The datacages behaviour is defined in the file conf/meta-data.xml .

The datacage serves two purposes.
It handles automatic 'recommendations', which are instructions
sent by the client to add newly created artifacts to the collection.
From a user perspective, these artifacts mainly represent curves or data
points in the resulting diagrams.
The second task is to let the user add already existing artifacts (i.e.
previous calculations) or new artifacts with access to related data.

Irrelevant of the type of elements (recommendations or user picked data) the
datacage can iterate over possible artifacts by accessing its own database.
Thus, to create a list of matching entries, database queries are used.

In meta-data.xml, database queries are defined as <dc:statement> elements,
for example
                <dc:statement>
                  SELECT id          AS prot_id,
                         description AS prot_description
                  FROM wsts WHERE kind = 1 AND river_id = ${river_id}
                </dc:statement>

As can be seen from the example, the datacage configuration file can maintain
its own stack of variables (${river_id} in above example).