LDAP is a network protocol. It supports a lot of backends (SQL and NoSQL alike) (openLDAP has nice proxy features) It also supports role based authentication.

LDAP is a network protocol. It supports a lot of backends (SQL and NoSQL alike) (openLDAP has nice proxy features) It also supports role based authentication. But at the opposite of actual NoSQL data bases, even though it has no structures, it is strongly typed and secure. You map your NoSQL structures to structured backend through a mapping. It is awesome but most devs hates it, because they are (at my opinions) ignorants. And developers hates it because they are lazy.