Here's the basic reason two-hase commit is needed whenever updates are done.
In a ohe-phase commit, all the participants in the commit are told to comiit.
If any of them have a problem., it's too bad. Go back to the log and
reconstruct the old state of the data. IN a two-phase commit, the scenario is
idfferent. In Db2MVS, for example, for which Iuseed to own part of the commit
code, a broadcast is ent out to all the components that need to particiapte,
like log manager, lock manager, etc, to see if they are all ready. If any says
it isn't, the commit is not done. If and only if all the components say they
are ready,is the second phase of the commit done. It'smuch safer, always.
Ken Litwak
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