Data warehousing is the process of taking data from legacy and transaction database systems and transforming it into organized information in a user-friendly format to encourage data analysis and support fact-based business decision making. The process that involves transforming data from its original format to a dimensional data store accounts for at least 70 percent of the time, effort, and expense of most data warehouse projects. After implementing many data warehouses, we’ve determined that a data warehouse should have the following definition:
A data warehouse is a system that extracts, cleans, conforms, and delivers source data into a dimensional data store and then supports and implements querying and analysis for the purpose of decision making.
We’ve come up with this definition to alleviate confusion about data warehouse implementation costs. Historically, the most visible part of a data warehouse project is the data access portion—usually in the form of products—and some attention is brought to the dimensional model. But by spotlighting only those portions, a gaping hole is left out of the data warehouse lifecycle. Whenit comes time to make the data warehouse a reality, the data access tool can be in place, and the dimensional model can be created, but then it takes many months from that point until the data warehouse is actually usable because the ETL process still needs to be completed.
By bringing attention to building the back room data management component, data warehouse sponsors are better positioned to envision the real value of the data warehouse—to support decision making by the end users—and allot realistic budgets to building data warehouses.
Unanticipated delays can make the data warehouse project appear to be a failure, but building the ETL process should not be an unanticipated delay. The data warehouse team usually knows that the ETL process consumes the majority of the time to build the data warehouse. The perception of delays can be avoided if the data warehouse sponsors are aware that the deployment of the data warehouse is dependent on the completion of the ETL process. The biggest risk to the timely completion of the ETL system comes from encountering unexpected data-quality problems.
What the Data Warehouse Is Not
What constitutes a data warehouse is often misunderstood. To this day, you can ask ten experts to define a data warehouse, and you are likely to get ten different responses. The biggest disparity usually falls in describing exactly what components are considered to be part of the data warehouse project. To clear up any misconceptions, anyone who is going to be part of a data warehouse team, especially on the ETL team, must know his or her boundaries.
The environment of a data warehouse includes several components, each with its own suite of designs, techniques, tools, and products. The most important thing to remember is that none of these things alone constitutes a data warehouse. The ETL system is a major component of the data warehouse, but many other components are required for a complete implementation. Throughout our experiences of implementing data warehouses, we’ve seen team members struggling with the same misconceptions over and over again. The top five things the data warehouse is mistaken to be are as follows:
Industry Terms Not Used Consistently
In this section, we call out industry terms that are given different meanings by different writers. There is probably no realistic hope of getting the industry to settle on uniform definitions of these terms, but at least we can take a clear stand on how we use the terms here.
Other authors frequently define a data mart as an aggregated set of data prebuilt to answer specific business questions for a given department. Of course, thisdefinition contains its own criticism! Here and in our writings for thelast decade, we have consistently defined a data mart as a process-orientedsubset of the overall organization’s data based on a foundation of atomicdata, and that depends only on the physics of the data-measurement events,not on the anticipated user’s questions. Note the differences among datamart definitions:
Our data marts (call them dimensional data marts) look the same to all observers and would be implemented identically by anyone with access to the underlying measurement events. Furthermore, since dimensional data marts are always based on the most atomic data, these data marts are impervious to changes in application focus; by definition, they contain all the detail that is possible from the original sources. Data marts constructed according to the misguided definitions will be unable to handle changing business requirements because the details have been pre summarized.
Enterprise Data Warehouse (EDW)
EDWis sometimes used as the name the name for a specific design approach (as contrasted with the uncapitalized enterprise data warehouse, which refers generically to the data warehouse assets of a large organization). Many people also refer to the EDW as the CIF, or Corporate Information Factory. EDW embodies a number of related themes that need to be contrasted individually with the DW Bus approach. It may be helpful to separate logical issues from physical issues for a moment.
Logically, both approaches advocate a consistent set of definitions that rationalize the different data sources scattered around the organization. In the case of the DW Bus, the consistent set of definitions takes the form of conformed dimensions and conformed facts. With the EDW approach, the consistency seems much more amorphous. You must take it on faith that if you have a single, highly normalized ER model of all the enterprise’s information, you then know how to administer hundreds or thousands of tables consistently. But, overlooking this lack of precision, one might argue that the two approaches are in agreement up to this point. Both approaches strive to apply a unifying coherence to all the distributed data sources.
Even if we have a tenuous agreement that both approaches have the same goal of creating a consistent representation of an organization’s data, as soon as you move into physical design and deployment issues, the differences between the EDW and the DW Bus become really glaring.
Conformed dimensions and conformed facts take on specific forms in the DW Bus architecture. Conformed dimensions have common fields, and the respective domains of the values in these fields are the same. That guarantees that you can perform separate queries on remote fact tables connected to these dimensions and you will be able to merge the columns into a final result. This is, of course, drill across. We have written extensively on the steps required to administer conformed dimensions and conformed facts in a distributed data warehouse environment. We have never seen a comparable set of specific guidelines for the EDW approach. We find that interesting because even in a physically centralized EDW, you have to store data in physically distinct table spaces, and that necessitates going through the same logic as the replication of conformed dimensions. But we have never seen systematic procedures described by EDW advocates for doing this. Which tables do you synchronously replicate between table spaces and when? The DW Bus procedures describe this in great detail.
The denormalized nature of the dimensions in the DWBus design allows us to administer the natural time variance of a dimension in a predictable way (SCD types 1, 2, and 3). Again, in the highly normalized EDW world, we have not seen a comparable description of how to build and administer the equivalent of slowly changing dimensions. But it would seem to require copious use of time stamps on all the entities, together with a lot more key administration than the dimensional approach requires. By the way, the surrogate key approach we have described for administering SCDs actually has nothing to do with dimensional modeling. In an EDW, the root table of a normalized, snowflaked dimension would have to undergo exactly the same key administration (using either a surrogate key or a natural key plus a date) with the same number of repeated records if it tracked the same slowly changing time variance as the DW Bus version. The denormalized nature of dimensions in the DW Bus design allows a systematic approach to defining aggregates, the single most powerful and cost effective way to increase the performance of a large data warehouse. The science of dimensional aggregation techniques is intimately linked to the use of conformed dimensions. The shrunken dimensions of an aggregate fact table are perfectly conformed subsets of the base dimensions in the DW Bus architecture. The EDW approach, again, has no systematic and documented approach for handling aggregates in the normalized environment or giving guidance to query tools and report writers for how to use aggregates. This issue interacts with drilling down, described in a moment.
Most important, a key assumption built into most EDW architectures is that the centralized data warehouse releases data marts. These data marts are often described as built to answer a business question, as described in the previous subsection on data-mart definitions. A final, unworkable assumption of the EDW is that if the user wants to ask a precise question involving atomic data, he or she must leave the aggregated dimensional data mart and descend into the 3NF atomic data located in the back room. EVERYTHING is wrong with this view in our opinion. All of the leverage we developed in the DW Bus is defeated by this two level architecture: drilling down through conformed dimensions to atomic data; uniform encoding of slowly changing dimensions; the use of performance-enhancing aggregates; and the sanctity of keeping the back room data-staging area off limits to query services.
Resolving Architectural Conflict: The Hybrid Bus Approach
Is it possible to reconcile the two architectural approaches? We think so. Throughout here, we support the judicious use of normalized data structures for data cleaning. A really dirty data source benefits from the discipline of enforcing the many-to-1 relationships brought to the surface by the process of normalization. THEN we urge the ETL team to convert any such normalized structures into simple dimensional structures for the conforming and final handoff steps. This includes the atomic base layer of data. At this point, an IT organization that has already invested in normalized physical structures can leverage that investment. We can call this the Hybrid Bus Approach.
How the Data Warehouse Is Changing
As we write this, the data warehouse is undergoing a significant change, perhaps the most significant change since the beginning of data warehousing. Everything we have said here is about the data warehouse supporting decision making remains true, but the focus of new development in the data warehouse is in many cases drastically more operational and real time. Although the basic front room and back room components of the data warehouse are still very necessary for these real-time applications, the traditional batch-file-oriented ETL processing is giving way to streaming ETL processing, and the traditional user-driven query and reporting tools are giving way to data-driven and event-driven dashboards. We describe these new developments and how they extend the central concepts of the data warehouse in the following sections.
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