Which kind information to cache in memory?

Answer 1

Identify parts of your Web content that is static, potentially most popular, large (for instance animations, images). These are usually the parts you should consider first.

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Answer 2

One thing to analyze is if the information in your pages needs to be always up to date or if you could delay the updates holding them in a cache for period of time. This dependes on the kind of site.
There are some real big sites that reaches to the point of having inconsistent data for a while until all cache nodes are updated.

Answer 3

Caching makes sense if and only if you have some (spatial, temporal) locality, such as the most frequent queries (temporal locality). If you have a model where each page is fetched with uniform probability, then caching is a complete nonsense, yes.