Derived attributes has been in ECO for ages. To a SQL-guy derived attributes resemble calculated fields. Derived attributes are just like calculated fields that subscribe to all the values that it is calculated from. So whenever data, seen by the expression, is changed the derived attribute is marked as out of date. And the next time you or your UI tries to read it, the attribute is re-evaluated.

The key thing with derived attributes is that it is NOT calculated each time you access the attribute. If it were performance would suffer. It is calculated (or derived) as few times as theoretical possibly; only when read the first time after a change of anything that the derivation expression looks at.

The concept of derivation relies on the concept of subscription – everything in your domain layer has the ability to signal whenever it is changed (the publisher pattern), and subscribers subscribe to publishers to detect these changes. Although this is a different story I must mention that publishing event to catch changes is a key difference between the ECO approach to implementing a domain layer and the POCO way (plain old c#-objects) like NHibernate and some other frameworks use.