Router

The SPQR Router routes queries to shards and provides the interface for clients to a sharded cluster. Applications should never communicate directly with shards, but rather use the Router to route their queries.

The Router caches metadata from the Coordinator and guesses which shard a transaction should be routed to.

A Router is stateless, meaning it has no persistent state, and is typically run on dedicated resources. Or can be run anywhere else, including on the same host as the application.