I recall a couple years ago that it was rumored that Apple had bought FDB with the intention of replacing Cassandra (and I think, at the time, Apple had the largest Cassandra cluster ever known).

Combined with other statements in this thread, I think that may be true. I remember reading once that iMessage used to be served by Cassandra, but now its served by FDB.

This is all speculation though.

The FDB Record Layer white paper [0], section 8.1, does open with:

> 8.1 New CloudKit Capabilities

> CloudKit was initially implemented using Cassandra as the underlying storage engine.

So it seems this is what happened, for CloudKit at least.

[0] https://www.foundationdb.org/files/record-layer-paper.pdf

Apple's Cassandra footprint has grown to over 100 PB (https://twitter.com/jjirsa/status/1071357976454316033)