Q:

Cassandra db user's password complexity

How can we customize password complexity in Cassandra for database users ? 
Like the password must contains 9 character ,2 special character ,1 upper case ,1 lower case etc. 

A:

There is no such functionality in Cassandra.  Usually this is a responsibility of external authentication system, that is integrated via DSE's Unified Authentication, or recently open sourced LDAPAuthenticator.