“Apple really does store billions of databases in their extreme multi-tenant architecture.”
Apple runs over 300,000 Cassandra nodes holding hundreds of petabytes, and still built CloudKit’s record store on FoundationDB. Cassandra only does atomic updates inside one partition, and that slows as partitions grow. Every user gets a separate logical database on shared clusters. The design makes your data easy to move between Apple’s servers and hard to move anywhere else.