Building the Data Warehouse

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As mentioned earlier in the chapter, the business event that triggers a snapshot might be the occurrence of some notable activity, such as the making of a sale,

Figure 3.42 Every snapshot in the data warehouse is triggered by some event.

the stocking of an item, the placing of a phone call, or the delivery of a shipment. This type of business event is called an activity-generated event. The other type of business event that may trigger a snapshot is the marking of the regular passage of time, such as the ending of the day, the ending of the week, or the ending of the month. This type of business event is called a time-generated event.

Whereas events caused by business activities are random, events triggered by the passage of time are not. The time-related snapshots are created quite regularly and predictably.

Components of the Snapshot

Mentioned earlier in this chapter, the snapshot placed in the data warehouse normally contains several components. One component is the unit of time that marks the occurrence of the event. Usually (not necessarily always) the unit of time marks the moment of the taking of the snapshot. The next component of the snapshot is the key that identifies the snapshot. The third normal component of a data warehouse snapshot is the primary, nonkey data that relates to the key. Finally, an optional component of a snapshot is secondary data that has been incidentally captured as of the moment of the taking of the snapshot and placed in the snapshot. As mentioned, sometimes this secondary data is called an artifact of the relationship.

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