Tuesday, February 22, 2005

Proactive Caching

Another question that came up at the Issaquah Ascend class was related to Proactive Caching. Specifically, where is it stored? In case you haven’t heard about it yet, this is one of the interesting new features of Analysis Services 2005 that really changes maintenance strategies from the way you currently keep cubes up-to-date in AS2000. It’s a huge step towards real-time OLAP without the traditional issues associated with ROLAP storage. With proactive caching, you have MOLAP storage just as you always did with AS2000, but now you also have relational storage. (So unlike AS2000, where in theory you can toss your source database and run solely off of the cube – assuming you never wanted to update it again – you must maintain a relationship to the relational source in 2005 in order for proactive caching to work).

As changes to the relational source occur, updates to the MOLAP cache also occur—eventually (based on how you configure triggering events and frequency). However, as with AS2000, you still need to provide a query for the partition that correctly identifies new rows in the relational source. In theory, you could have two versions of the MOLAP cache concurrently - the cache responding to queries and the cache that is being rebuilt to conform to the changes in the relational source. When the rebuild is complete, the obsolete cache goes away.

- Stacia

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