Question

I have a task that takes quite a long time. So I would like to let several programs/threads/computers execute the same task to speed things up. Each task requires unique ids which are stored in a db – so I thought these ids could be obtained like this:

NHibernateSession.Current.BeginTransaction(IsolationLevel.Serializable);
list = NHibernateSession.Current.CreateCriteria<RelevantId>().SetFirstResult(0).SetMaxResults(500).List<RelevantId>();

foreach (RelevantId x in list)
{
RelevantIdsRepository.Delete(x);
}
NHibernateSession.Current.Transaction.Commit();

Unfortunately, this throws an exception after a while if several processes access the database (nr of deleted objects is not the same as batch size). Why is this? The isolation level of the db should be ok shouldn’t it? Thanks.

Best wishes,

Christian

Answer 1

I’m not sure that I understand what you are doing here. It looks like each process should take some ids and process them but no two processes should take the same.

It doesn’t work like you implemented it. All processes are reading the same ids. After committing the transaction they disappear from the database. Until then, they are visible to everyone. Isolation level only make sure that other transactions can’t read them after they got deleted. But until then, they all can read them.

It’s not so easy to distribute load. You could

• maintain ids in a table where each process is registering itself as the executer and commits it before starting (handling conflicts, eg. StaleObjectStateException). Make sure to clean it up even when a process crashes.
• write a central service which distributes ids.