While your personal computer only keeps one copy of a file, large-scale services like Facebook rely on what are called content delivery networks to manage data and distribution.
When faced with this idea, the temptation is to assume that you need some sort of distribution or replication mechanism that will copy the key/value pairs between the machines.
有了这个想法后，还有一种想当然是需要使用某种分配或复制机制来在机器之间复制键/值对。
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Success in the free-copy world is not derived from the skills of distribution since the Great copy Machine in the Sky takes care of that.