NHibernate multi-threading issue - nhibernate

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.