The preceeding assumed a seperate asynchronous process could be running determining what the best server is for a particular type of request. Other reverse proxy configurations might include:

Squid

The Squid proxy server can be configured to pass through selected URL's though not with nearly the flexibility of mod_rewrite & mod_proxy
Nonetheless, Squid is optimized as a fast caching engine and as such is reasonable choice where one can have it cache static elements but pass through requests that require dynamic content.

Eddie

Eddie is an Open Source load balancing project. One component of Eddie is the quot;Intelligent HTTP Gateway" package. It also allows mapping specific requests to specific servers on the back end. One clever aspect of Eddie's behavior is it can split multiple requests within an HTTP/1.1 persistent connection amongst the back ends!
(see diagram)
We'll revisit Eddie shortly.

Without some kind of failover framework for the front end server, these reverse proxy configurations constitute single points of failure.