This PDF is a lot more technical, but reveal some challenges WikiPedia is facing in order to maintain its infrastructure/response time.
WikiPedia is simply the biggest multilingual free-content encyclopedia on the Internet. Over 7 million articles in over 200 languages, and still growing.

[...] Started as Perl CGI script running on single server in 2001, site has grown into distributed platform, containing multiple technologies, all of them open. The principle of openness
forced all operation to use free & open-source software only. Having commercial alternatives out of question, WikiPedia had the challenging task to build efficient platform of freely
available components. [...]

One more time, worth reading if you are into web development, performance and scalability. It seems that lighttpd is more and more used for serving static files (html, images, js, css, pdf ...) instead of the venerable Apache

If you have still some new fresh neuron to burn, you can read the  Overall system architecture and more HERE
The most important news here is that Wikipedia currently uses APC, so I choose the right PHP cache ;-)

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