04 Sep 1997

Methodology

DataArchitect separates data models into conceptual (CDM) and physical (PDM) layers. Each model is saved as a plain ASCII text file on whatever path you choose. Model files have no more cleverness or multi-user facility than any text file you might open in Notepad.

Product help says "A CDM represents the overall logical structure of a database, which is independent of any software or data storage structure. A conceptual model often contains data objects not yet implemented in the physical databases. It gives a formal representation of the data needed to run an enterprise or a business activity." Hmmm. Not very enlightening. I suggest you take a look instead at our paper on Conceptual, Logical, Physical Models.

We users generally regard DataArchitect's CDM as a logical model the way we have defined that type. In any case, a CDM represents concepts of information organization and not necessarily their actual form in a database. The CDM is not specific to any DBMS product or installation. Those specifications are made in a separate PDM.

In DataArchitect you can generate a PDM from a CDM at any time (presuming you pass a few basic validation checks). This generation demands the choice of a target DBMS product. It then handles translation of conceptual datatypes into the target product's types and constructs the PDM with a new set of symbols for physical database objects (i.e., tables and foreign key references).

Multiple PDMs, perhaps targeted to different DBMS products or different server configurations or different denorlaization, can be generated from the same CDM. It is in the PDM that you create and specialize all implementation objects such as tablespaces, indexes, triggers, declarative constraints, etc. Thus managing the connection of a CDM to one or more PDMs is crucial to working with the product. Yet PowerDesigner does little to assist or automate this management. It is up to the user to associate the appropriate CDM and PDM(s).

Most new users facing this architecture ask "Why should I use the conceptual model?" That's a fair question, especially in that you can model quite well in the PDM (better, on the whole than ERwin at any level). But the CDM provides power in more abstract forms which allow fewer objects to define more model. Read on about data items and inheritances.

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