Quorum

1. The Incident

First, a change of level: the last two lessons coordinated different services doing different jobs. This lesson and the next coordinate copies of the same data trying to agree on what it is.

The store's shopping cart has outgrown the primary-replica setup, for one specific reason: failover. When the cart's primary dies, promotion takes about 30 seconds: and for those 30 seconds, nobody in the store can add anything to any cart. For most data, an acceptable pause