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"Writing cached data back to persistent storage is bad", what is the "right" way to write data? I'm not terribly familiar with distributed systems, just curious. Is this referring to write-back vs. write-through?

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I believe he's suggesting that caches should only be allowed to read from persistent storage and that values read from cache should not be assumed to be current. For example, if I read $FirstName, $LastName from some cache lookup for a user and then go to update $Address I should not write $FirstName and $LastName along with it as a "whole user update" sort of thing; just update the value that you need to.

I think this is referring to write-back caching, where the write operation is confirmed when the cache accepts it, but a separate process is responsible for replicating the write operation from cache to the persistent storage.

Write-through is more widely accepted, since the operation is not confirmed until it's successful on both the cache and the persistent storage.

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