I believe what we spanner calls 'external consistency' we call strict serializability. Read-write transactions in FaunaDB are strictly serializable, whereas reads default to serializable. This allows us to serve reads independently from the closest region to the client, cutting down on latency in geo-distributed clusters.

This is a bit weaker than external consistency or strict serializability; Peter Bailis has a good writeup on the meaning of serializability vs strict serializability here: http://www.bailis.org/blog/linearizability-versus-serializab...