6.6 Finding Hot Keys and Big Keys

Adding a fourth primary to a three-primary Redis Cluster does nothing for a key that receives 200,000 reads per second. The key still maps to one slot, the slot still lives on one primary, and that primary still serves every one of those reads. Sharding spreads load only when the load is spread across many keys, and two kinds of key break that assumption.

A hot key receives a disproportionate share of requests, so the node that owns it saturates while the other nodes are idle