They may refer to FoundationDB layers. [1] While Spanner may be built on a key-value store equivalent, Google does not expose it as a service. [1] https://appl

They may refer to FoundationDB layers. [1] While Spanner may be built on a key-value store equivalent, Google does not expose it as a service. [1] https://apple.github.io/foundationdb/layer-concept.html https://apple.github.io/foundationdb/layer-concept.html