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(6) Unlicensed user, please check to make sure key/value pairs can be handled in both JSON and XML
Second thoughts
(7) I'm thinking that rather than being subclass handlers, all 3 definitions and all 3 requests should be first-class resources. They would each contain all the current superclass fields (except Type) plus their own private fields. That way, we could at some later point incorporate their subclasses (e.g. AgeCohortDefinition, EncounterDatasetDefinition, PeriodIndicatorReportDefinition) into the model so they can be CRUDed through the REST interface.
(8) Re key-value pairs and (6) above, I am thinking that DatasetDefintions[] and Datasets[] can be represented as arrays of links, with the "rel" field being the key and the "uri" field being the uri of the resource. DataRows[] can be as Darius proposes.