Existing backend services will be opened for more clients (adding a Web Service Layer to the Web-Frontend client).
Currently, XML Schema is used for validation of DTOs, but since the frontend is out of our control, we will have to move extended tests (eg. interdependency tests) from the frontend layer to a new "validation layer".
Now our question is if there exist a common pattern for this, eg. something similar to using RELAX NG or a rule engine? The requirement is to be able to check datatypes (we would stay with XML Schema here) and interdependencies, but these interdependencies can get quite complicated, ie. they might involve checks on data which is not available in the input data (but in some external system).
So we are looking for a patterns as well as a framework which makes it easy to implements the patterns.