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HBase is a column database. As a general rule of thumb, you need to know your queries in advance to use a column database effectively. That's because you must structure your data in a manner that reflects the query needs, unlike say a relational database where you just normalise and then construct queries and indexes later. Because of this, it's hard to support directly in Roo. Certainly you could put some abstractions in that would make it possible (eg Google App Engine monitors development-time queries and thus can register indexes; Amazon SimpleDB indexes everything so you can construct SQL-like queries later on etc) but there are trade-offs which generally limit performance or scalability of such models. If you're using a column database, you generally have a sophisticated enough problem to require scale and therefore you'll need to design accordingly.
As an aside, I have done a lot with Cassandra and highly recommend it as a column database. It's the database that underpins Spring UAA for example.
Actually we're trying to use a column database because of the very big amout of data that we could have. The performance of the application is one of the most important things.
We've just tried spring Roo with RDBMS and we were amazed of how developpement can go fast. This is why we're asking if we can use it with a Nosql database. We saw that it's possible with Mongodb but it seems that Mongodb is not as much performent as Cassandra or HBase with large scale data (because of how it uses the memory I guess).
You're answer is clear
Is it possible with Roo to use 2 databases, an RDBMS and Mongodb in the same time?