Unfortunately I don't see much material written in English at the moment. To your technical question, I'm not that familiar with FoundationDB to comment, but I

Unfortunately I don't see much material written in English at the moment. To your technical question, I'm not that familiar with FoundationDB to comment, but I can tell the author's intent is building an open source solution for Spanner/F1. So I'd say TiKV/TiDB is really a drop-in replacement for Spanner/F1, not FoundationDB. So in that case, I think the guarantees they want to meet include at least distributed transactions on a KV store(Spanner), and zero downtime SQL schema changes(F1).