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And after this split of USERS table, the system has achieved perfectly linear scalability.

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Start with a simple single-write-connection DB, with reporting running off the same DB

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Dealing with transaction isolation is very far from being a picnic

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One of such real-world systems was consistently processing over 30M real-world write transactions/day over one single DB connection, supporting ~100K simultaneous players.

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in real world, after deployment, most of the changes in DB structure are about widening columns and adding the new ones

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For documents and BLOBs, NoSQL is a natural habitat

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The worst case of inconsistency happens when row X in database A is modified (by a Client connected to datacenter A), and the same row X in database B is independently modified too (by a Client connected to datacenter B).

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One way to avoid reports affecting operational DB, is via creating a read-only replica of our main operational DB – and running our reports off that replica.