In light of your earlier blatant misstatements about Hadoop, I don't think I'm the one with a knowledge problem here.
The original post here is entitled "Scaling Etsy". The ultimate solution that Etsy used, to solve the problem described in those tweets, was to move to sharded MySQL. With no database-side foreign key constraints. This architecture has served them well for over a decade now.
By your rigid definitions, Etsy isn't using an RDBMS, which would seemingly make all your RDBMS-related comments off-topic.
The original post here is entitled "Scaling Etsy". The ultimate solution that Etsy used, to solve the problem described in those tweets, was to move to sharded MySQL. With no database-side foreign key constraints. This architecture has served them well for over a decade now.
By your rigid definitions, Etsy isn't using an RDBMS, which would seemingly make all your RDBMS-related comments off-topic.