7.5 ZooKeeper, etcd, and Choosing a Lock

Order 42 in a checkout service remains the running example. The previous lesson sorted locks by the cost of a double hold: an efficiency lock only prevents wasted work, such as two rebuilds of the cached order summary, while a correctness lock protects data, such as a single charge to the card. It showed that a Redis lock is a lease that a paused or delayed holder can outlive, so it is safe for correctness only with a fencing token that the resource checks.

It also showed that row locks, advisory locks, and unique constraints enforce the rule inside the database that holds the data