But when you have a huge system like this, there's always engine-level rules (e.g. referential integrity) and business-rules that need to apply: for example, a Location-object can't "follow" a Comment and a PM cannot "check-in" to a User, and Human Users can't "friend" a declared bot User (I think?), given their overall design I assume the rules about what objects and associations can and cannot exist are enforced by the Association APIs when an association is created - so rejecting an invalid create-association request is straightforward, but how does it handle ever-changing business-requirements that can retroactively apply to existing objects and associations (e.g. supposing that FB decides to allow Human users to befriend bot users, so suddenly Mark Zuckerberg suddenly now has real Friend associations (shots fired), and then they decide to undo that policy change, how is the rule-change applied? What happens to the existing objects? What happens if two different physical FB servers running different versions of the software want to apply conflicting business rules?
Another thing I don't yet understand about using MySQL (or any SQL-first RDBMS for that matter) as a storage layer for an ostensible object/document system (hierarchical structured data) is that now you run into the object-relational impedance mismatch problem - which has many unpleasant solutions (e.g. using the EAV anti-pattern, which defeats the point of using an RDBMS in the first place - or going all-in with Codd's normalization and ending up with a rigid, strict data model schema design which is very difficult to update when business requirements change (i.e. using a table for each Codd relation (tuple) type to for each TAO object type and having to run ALTER TABLE ADD/DROP COLUMN every time you want to update the TAO object schema - which obviously does not scale. I assume each TAO object can be treated as a keyed blob with type-name (i.e. a 3-tuple of (id, type, blob) ) - in which case using an RDMBS is overkill and introduces many overheads, so why use an RDMBS? Especially as I understand that pretty much every major web-service will disable things like referential integrity constraints for the sake of performance (and you can't have a constraint w.r.t data in a blob column anyway).
...so I assume I'm missing something, but what?