The prefork MPM implements a nonthreaded, preforking Web server. It makes the Web server behave similarly to Apache version 1.x in that it isolates each request and handles it by forking a separate child process. Thus problematic requests cannot affect others, avoiding a lockup of the Web server.

While providing stability with this process-based approach, the prefork MPM consumes more system resources than its counterpart, the worker MPM. The prefork MPM is considered the default MPM for Unix-based operating systems.

	MPMs in This Document
This document assumes Apache is used with the prefork MPM.

The worker MPM provides a multithreaded web server. A thread is a “lighter” form of a process. The advantage of a thread over a process is its lower resource consumption. Instead of only forking child processes, the worker MPM serves requests by using threads with server processes. The preforked child processes are multithreaded.

This approach makes Apache perform better by consuming fewer system resources than the prefork MPM. One major disadvantage is the stability of the worker MPM: if a thread becomes corrupt, all threads of a process can be affected. In the worst case, this may result in a server crash. Especially when using CGI (described in Section 46.5.1.2, “Common Gateway Interface: mod_cgi ”) with Apache under heavy load, internal server errors might occur due to threads unable to communicate with system resources.

Another argument against using the worker MPM with Apache is that not all available Apache modules (see Section 46.5, “Apache Modules”) are thread-safe and thus cannot be used in conjunction with the worker MPM.

	PHP as an Apache Module (mod_php)
Not all available PHP modules are thread-safe. Using the worker MPM with mod_php is strongly discouraged.

Most of the executable files in SUSE Linux Apache have a 2 appended to them. This simplifies differentiation of binary files for a parallel installation of Apache 1.x and Apache 2.x.

Most of the configuration files reside under /etc/apache2. For information about how to change configuration settings, refer to Section 46.3, “Configuration”.

By default, Apache provides various information about its runtime status in the following files:

The physical directory /srv/www/htdocs is the default location from which Apache serves Web pages. It acts as “root directory” for a client request. To publish Web pages with Apache, store the files hierarchically in or under that directory.

A URL like http://www.example.com/index.html refers to /srv/www/htdocs/index.html in the default Apache configuration in SUSE Linux for a domain named example.com.

To be able to develop modules for Apache or compile third-party modules, the package apache2-devel is required along with the corresponding development tools. apache2-devel also contains the apxs2 tools, which are necessary for compiling additional modules for Apache.

The apxs2 binaries are located under /usr/sbin:

apxs2 installs modules so they can be used for all MPMs. The other two programs install modules so they can only be used for the respective MPMs. apxs2 installs modules in /usr/lib/apache2 and apxs2-prefork installs modules in /usr/lib/apache2-prefork.

apxs2 enables the compilation and installation of modules from source code (including the required changes to the configuration files), which creates dynamic shared objects (DSOs) that can be loaded into Apache at runtime. Install a module from source code with the commands cd /path/to/module/source; apxs2 -c -i mod_foo.c. Other options of apxs2 are described in the apxs2(1) man page. The modules should then be activated in /etc/sysconfig/apache2 with the entry APACHE_MODULES as described in Section 46.3.2, “Configuring Apache Manually”.