No, they both matter. The database can handle X shops per partition, and the rails host can handle Y shops per partition.
If rails were half as fast, you'd need twice as many rails hosts (but no more databases).
If rails were half as fast, you'd need twice as many rails hosts (but no more databases).
Sort of. Twice as many rails hosts means more DB connections which generally means more load/memory on the DB or more load/memory on the external connection pooler.
It's only a bit of incremental load, but it's easy to overlook how many other systems need to run to make Rails scale.
Having 1000 connections doing 1 request per second isn't the same as having 1 connection doing 1000 requests per second.
But, and I could be wrong, the larger factor is the number of requests per second.