How to approach caching in a web application?

this might be a more general question, not just Rails related. I'm about to start working on an application that will need to employ some kind of caching strategy sooner or later.

How to approach caching when programming a web application? Should I think it through up front or is it a matter of a later decisions? And then, how to approach testing of such appli开发者_JAVA百科cation? Do you test it with caching turned on (after all, that's how it's going to be run in production) or off?

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There's a well done serie of screencasts about the subject here: http://railslab.newrelic.com/2009/01/22/introduction whatch all the cache related episodes,they show both concepts and what options you have in rails