The Data Warehouse ETL Toolkit

Practical Techniques for Extracting, Cleaning, Conforming, and Delivering Data

Ralph Kimball, Joe Caserta

• Éditeur: Wiley
• Année: 2011
• ISBN:
  9781118079683
• Édition: 1
• Sujet: Computer Science
• Sous-sujet: Data Warehousing

• Cowritten by Ralph Kimball, the world's leading data warehousing authority, whose previous books have sold more than 150, 000 copies
• Delivers real-world solutions for the most time- and labor-intensive portion of data warehousing-data staging, or the extract, transform, load (ETL) process
• Delineates best practices for extracting data from scattered sources, removing redundant and inaccurate data, transforming the remaining data into correctly formatted data structures, and then loading the end product into the data warehouse
• Offers proven time-saving ETL techniques, comprehensive guidance on building dimensional structures, and crucial advice on ensuring data quality

PART I

Requirements, Realities, and Architecture

CHAPTER 1

Surrounding the Requirements

Ideally, you must start the design of your ETL system with one of the toughest challenges: surrounding the requirements. By this we mean gathering in one place all the known requirements, realities, and constraints affecting the ETL system. We’ll refer to this list as the requirements, for brevity.

The requirements are mostly things you must live with and adapt your system to. Within the framework of your requirements, you will have many places where you can make your own decisions, exercise your judgment, and leverage your creativity, but the requirements are just what they are named. They are required. The first section of this chapter is intended to remind you of the relevant categories of requirements and give you a sense of how important the requirements will be as you develop your ETL system.

Following the requirements, we identify a number of architectural decisions you need to make at the beginning of your ETL project. These decisions are major commitments because they drive everything you do as you move forward with your implementation. The architecture affects your hardware, software, coding practices, personnel, and operations.

The last section describes the mission of the data warehouse. We also carefully define the main architectural components of the data warehouse, including the back room, the staging area, the operational data store (ODS), and the presentation area. We give a careful and precise definition of data marts and the enterprise data warehouse (EDW). Please read this chapter very carefully. The definitions and boundaries we describe here drive the whole logic of this book. If you understand our assumptions, you will see why our approach is more disciplined and more structured than any other data warehouse design methodology. We conclude the chapter with a succinct statement of the mission of the ETL team.

PROCESS CHECK

Planning & Design: Requirements/Realities → Architecture → Implementation → Test/Release

Data Flow: Haven’t started tracing the data flow yet.

Requirements

In this book’s introduction, we list the major categories of requirements we think important. Although every one of the requirements can be a showstopper, business needs have to be more fundamental and important.

Business Needs

Business needs are the information requirements of the end users of the data warehouse. We use the term business needs somewhat narrowly here to mean the information content that end users need to make informed business decisions. Other requirements listed in a moment broaden the definition of business needs, but this requirement is meant to identify the extended set of information sources that the ETL team must introduce into the data warehouse.

Taking, for the moment, the view that business needs directly drive the choice of data sources, it is obvious that understanding and constantly examining business needs is a core activity of the ETL team.

In the Data Warehouse Lifecycle Toolkit, we describe the process for interviewing end users and gathering business requirements. The result of this process is a set of expectations that users have about what data will do for them. In many cases, the original interviews with end users and the original investigations of possible sources do not fully reveal the complexities and limitations of data. The ETL team often makes significant discoveries that affect whether the end user’s business needs can be addressed as originally hoped for. And, of course, the ETL team often discovers additional capabilities in the data sources that expand end users’ decision-making capabilities. The lesson here is that even during the most technical back-room development steps of building the ETL system, a dialog amongst the ETL team, the data warehouse architects, and the end users should be maintained. In a larger sense, business needs and the content of data sources are both moving targets that constantly need to be re-examined and discussed.

Compliance Requirements

In recent years, especially with the passage of the Sarbanes-Oxley Act of 2002, organizations have been forced to seriously tighten up what they report and provide proof that the reported numbers are accurate, complete, and have not been tampered with. Of course, data warehouses in regulated businesses like telecommunications have complied with regulatory reporting requirements for many years. But certainly the whole tenor of financial reporting has become much more serious for everyone.

Several of the financial-reporting issues will be outside the scope of the data warehouse, but many others will land squarely on the data warehouse. Typical due diligence requirements for the data warehouse include:

Archived copies of data sources and subsequent stagings of data

Proof of the complete transaction flow that changed any data

Fully documented algorithms for allocations and adjustments

Proof of security of the data copies over time, both on-line and off-line

Data Profiling

As Jack Olson explains so clearly in his book Data Quality: The Accuracy Dimension, data profiling is a necessary precursor to designing any kind of system to use that data. As he puts it: “[Data profiling] employs analytic methods for looking at data for the purpose of developing a thorough understanding of the content, structure, and quality of the data. A good data profiling [system] can process very large amounts of data, and with the skills of the analyst, uncover all sorts of issues that need to be addressed.”

This perspective is especially relevant to the ETL team who may be handed a data source whose content has not really been vetted. For example, Jack points out that a data source that perfectly suits the needs of the production system, such as an order-taking system, may be a disaster for the data warehouse, because the ancillary fields the data warehouse hoped to use were not central to the success of the order-taking process and were revealed to be unreliable and too incomplete for data warehouse analysis.

Data profiling is a systematic examination of the quality, scope, and context of a data source to allow an ETL system to be built. At one extreme, a very clean data source that has been well maintained before it arrives at the data warehouse requires minimal transformation and human intervention to load directly into final dimension tables and fact tables. But a dirty data source may require:

Elimination of some input fields completely

Flagging of missing data and generation of special surrogate keys

Best-guess automatic replacement of corrupted values

Human intervention at the record level

Development of a full-blown normalized representation of the data

And at the furthest extreme, if data profiling reveals that the source data is deeply flawed and cannot support the business’ objectives, the data-warehouse effort should be cancelled! The profiling step not only gives the ETL team guidance as to how much data cleaning machinery to invoke but protects the ETL team from missing major milestones in the project because of the unexpected diversion to build a system to deal with dirty data. Do the data profiling up front! Use the data-profiling results to prepare the business sponsors for the realistic development schedules, the limitations in the source data, and the need to invest in better data-capture practices in the source systems. We dig into specific data- profiling and data-quality algorithms in Chapter 4.

Security Requirements

The general level of security awareness has improved significantly in the last few years across all IT areas, but security remains an afterthought and an unwelcome additional burden to most data warehouse teams. The basic rhythms of the data warehouse are at odds with the security mentality. The data warehouse seeks to publish data widely to decision makers, whereas the security interests assume that data should be restricted to those with a need to know.

Throughout the Toolkit series of books we have recommended a role-based approach to security where the ability to access the results from a data warehouse is controlled at the final applications delivery point. This means that security for end users is not controlled with grants and revokes to individual users at the physical table level but is controlled through roles defined and enforced on an LDAP-based network resource called a directory server. It is then incumbent on the end users’ applications to sort out what the authenticated role of a requesting end user is and whether that role permits the end user to view the particular screen being requested. This view of security is spelled out in detail in Data Warehouse Lifecycle Toolkit.

The good news about the role-based enforcement of security is that the ETL team should not be directly concerned with designing or managing end user security. However, the ETL team needs to work in a special environment, since they have full read/write access to the physical tables of the data warehouse. The ETL team’s workstations should be on a separate subnet behind a packet-filtering gateway. If the ETL team’s workstations are on the regular company intranet, any malicious individual on that intranet can qui...