The high level architecture of the Apache web server is not very different from the high level conceptual architecture, in the sense that we still find the same splitting of functionality between the core and the modules.

In addition to the Apache core and the modules we can identify some new small modules. Two of them (ap, regex) are essentially libraries of utility functions, used by both the core and the modules. The third one (os) is the one that ensures the independence of the operating system from the Apache core and the standard modules.

The Apache Module

As you can see the designers of Apache decided to take a modular approach so that anyone can add to the basic functionality of the server without disturbing the basic Core implementation.

• Web servers that can handle simultaneous requests, manage the requests in two ways. Either they start a new process or they start a new thread within a process. Web servers that start a new process on each request are called multi-processed Web servers, while those that start a new thread within the main process are called multithreaded Web servers.
• IIS (Internet Information Services) on the Windows platform is an example of a multithreaded Web server. Apache on a Unix platform is a multi-processed Web server.
• Windows platform lack forking for server client request interaction which put Apache on a Unix platform more efficient. Unix is the software for the hardware architecture of a web based server.

  Apache_Request_Phases.vsdx

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