Vector Clocks

1. The Pattern

Version each key with a small table of counters, one per writer. Every write increments its writer's counter. Now any two versions can be compared. If one's counters are all greater than or equal to the other's, it knew about the other and supersedes it. If each is ahead somewhere, the writes were concurrent: neither knew about the other. That is a true conflict, detected instead of silently lost.

The quorum lesson ended with an open problem. Two devices wrote the same cart at the same moment