Two kitchens, two cities (multi-region and active-active)

The cut line between two books. That board cuts the line inside one setup. This lesson cuts it between two whole cities and asks what a company does when the building holding everything goes dark.

Before you start. You need The cut phone line (CAP, done properly): when the wire between two copies of the data goes quiet, a clerk can only answer from her own book (and risk being stale) or refuse until she can check. You need The copy clerks (database replication), where a second book copies from the first and runs a moment behind (lag), and Splitting the ledger (sharding), where the records were cut across more than one book. Every other word here is shown working before it is named.