Conceptual modeling, or semantic modeling if you like, is a rather nebulous area in data management. There seems to be a lot of agreement that it is needed, some disagreement about what it is, and little understanding of how to do it. Yet I believe we are now at a point where we will be forced to deal with it in a far more serious way than we have in the past.
I define a conceptual model as "a model of business information purely as information without any concern as to how it might be stored as data."
To me a conceptual model is not a data model in any sense because it is not part of any effort to design a data storage solution. It is a model that captures information used in a particular area of the business.
Other definitions of "conceptual model" exist. Confusingly, the ANSI/SPARC definition of "conceptual schema" is something that describes "... all the data items and relationships between them, together with integrity constraints (later). There is only one conceptual schema per database." This is essentially what is commonly called a "logical data model."
Then there is the "conceptual data model," defined by Tom Haughey as "a high level or coarse data model which is preliminary in structure, possibly abstract in content and sparse in attributes, that is intended to represent a business area. It is preliminary in structure because it may contain many-to-many relationships."
I do believe that a conceptual data model has a place in data management, as a preliminary to a logical data model. However, it lacks the detail I would expect of a real conceptual model and suffers from being oriented to a data storage design rather than a full description of a business reality.
Data Models and the Relational Paradigm
There is strong evidence that conceptual models are becoming more important today than they have ever been. Essentially, conceptual models are becoming divorced from traditional data models, and the divorce is likely to be a messy one because of the way that data models and data modelers have grown up since the 1970s.
Data modeling as we know it today is inextricably linked with the relational database paradigm - the way in which the columns of database tables are all "related" together. The relational paradigm is so ubiquitous that data modelers do not realize just how much data modeling presupposes it. And the relational paradigm has been enormously successful. It has been tempting to think, therefore, that a logical data model can truly represent the business - to think that a logical data model is the same as a conceptual model.
Enter Big Data
But now things are changing. The success of columnar databases in ultra-large scale data environments has presented a challenge to the relational paradigm. Of course there is enormous hype about big data, but it is also enough of a reality to demand attention. To use the columnar databases successfully you have to unlearn the relational paradigm. I have seen this on a petabyte-scale project I worked on, and it can be ugly. Once the relational paradigm is jettisoned, data modeling as we have known it goes out the window, too. Yet the need to understand what to make of the data in business terms remains. The challenge of managing big data is to distill it into forms that fit the models that business users have of their information requirements - to distill it into conceptual models. Of course it is also true that data models are needed to design a big data dataspace, but these are also unrelational and must come after detailed conceptual models. The reason is that in big data there is no approximation of the conceptual model and logical data model as there can be in the relational paradigm.
And so it was too in the era before relational. ISAM, VSAM, IMS, ADABASE, IDMS and the prerelational data stores could not be designed using ER-based data modeling techniques built on the foundation of the relational paradigm.
And, if truth be told, this is also true when relational databases use generic patterns to hold data. For instance, I recently spent several weeks producing conceptual models for different types of institutional customer housed in a "party model" generic database. My conceptual models bore no resemblance to the design of the relational data store.
What Data Modeling Cannot Do
If we truly model business information in full detail and compare it to what we find in typical data models, there are significant divergences. There are things we need to represent in conceptual models that either are not represented or cannot be represented in traditional data models. These include:
- Relationships between non-key attributes in an entity. For example, Total Sales Amount is related to Total Sales Amount Currency by the relationship "is denominated in," but the relationship cannot be expressed in a data model.
- The use of code tables. Wherever a code table is used, the physical records it contains represent business concepts that have not been captured and defined in the data model used to design the database in which the code tables are housed. I submit that half or more of the business concepts required for a database can easily exist as code table records and thus be missing from the corresponding data model.
- Levels of abstraction. Total Sales Amount as a column in a database represents a piece of business information. But Date-Time of Last Update is metadata about a record. If an entity contains both attributes that are data and attributes that are metadata, I cannot represent them as being at different levels of abstraction. At best I can use devices such as naming conventions, but these are not really satisfactory.
Conceptual models must capture all business concepts and all relevant relationships. If instances of things are also part of the business reality, they must be captured too. Unfortunately, there is no standard methodology and notation to do this. Conceptual models that communicate business reality effectively require some degree of artistic imagination. They are products of analysis, not of design.
Today, big data environments present us with a gap between data storage design and business information that cannot be bridged in a single type of model. The reality is that this problem has always been present to some degree, even during the heyday of the relational paradigm. How the problem will be solved is a different matter, but it will eventually be solved.
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