Hubs, Spokes and Buses: How to Get to a Better Data Warehouse - InformationWeek
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3/10/2008
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Hubs, Spokes and Buses: How to Get to a Better Data Warehouse

Are you frustrated by the inefficiency, rigidity and latency of a first-generation, hub-and-spoke-architecture data warehouse? Here's a six-step guide to evolving to a streamlined, robust Kimball Dimensional Bus Architecture that will reduce time to reporting, lower data latency, and deliver more detailed, analytically useful information.

Now that data warehousing has become ubiquitous in the corporate world, "clean slate" designs of entirely new decision support systems are becoming somewhat rare. Instead, designs and roadmaps are more often intended to improve, modernize or otherwise evolve existing production data warehouses that may be showing signs of age.

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Hub-and-Spoke Architecture
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Many first-generation data warehouses have a hub-and-spoke architecture, an approach that presents aggregated and departmentally-aligned information to end users in dimensional form. These "departmental data marts" can be either centralized or distributed, but a foundational tenet of this approach is that they must be sourced from a centralized and normalized repository — often referred to as a Third Normal Form Data Warehouse (3NF DW), as depicted in the diagram at right — that contains the most detailed information available. In this architecture, Sales, Inventory and Payment information might appear in several departmental data marts, aggregated as needed by each department and reflecting each department's preferences for metric calculations or dimensional attribution.

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Kimball Bus Architecture
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The Kimball Dimensional Bus Architecture, in contrast, makes the most detailed data directly available to end users (also in dimensional form) but in a business-process-aligned (rather than departmentally aligned) manner. Thus, Sales, Inventory and Payment information is stored only once — each in a single Business Process Dimensional Model, as pictured at right — rather than appearing in various incarnations within several hub-and-spoke departmental data marts. Dimensions are conformed in an organization-wide manner, tied to atomically-grained and single-business-process facts, allowing users to creatively sum the data any which way, using any combination of the attributes from the (conformed) dimensions. Special departmental needs are handled by adding department-specific attributes to dimensions, or by creating department-specific measures in the facts, which retains the benefits of a unified cross-department perspective.

Notice that the hub-and-spoke architecture's normalized data warehouse (3NF DW) and its associated ETL are completely eliminated, simplifying and streamlining the solution. (A more thorough discussion of these two data warehousing approaches can be found in the article "Differences of Opinion," by Margy Ross and Ralph Kimball.)

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