HOW APPLE BUILT ICLOUD TO STORE BILLIONS OF DATABASES

Summary from the TLDR Web Dev newsletter

Apple's iCloud and CloudKit were built using a combination of Cassandra and FoundationDB. Cassandra, which is being actively improved by Apple engineers, handles exabytes of data and millions of queries. FoundationDB powers CloudKit, Apple's cloud backend service. It provides key features like transactional guarantees and solves scalability issues that Cassandra alone could not address. Each iCloud user gets their own database, resulting in billions of databases. This is made possible by the Record Layer's stateless architecture and resource allocation using a “record store” abstraction.