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

Answer 2

i think you are just crossed up on the status you are using to set that you are processing and to check that you are already processing. first one in sets ThingStatusEnum.Processing, but the next guy is checking for something different - ThingStatusEnum.PreProcessing. because ThingStatusEnum.Processing.= ThingStatusEnum,PreProcessing, your locking means two threads are not