadStackSize has been added to set the stack size on all threaded MPMs. This is required for some third-party modules on platforms with small default thread stack size.

Protocol handling for output filters

- In the past, every filter has been responsible for ensuring that it generates the correct response headers where it affects them. Filters can now delegate common protocol management to mod_filter, using the ap_register_output_filter_protocol or ap_filter_protocol calls.

Monitor hook added

- Monitor hook enables modules to run regular/scheduled jobs in the parent (root) process.