Consistency and Consensus

A precise map of what a group of machines can still promise each other once the network, the clocks, and time itself can’t be trusted: one strong guarantee, and the vote underneath it. The cut phone line (CAP, done properly) and Who holds the pen (consensus & Raft) drew the same two ideas on the System Design boards first; this lesson gives them their formal names.

Before you start. The previous lesson, When Machines Fail, showed why no machine in a group can fully trust the network: a message can be late, lost, or arrive after the machine that sent it has already been declared dead. This lesson asks what a group of such machines can still promise each other, and the answer needs only one more idea: a vote. If a network split forcing a choice between staying current and staying up is new, The cut phone line (CAP, done properly) in System Design shows it in one read.