NHibernate Concurrency problem

Question

I have a S#arp Architecture app that implements a lightweight queue-processing thing whereby various threads pull entities from a list and set their status to mark the fact that processing has started on those items.

Despite wrapping the start-processing bit in explicit transactions and using a C# lock(), I still get them starting at the same time sometimes.

Do I regret not using MSMQ ... well, yeah, but now this concurrency behaviour has got me baffled. Evidently there's something that I don't understand about NHibernate transactions and flushing. Can you help me out?

Here's the relevant bits of code:

private static object m_lock = new object();

private bool AbleToStartProcessing(int thingId)
{
    bool able = false;
    try
    {
        lock (m_lock)
        {
            this.thingRepository.DbContext.BeginTransaction();
            var thing = this.thingRepository.Get(thingId);
            if (thing.Status == ThingStatusEnum.PreProcessing)
            {
                able = true;
                thing.Status = ThingStatusEnum.Processing;
            }
            else
            {
                logger.DebugFormat("Not able to start processing {0} because status is {1}",
                        thingId, thing.Status.ToString());
            }
            this.thingRepository.DbContext.CommitTransaction();
        }
    }
    catch (Exception ex)
    {
        this.thingRepository.DbContext.RollbackTransaction();
        throw ex;
    }
    if (able)
        logger.DebugFormat("Starting processing of {0}",
                        thingId);
    return able;
}

I would have expected this to guarantee that only one thread could change the status of a 'thing' at one time, but I get this in my logs pretty regularly:

2011-05-18 18:41:23,557 thread41 DEBUG src:MyApp.Blah.ThingJob - Starting processing of 78090
2011-05-18 18:41:23,557 thread51 DEBUG src:MyApp.Blah.ThingJob - Starting processing of 78090

.. and then both threads try and operate on the same thing and create a mess.

What am I missing? Thanks.

edit: changed code to reflect how my logging works in the real version

Answer 1

Setup concurrency in your NHibernate mappings, this post should help you get started.

http://ayende.com/blog/3946/nhibernate-mapping-concurrency