If you're going to cache some really important data, you should only cache it in one place. Not two, and certainly not three...while you're at it, provide a way to manually flush the cache, or at least make the logic for when the cache gets flushed pretty obvious. Please don't cache the same thing in three different places (memory, file system, and database), then embed the update checks in a complicated, nested fashion.
And then when you are done not doing all that, make sure that you use something about the cached data itself to determine staleness; not something else that can be totally independent of the data.
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