Bernard replies to my reply:
Me: My thoughts were that it is easier to transform bare data as it is less to transform. You take the old data and insert it into the new code, which is all you need because you don’t want the old implementation anyway.
Bernard: I guess you are somehow comparing apples and oranges. If you want to compare data storage then instead of storing Java objects with data and business logic, just store some objects having data only. Sure, if you store objects with only data you are quite close to the tabular data model. But remember, I only want to be able to “offer” it for OLAP purposes and for upgrades, if then.
I don’t want to degrade my business objects, I want them to carry as much info as possible. This to ensure that the state is all set and verified before the Command executes, as discussed on the Prevayler site.
To sum up the upgrade story: when moving an application between version, the persistent state needs to be transformed. The less info in the persistent model, the less to transform. That is why I, initially, believe that the tabular data model is good for upgrades. But as mentioned, I’m intrigued by the XSL approach, and we will most certainly try it out.
Me: However, we may have to provide a tabular data model for analysis purposes outside the scope of our application. The standard way to access this is using SQL. This is the only reason I want it.
Bernard: At least if your external application could use JCA you could think of a JCA implementation for Prevayler. Very nice thought. We want to provide as a broad support for data extraction as possible which probably would make a connector a good choice.
Me: This morning Jon had a good way of formulating what I wrote on the two persistence needs: “RDBMS do two things: OnLine Transaction Processing and OnLine Analysis Processing. After Prevayler we only need OLAP.”
Agree. This is an empty room for Prevayler. We might have a look at some integration with Mondrian which is rougly a Java OLAP implementation working in-memory. Mondrian looks way cool. Will look into it more.