Together, the Record Layer and FoundationDB form the backbone of Apple’s CloudKit. We wrote a paper describing how we built the Record Layer to run at massive scale and how CloudKit uses it. Today, you can read the preprint to learn more.
From an anonymous FoundationDB blog post introducing relational database capabilities built atop FoundationDB’s key-value store. The paper about CloudKit (PDF) is also worth a read. CloudKit is Apple’s free at any legitimate scale back-end as a service for all iOS and MacOS apps.