I'm not a programmer and don't pretend to be one so I won't be the one to effectively address your COM+ transaction questions. I could address the issue at a very general level, but I am not familiar with all the details of how COM+ or .Net handle transactions.
Generally speaking though, I think it is best to control transactions at the DBMS rather than in the application, that's one of the major functions of the DBMS and ultimately that is where the rubber meets the road so to speak when it comes to transactions. In my experience, controlling transactions at the application layer results in more network traffic and less reliable results.
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If most people are not willing to see the difficulty, this is mainly because, consciously or unconsciously, they assume that it will be they who will settle these questions for the others, and because they are convinced of their own capacity to do this. -Friedrich August von Hayek
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