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Message Driven Beans and Transaction Attributes

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Hi all,
The EJB 2.0 spec says that Message Driven Beans (MDB) are only allowed a transactional attribute of either Required or NotSupported. The reasons it gives for this is as follows :
1) Since a MDB never gets called by a client so to speak, there is never a pre-existing transactional context, hence, RequiresNew and Supports, both which deal with client transactions, are meaningless to MDBs
2) Also, since Mandatory and Never throw exceptions to clients if the methods are executed in a transaction, and since there is never an MDB client, these 2 are also meaningless.
Now, I understand these 2 points totally. But in the same vein, why allow Required and NotSupported ??? Required also deals with client transactions (if one exists, Required means container will execute the transaction under the client transaction) and NotSupported suspends a client transaction (if one exists)...and since a MDB never has a client, these 2 also should be meaningless in the context of a MDB. So why allow these 2 but not the rest ??
I know that the container, according to the specs, creates a new transaction everytime for a MDB as it has no client, but I do not understand why it allows Requires and NotSupported, but not the others ???
Any ideas ??
mister krabs
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