The cut phone line (CAP, done properly)

The law under The copy clerks (database replication) and Splitting the ledger (sharding). Both left you holding copies that can disagree. This is the rule about what you may do when they do.

Before you start. The copy clerks gave you a second book that copies from the first and can fall behind by seconds; splitting the ledger spread the stall’s records over more than one branch. That is the whole prerequisite: two places holding writing meant to say the same thing, and a wire between them.