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> 12 web servers with separate MySQL instances local to each sharded on primary key IDs.

I don't understand this part. Hopefully you can clarify this to me.

If you're sharding by primary key, doesn't that mean that there's a high chance that the shard in your local DB instance won't have the data the web server is requesting?

I'm not familiar with DB management.

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Imagine you have a system which services 50 states. In the vast majority of cases, states only look at or mutate information on their own state.

In that case, you can easily split the data between shards based on ranges of an integer key. It's very easy to code, test, deploy and understand such a design.

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