What you think about a client-side cookie cache? For example, one could store the rating a user gave for a comment. This will save database-requests. I see many big sites doing it for different data.
Benefits:
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Probably a little faster than other caches like file-cache (no disk-io).
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If a usual cache like XCache/Memcache flushes (eg server restart), the cookie will remain valid.
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Websites with very high load will benefit the most.
Of course this is only useful for very small data since a cookie can’t be bigger than 4kb. But one may setup several cookie-caches for different usage. Usually one will save arrays with id’s/integers.