Immediate update of content on reverse proxy

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http://serverfault.com – I have a couple caching proxy servers in front of my web server. When the content is updated on the web server I want to the old cached content removed, and to have them get the updated content ASAP. My setup: load balancer pointing to three caching proxy servers proxy servers are in front of a single Apache web server. I have looked at Squid, Nginx, a few others, but I don't see any obvious way to accomplish this. I would prefer to have the cache always return the most up-to-date information, with little, or preferably no steps required on the part of the person updating the web site (HowTos)