Question

I have a ASP.NET MVC application for which I want to write some stress tests in sense of concurrent database access from multiple threads. I wrote it as a unit test using Parallel.ForEach(), but was not able to make it work since I was getting the following exception most of the time:

There is already an open DataReader associated with this Command which must be closed first

So I simplified the test as much as possible and here it is

[Test]
public void Can_Access_DB_Concurrently()
{
    Parallel.ForEach(Enumerable.Range(0, 9), x =>
    {
        try
        {
            var sessionBuilder = new HybridSessionBuilder();             
            var session = sessionBuilder.GetSession();

             using (ITransaction transaction = session.BeginTransaction())
             {
                 var job = session.Query<Job>().Where(y => y.Name == "TestProject").SingleOrDefault().Name;
                 Trace.WriteLine(Thread.CurrentThread.ManagedThreadId + ": Job name is " + job);
                 transaction.Commit();
             }                
        }
        catch (Exception e)
        {
            Trace.WriteLine(Thread.CurrentThread.ManagedThreadId + ": Exception: " + e.Message);
        }
    });
}

Typical output:

13: Exception: Object reference not set to an instance of an object.
16: Exception: There is already an open DataReader associated with this Command which must be closed first.
9: Exception: There is already an open DataReader associated with this Command which must be closed first.
16: Exception: There is already an open DataReader associated with this Command which must be closed first.
14: Exception: There is already an open DataReader associated with this Command which must be closed first.

The HybridSessionBuilder looks like this:

public class HybridSessionBuilder : ISessionBuilder
{
    private static ISessionFactory _sessionFactory;
    private static ISession _currentSession;

    public ISession GetSession()
    {
        ISessionFactory factory = getSessionFactory();
        ISession session = getExistingOrNewSession(factory);
        return session;
    }

    private ISessionFactory getSessionFactory()
    {
        lock (this)
        {
            if (_sessionFactory == null)
            {
                Configuration configuration = GetConfiguration();
                _sessionFactory = configuration.BuildSessionFactory();
            }

            return _sessionFactory;
        }
    }

    public Configuration GetConfiguration()
    {
        string connectionString = WebConfigurationManager.ConnectionStrings["StagingDatabase"].ConnectionString;

        Configuration configuration = new Configuration();
        configuration = PostgreSQLConfiguration.PostgreSQL82
              .ConnectionString(connectionString)
              .Dialect("NHibernate.Dialect.PostgreSQL82Dialect")
              .UseReflectionOptimizer()
              .AdoNetBatchSize(50)
              .ConfigureProperties(new Configuration());

        configuration.AddMappingsFromAssembly(typeof(Job).Assembly);
        Fluently.Configure(configuration);
        return configuration;
    }

    private ISession getExistingOrNewSession(ISessionFactory factory)
    {
        {
            if (HttpContext.Current != null)
            {
                ISession session = GetExistingWebSession();
                if (session == null)
                {
                    session = openSessionAndAddToContext(factory);
                }
                else if (!session.IsOpen)
                {
                    session = openSessionAndAddToContext(factory);
                }

                return session;
            }

            if (_currentSession == null)
            {
                _currentSession = factory.OpenSession();
            }
            else if (!_currentSession.IsOpen)
            {
                _currentSession = factory.OpenSession();
            }
        }

        return _currentSession;
    }        

    public ISession GetExistingWebSession()
    {
        return HttpContext.Current.Items[GetType().FullName] as ISession;
    }

    private ISession openSessionAndAddToContext(ISessionFactory factory)
    {
        ISession session = factory.OpenSession();
        HttpContext.Current.Items.Remove(GetType().FullName);
        HttpContext.Current.Items.Add(GetType().FullName, session);
        return session;
    }        
}

Apparently I am doing something wrong with this concurrent access, but I am unable to spot the error. Thank you for any advice.

Answer 1

HybridSessionBuilder is storing the ISession in a static member and therefore reusing it for each thread. The simplest solution to fix your tests would be to remove the static keyword from _currentSession. Each instance of HybridSessionBuilder would then provide a different ISession.