Do you guarantee referential integrity in TAO? Last I heard the answer is no; complex queries are not supported and clients would have to do their own work with internal versions to achieve a consistent view on the whole graph (if such a consistent view even exists). But it seems to work fine since global referential integrity doesn't seem to be a big deal; there aren't a lot of cases where dependency chains are long enough or actions quick enough that it matters (e.g. a group admin adds a new user, makes them an admin, who adds an admin, and then everyone tries to remove the admin who added them [always locally appearing as if there is >=1 remaining admin], but causes the group to ultimately have no admins when transactions settle). Run into any fun issues like that?

Contrasting that with Spanner where cache coherency is solved by reading from the recent past at consistent snapshots or by issuing global transactions that must complete without conflict within a brief truetime window. I am guessing the cost of Spanner underneath the social graph would be a bit too much for whatever benefits might be gained, but curious if anyone looked into using something similar.