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. The copies must agree on what the data is.

The store's shopping cart no longer fits the primary-replica setup, for one specific reason: failover. Failover is what happens when the primary dies and a replica is promoted to replace it. For the cart, promotion takes about 30 seconds. For those 30 seconds, nobody in the store can add anything to any cart