User write-back is a facility which helps users to perform what-if analysis and business modeling using OLAP server capabilities. The data which OLAP gets from the Data-Warehouse relational database is the 'real-actual' data. OLAP has a unique advantage of providing the write-back facility, so that analysis capabilities are not limited to real-life data only, but also is extended to various current and the future scenarios. This is done without impacting the 'real' data lying in the Data-Warehouse. This feature is a unique differentiator between Data Warehouse and OLAP.
How it works is that, when user creates different scenarios in what-if analysis, OR writes the future budgeting scenarios, the data is stored in the OLAP for doing various OLAP operations (just like actual data). As a user changes the data in the scenarios, the data is kept on updating OR/and adding to the OLAP database. Typically OLAP systems take care that this written back data is not over-writing the real data, but is appended to it in terms of new instances.
One needs to go through on OLAP- What-if analysis topic in OLAP Analysis in the subject domain, to understand how this fits into the OLAP tool evaluation.
In-terms of allowing the user OLAP server write-backs, here are typical OLAP capabilities:
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You will typically use user write-back in conjunction with the end-user BI tools. One should be able to have entire OR partial data stored in OLAP and the rest of it to be stored in the end-user BI tools. For example, if you are using excel as an end-user BI tool, you will be able to have some scenarios stored in excel and some of them (which you want to share with other users) in OLAP. An OLAP tool should be able to intelligently allow you to manage these different cuts of storage.
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OLAP should be able to maintain versions and track changes to the value changes (of-course- not to the infinite level) in the user-write back data. For example- a user should be able to specify, if the data related to a scenario, needs to be over-written OR added as a separate scenario.
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OLAP server should be able to allow the tagging of various scenarios in terms of various categories.
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OLAP server should be able to allow the write-backs at any levels of dimensions (refer the chapter of Hierarchies in OLAP section).
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Execution-MiH does not recommend OLAP server to allow write-back on the real-data. However, some products allow it. In this case the write-back needs to be carefully managed.
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