> The part that gets me is that I�ve seen references (in both books and here) of the
>same entity bean (of, say record A) can have multiple instances (all are record A,
>not record A, B, C, etc) referenced by multiple clients. But I have seen no examples
>of such to illustrate the scenarios where having multiple instances of the same bean
>are desirable.
First, forget all that you have learned about OODBMS. OODBMS enforces
single object per identity (database record), while O/R mapping can
support multiple objects viewing the same identity.
EJB is neither an OODBMS, not O/R mapping, not assumes persistence. But
it is modeled after the O/R mapping, and in O/R mapping (as in all RDBMS
activitie) it is desirable to have multiple access to the same identity.
Consider the situation where two customers are interested in buying a
ticket on the same flight. Each customer will have a single instance of
a ticket bean, but both will use the same instance of the flight/plane
bean. They may be served by the same, or by different EJB servers.
> Is this flexibility in EJB (compared to OODB or persistent Java) purely for
>performance optimization?
No, this flexibility is part of the relational database world way of
working. This is something OODBMS never managed to do right and in my
opinion the reason why they never succeeded in the enterprise world.
arkin
>
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