KV: quorum

One book copied by clerks who each run a moment behind. In that picture one book is the master and the clerks follow it. In this build there is no master: three copies, all equal, and a vote decides what counts as saved.

Before you start. KV: the ring built the ring: every key (the name a value is stored under) hashes to a spot on a clock face, and walking clockwise from it gives an ordered list of three different machines, its preference list, which is where the key’s copies live. The copy clerks (database replication) and Replication showed why copies disagree: a copy is updated over a network, a moment later than the original, so for a moment it holds the old value. This lesson decides how many copies you wait for when you write, and how many you ask when you read.