The reference web server
Apache HTTP Server serves over 60% of websites worldwide. Born in 1995 as a collection of patches applied to the NCSA HTTPd server — hence the name, “a patchy server” — it is now the most widely deployed open source web server and the software that made the growth of the World Wide Web as we know it possible.
The current production version is 1.3.x, stable and mature, running on Unix, Linux and Windows systems. Development of version 2.0, with a new multi-process architecture, is underway.
Prefork architecture
Apache 1.3 uses a concurrency model based on prefork: at startup, the parent process spawns a pool of child processes, each capable of handling one client connection at a time. When a child process finishes serving a request, it becomes available for the next one. The number of active processes is adjusted dynamically based on load.
This model has the advantage of stability: an error in one child process does not affect the others or the parent process. The trade-off is memory consumption, since each process maintains its own address space. For most web workloads, the compromise is acceptable.
Modules and configuration
Apache’s strength lies in its modular architecture. Functionality is distributed across loadable modules that extend the server without modifying its core:
- mod_rewrite: URL rewriting based on regular expressions
- mod_ssl: HTTPS connection support via OpenSSL
- mod_php: PHP script execution within the Apache process
- mod_proxy: reverse proxy and forward proxy capabilities
- mod_vhost_alias: dynamic virtual host management
Virtual hosts allow a single physical server to host hundreds of distinct websites, each with its own domain, document root and access rules. This feature is essential for shared hosting providers.
Configuration is handled through the httpd.conf file and .htaccess files distributed across directories, enabling decentralised configuration changes without restarting the server.
The Apache Software Foundation
Development of Apache HTTP Server is coordinated by the Apache Software Foundation (ASF), established in 1999. The ASF follows a meritocratic governance model: contributors earn commit rights and a voice in decisions based on the quality and consistency of their contributions. No single company controls the project. Decisions are made by consensus on the developer mailing list, with formal voting when consensus cannot be reached.
This model has ensured the continuity and independence of the project, and the ASF has extended it to other Open Source projects hosted under its umbrella — from Jakarta to XML — forming an ecosystem of infrastructural tools for the web.