Yesterday we began our meal at the ETL restaurant with a look at the need to be open convenient hours and serve fresh food. We'll pick up now during the meal.
As courses are brought to the table, if the same food were brought out every time and you had to wade through all menu items each time, that would be an inconvenience. When it comes time for dessert, there is no need to show samples of the main courses again. Just the desserts please. If the ETL is not bringing over just data that it hasn't brought over yet (new or changed data only), it is inefficient and causing wasteful energy and processes.
It's time for the bill.
While a 4-star restaurant is not considered inexpensive, obviously the more of these meal preferences we can achieve at a lower price, the better. The impact on operational systems for an ETL should be minimal as well. For most folks, this means taking the data in the operational batch window. While this works for nightly loads, as previously mentioned many are moving to more frequent loading. And you can't load unless you have extracted. Fortunately, many operational systems being put into use today are "data warehouse aware" and have built-in concurrent operation and load and have capabilities for pushing data out on a frequent, perhaps instant, basis.
Unlike restaurants, where there is none that is inexpensive with fresh food ready exactly when you want it and open 24 X 7, we can strive for an up-to-date ETL capturing changed data only, with minimal impact to the operational system(s) that can be run anytime.
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