Wednesday, May 19, 2010

Kimball's Approach is Top-Down

Ralph Kimball's approach to data warehousing is frequently mis-characterized as being "bottom-up." This post aims to clear up that misconception.

Bus Architecture

Kimball's bus architecture (or dimensional data warehouse architecture) is an enterprise architecture.  At its core, a set of conformed dimensions ensure a consistent representation of standard terms and data elements across multiple subject areas.  The conformed dimensions describe important things like products, customers, locations, or anything of significance to the business.

The subject areas  are called data marts.  They represent things like manufacturing, sales, invoicing, receivables and so forth.  Data marts don't need to be implemented all at once.  They can be implemented one at a time, as part of an incremental program. Data marts also don't need to be stored in a single database (although they may.)  When they are stored in different databases, the conformance bus ensures consistency and compatibility.


Kimball advocates planning a set of conformed dimensions as an up-front (i.e. strategic) activity.  The conformance bus then serves as the blueprint for a set of integrated data marts, which can be built on whatever schedule makes the most sense.

Kimball and Ross put it this way:
During the limited-duration architecture phase, the team designs a master suite of standardized dimensions and facts that have uniform interpretation across the enterprise...We then tackle the implementation of separate data marts in which each iteration closely adheres to the architecture.

- From The Data Warehouse Toolkit, Second Edition
by Ralph Kimball and Margy Ross (Wiley, 2002)
Because it begins with an enterprise-level framework, then delivers departmental functionality, this is a top-down approach. 


A bottom-up approach is one that moves in the opposite direction, beginning with a departmental focus and later evolving into one that has an enterprise focus. This occurs when organizations build stand-alone data marts, then later decide to integrate them.  

Stand-alone data marts are designed and built for departmental use, without an enterprise context. They are cheaper in the short-run, offering a fast path to quick results.  Stand-alone data marts also arrive due to mergers and acquisitions, or through packaged software. 

When there is more than one stand-alone data mart, however, they are likely to exhibit incompatibilities and inconsistencies.  They are sometimes labeled "stovepipes." Faced with these inconsistent data marts, some organizations resolve to retrofit them into a conformance framework.  This can be a difficult and expensive process, requiring extensive rework.

When stand-alone data marts are successfully brought into conformance, a bottom-up path has been followed--one that starts with a departmental solution and moves to enterprise capability.   Bottom-up development is cheaper in the short term but more expensive in the long term. 

While the end result may be compatible with Kimball's vision, clearly the route is not.  If this is news to you, you might want to check out his book. (The link appears beneath the quotation above.)  You can also consult posts on data warehouse architectures and common misconceptions.

-- Chris

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