Spanner is not a non-relational database.
As for Firestore, it’s not clear whether it supports cross-collection transactions. Cloud Datastore does not support cross-namespace transactions AFAICT.
As for Firestore, it’s not clear whether it supports cross-collection transactions. Cloud Datastore does not support cross-namespace transactions AFAICT.
b) Given that the primary use case for namespaces was/is multitenancy, it's not clear to me why you'd want to transact across them. Nevertheless, you can. What's leading you to draw this conclusion?