Edge-Side Includes (ESI) is a new XML specification for inclusion of dynamic content. Akamai was one of the developers, and you’re familar with how they proxy content for busy sites like CNN. ESI markup in a document works like XInclude, telling the processor to go get content from somewhere else, like an Akamai proxy server. Going beyond XInclude, it also allows for fail-over handling (if that cache doesn’t reply, use this one, and if that fails, ignore the include request.)
Another thing which ESI adds, is access to the original HTTP request variables (headers, cookies, path info, etc.) So the proxy can do some processing on its own. That makes Akamai more valuable than a place to stash front page photos for CNN.
The other piece of the ESI protocol is a messaging spec for telling proxies when to expire their caches.
The ESI site has a JSP taglib if you want to try playing with this immediately.
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