Example: LDAP-like publication model (semi-structured)
a publication described via a set of topics; a topic described via a set of attributes
set of attributes:
- an attribute has an unique name
- its domain is: numeric, string, enumerated, or hierarchical
Numeric: price string: email
Hierarchical: auction_category
- domain is single or multi-valued.
Single: price, auction_category
Multi-valued: email, description by keywords