NHibernate session.flush() fails but makes changes

We have a SQL Server database table that consists of user id, some numeric value, e.g. balance, and a version column.

We have multiple threads updating this table's value column in parallel, each in its own transaction and session (we're using a session-per-thread model). Since we want all logical transaction to occur, each thread does the following:

1. load the current row (mapped to a type).
2. make the change to the value, based on old value. (e.g. add 50).
3. session.update(obj)
4. session.flush() (since we're optimistic, we want to make sure we had the correct version value prior to the update)
5. if step 4 (flush) threw StaleStateException, refresh the object (with lockmode.read) and goto step 1

we only do this a certain number of times per logical transaction, if we can't commit it after X attempts, we reject the logical transaction.

each such thread commits periodically, e.g. after 100 successful logical transactions, to keep commit-induced I/O to manageable levels. meaning - we have a single database transaction (per transaction) with multiple flushes, at least once per logical change.

what's the problem here, you ask? well, on commits we see changes to failed logical objects. specifically, if the value was 50 when we went through step 1 (for the first time), and we tried to update it to 100 (but we failed since e.g. another thread changed it to 70), then the value of 50 is committed for this row. obviously this is incorrect.

What are we missing here?

Well, I do not have a ton of experience here, but one thing I remember reading in the documentation is that if an exception occurs, you are supposed to immediately rollback the transaction and dispose of the session. Perhaps your issue is related to the session being in an inconsistent state?

Also, calling update in your code here is not necessary. Since you loaded the object in that session, it is already being tracked by nhibernate.