QueryBackendProvider
The registration SPI of a query storage (design §5.5): under one name, the factories that pair a query backend with its schema provider for each aggregate, one per read model the storage serves.
Hosts collect every provider and route by name, so a new storage only implements its backends and registers a provider; the host needs no change. Several providers may share a name when they supply different read models, as when a storage's snapshot and event-stream backends are configured independently; two providers of one name must not supply the same read model.