The user application does not access case data or metadata directly. Instead, it passes a query to the Provider, which presents the data in the form of one or more virtual tables. These tables resemble a standard relational database of tables, columns, and records, but they are not stored in a database and are just a view of the data that is held temporarily in memory. The Provider can process queries written in a subset of SQL syntax, which means that you can use any OLE DB-enabled application to access the data presented by the Provider.

The virtual tables are populated by the CDSC, which reads the case data from the underlying data store, and by the metadata from the MDM Document. The CDSC processes the data and passes the results to the Provider in the form of the virtual tables. These are the tables presented by the Provider. If these tables are modified in any way by the user application, the Provider passes the changes on to the CDSC, which modifies the underlying data appropriately (assuming that the CDSC is write-enabled).

When necessary, the Provider can construct case data “on the fly” by interpreting the metadata. For example, a derived variable need not be stored in the case data, if the formula used to calculate the variable is stored in the metadata. The Provider reads the formula from the metadata and calculates the derived variable dynamically, passing the derived data to the application. Even though the derived variable is not stored in the case data, the Provider presents it to the user application as if it is.

If the project’s metadata is stored in an .mdd file, the Metadata Model loads the project metadata into memory from the .mdd file and uses it to populate an MDM Document. If the project's metadata is stored in another format, the MDSC populates the MDM Document. The MDM Document presents the metadata as an object model that can more easily be accessed by the user. The MDM Document contains all of the metadata associated with a project but presents only the metadata required by the user, filtering out everything else. For example, if you need to work with a particular version of a questionnaire, the MDM Document will present only the texts and variables that were present in that version. The software interfaces of the MDM Document are public so that you can adapt existing applications or create new ones to work with the MDM Document.