DATA WAREHOUSING BASICS

As told by Ralph Kimballs in -The Datawarehouse Toolkit

These are the operational systems of record that capture the transactions of the business. The source systems should be thought of as outside the data warehouse because presumably we have little to no control over the content and format of the data in these operational legacy systems. The main priorities of the source systems are processing performance and availability. Queries against source systems are narrow, one-record-at-a-time queries that are part of the normal transaction flow and severely restricted in their demands on the operational system. We make the strong assumption that source systems are not queried in the broad and unexpected ways that data warehouses typically are queried. The source systems maintain little historical data, and if you have a good data warehouse, the source systems can be relieved of much of the responsibility for representing the past. Each source system is often a natural stovepipe application, where little investment has been made to sharing common data such as product, customer, geography, or calendar with other operational
systems in the organization. It would be great if your source systems were being reengineered with a consistent view. Such an enterprise application integration (EAI) effort will make the data warehouse design task far easier.