You need to add code to the BeginTrans, CommitTrans, and RollBackTrans methods of your Source class. BeginTrans should set some kind of bookmark to record the state of the data before a set of changes is made. RollBackTrans should do whatever is necessary to make sure the data is left in this bookmarked state, whether that means undoing the intervening changes or simply not applying them, if they are deferred. CommitTrans should make sure the changes are finally applied. Only one of CommitTrans or RollBackTrans is called to complete a set of changes, and both functions should remove the initial bookmark, whether the changes have been applied or discarded.