When to use LDAP over a database?

Question

When should I use LDAP vs. database/key-value-store/column-oriented-database/etc?

Answer 1

In addition to what Preet Sangha has said, you should also note that LDAP is non-transactional. The server can delay updates arbitrarily, so the next read of updated data may not reflect the update. If you have transactional requirements you can't use LDAP; if you don't, you can.