OctoberTime

I've only ever tinkered with tikv, but one advantage that it currently has over fdb is its support of co-processors --the ability for each node in the cluster to run some piece of query logic locally on its own shard of data instead of having all of the data shipped to one node (or even all the way up to the application level) for processing, which is quite bandwidth intensive.

I'd love to know how Apple distributes its CloudKit data across all of (what I assume are) its dozens of globally distributed FoundationDB clusters.

As of version 6.0, a single FoundationDB cluster currently only supports one active, writable 'master' region + one hot failover region. Transactions are all essentially LAN so performance characteristics are great. It doesn't support the slower TiDB/CRDB style active/active configuration where all nodes are writable, data is globally distributed and transactions can span across all nodes in the cluster.

Do they have some kind of scheme whereby ACID is preserved for local transactions only, while far away regions are asynchronously replicated to with less guarantees on data consistency?