Apache has several modules that allow distribution of requests among servers, for redundancy, increased availability, etc.
Reverse proxying + mod_rewrite: There is nothing in Apache that you can not do with
mod_rewrite ... :) This technique consists of having an Apache front-end server acting
as a proxy for the backend servers. You can find more information
here
Mod_redundacy: Takeover web and ip in case of failure. You can find more information
here.
Mod_backhand: Allows seamless redirection of HTTP requests from one web server to another. This redirection
can be used to target machines with under-utilized resources, thus providing fine-grained, per-request load balancing of web
requests. More information at
http://www.backhand.org/.
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