6.5 Hash Tags, Resharding, and Choosing a Setup

Sharding a dataset changes how its keys must be designed, because one command can use several keys only when they live in the same slot. The previous lesson ran the user-profile service, with keys such as user:1001, in two ways. Under Sentinel, one primary holds the whole dataset and Sentinels promote a replica when it fails. In a three-primary cluster, each key maps to one of 16,384 hash slots, and each primary owns a range of them.