So, what is the BI semantic Model (BISM)?

BISM is a marketing concept just like ‘UDM’ unified dimension modeling introduced with SQL server 2005. Many people were confused about ‘UDM’. It was just another name for an Analysis Services Cube, but it also represented the nebulous concept of Analysis Services being a semantic layer suitable for all your reporting needs; however people often thought it was a new product that somehow replaced Analysis Services cubes and got themselves tied in all sorts of knots as a result. Thankfully, this terminology died out pretty quickly as everyone started referring to Analysis Services as, well, Analysis Services.

Present days we can see a similar amount of confusion about the term ‘BI Semantic Model’ or BISM for many of the same reason; what is the BI semantic Model exactly? It is Analysis services 2012 plus Power Pivot. Let’s be clear here; it is not just the new Analysis Services tabular model, it’s not even a specific bit of software. Analysis Services 2012 consists of two parts, the Multidimensional Model which is the Analysis Services of Cubes and dimension that we already had in 2008 R2 and earlier version, and the new Tabular model which is the Analysis Services of Table, relationships, in-memory storage and column store. BISM refer to the both models plus Power Pivot, or rather it refers to the way that Analysis Services and Power Pivot can be used as a semantic layer on top of other data for reporting purpose.

So what’s the point of a term like BISM then if it doesn’t refer to something concrete? Why not just call Analysis Services “Analysis Service and Power Pivot “Power Pivot”. BISM is just marketing concept like ‘UDM’, for three reasons:

• A single term like BISM suggests that Analysis Services 2012 and Power Pivot are a single, cohesive product, whereas the tabular and Multidimensional Model are actually very different monsters. If you are going to be working with Analysis Services 2012 on a project the first decision, you will have to make is which type of model to use, and if you change your mind later you will have to start development from scratch and learn lot of new skills. We can think that one day the two models will merge but it won’t happen soon.

• Microsoft has currently identified that many people what do BI but due to the complexity of building Multidimensional models. The simplicity of Tabular Model goes long way to solving this problem; Tabular also replaced report builder model which were really just a semantic layer people didn’t like SSAS or had valid reason to stay with relational reporting.

• Calling something a “semantic layer” suggests that it’s a nice, light, thin, easy to implement layer on top of your relational data warehouse, with no data duplication involved. In actual fact anyone has used the multidimensional model will know you almost always use MOLAP storage which involves all the data being copied in Analysis Services; and I suspect when people start using the Tabular model they will be using it in Vertipaq mode (where again all the data gets copied into Analysis Services) rather than in DirectQuery mode (where all queries are translated to SQL which is then run against SQL Server).

This is my understanding about BISM and may be different with others understanding but I don’t think using the term BISM is useful in any technical context, just as the term UDM wasn’t in the past, because it is a blanket term for several different things.