Richard,
Sorry, but you are wrong.
It is possible to have two servers access a single database
in a single transaction. Two phase commit is not required for
this (as there is only a single resource), although XA is
required (so that the TM can determine that the two resources
are in fact the same, in that they have the same XA id).
IAS 4.1.x provides such support out-of-the-box. That is, you
can have any number of EJBs access the same database from any
number of containers, all in a single transaction, and not
need 2PC.
Many people don't realize this is possible because almost every
TM on the market does not support this optimization. For example,
neither Tuxedo nor Encina can pull off this trick.
Of course, if you actually have two resources (that is, an Oracle
and a Sybase) both enlisted in the same transaction, then you
will need 2PC. And of course our product provides this capaibility
too, although it is more expensive due to the logging overhead
required to make the 2PC recoverable.
-jkw
Richard Monson-Haefel wrote:
>
> Although you are access the same database (resource) you will still need application
>servers that support two-phase commit (2PC) in order to execute operations
> across servers within a single transaction. The this is required because you are
>using two separate connections to the database that would otherwise be unaware of
> each other. You need to 2PC to inform the database that these two database
>connections are working together within the same transaction.
>
> It's feasible that a vendor could provide connection sharing through some type of
>database gateway -- that would eliminate the need for 2PC -- but I'm not aware of
> one that does this.
>
> Richard
> --
> Richard Monson-Haefel
> Author of Enterprise JavaBeans, 2nd Edition (O'Reilly 2000)
> Co-Author of Java Message Service (O'Reilly 2000)
> http://www.EjbNow.com
>
> "Maliyackel, Anup Kumar (CTS)" wrote:
>
> > hi
> >
> > I have read that when developinng J2ee applications using two app servers or
> > two databases, the two phase commit protocol is necessary to maintain data
> > consistency.
> >
> > If I am using two app servers , but writing to the same database how
> > important is this?
> >
> > Regards
> > Anup
> >
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