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Reading this brief example of using memcached with PHP, I was wondering how memcached knows when a request for data needs to actually come from the database instead of coming from the cache.
It doesn’t. It comes down to your caching strategy. That is so with all forms of cache, a tradeoff between getting the latest data and getting some data quickly. If you need to have the data up to date, invalidate (delete) the cache when updating the original. If performance is more important, let the cache expire by itself, at which point it will be renewed. Or something somewhere in-between. It depends on your restrictions and goals.