My first Publication Agile-Data-Warehouse-Design-eBook | Page 65

44 Chapter 2 Determining the Story Type Knowing the story type helps you to discover more details Recurring events contain a when detail with an every preposition If an event has at least one changeable when detail it is evolving After you have identified all the when details and documented them with example data, you use this information to determine the story type which in turn will give you strong clues about subsequent detail types you can expect especially the how many details. Recurring Event If the event contains a when detail with an every preposition and the example stories confirms that it occurs on a regular periodic basis then the event is recur- ring. If so, it will often contain balance measures. You should check for these when you ask your how many questions. Evolving Event If you have two or more when details you may have an evolving event. If any of the when details are initially unknown and/or can change after the event has been created (and loaded into the data warehouse) then it is definitely an evolving event. If so, you should look out for changeable duration measures that make use of the multiple whens. If an event is evolving you should ask the stakeholders for example stories that illustrate the initial and final states—the emptiest event story possible, and a fully completed one—to help explain how the event evolves. Imagine for a moment that the stakeholders had responded to your additional when question with: Orders are delivered on delivery date and paid on payment date. This would make the event evolving if the actual delivery dates and payment dates are unknown when orders are loaded into the data warehouse. Evolving events contain additional verbs that may be modeled as discrete events in their own right Notice that the “on” prepositions for these when details are preceded with the verbs “deliver” and “pay”. These verbs are events in their own right that occur some time after the initial order event. However, if stakeholders respond in this way they view them primarily as order event milestones. Therefore, you should continue to model them as when details of an evolving order event but you may also want to model delivery and payment as separate discrete events too: You would ask “Who delivers what?” and “Who pays what?” to discover if there are important details that will be lost if deliveries and payments are only available at the order level. If stakeholders provide multiple when details, pay attention to the verbs used in prepositional phrases. The multiple verbs can identify a process sequence of related milestone events. These events can be modeled as part of the current evolving event and as discrete events in their own right if you suspect they have more details. See Chapters 4 and 8 for more details on modeling evolving events.