NHibernate ISession Flush: Where and when to use it, and why?

One of the things that get me thoroughly confused is the use of session.Flush,in conjunction with session.Commit, and session.Close.

Sometimes session.Close works, e.g., it commits all the changes that I need. I know I need to use commit when I have a transaction, or a unit of work with several creates/updates/deletes, so that I can choose to rollback if an error occurs.

But sometimes I really get stymied by the logic behind session.Flush. I have seen examples where you have a session.SaveOrUpdate() followed by a flush, but when I remove Flush it works fine anyway. Sometimes I run into errors on the Flush statement saying that the session timed out, and removing it made sure that I didn’t run into that error.

Does anyone have a good guideline as to where or when to use a Flush? I’ve checked out the NHibernate documentation for this, but I still can’t find a straightforward answer.

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