public interface ICustomSessionLocator : IDependency
{
    ISession GetSession();
}

public class CustomSessionLocator : ICustomSessionLocator, ICustomTransactionManager, IDisposable
{
    private readonly ISessionFactoryBuilder _sessionFactoryBuilder;
    private ISession _session;
    private ITransaction _transaction;
    private bool _cancelled;

    public CustomSessionLocator(ISessionFactoryBuilder sessionFactoryBuilder)
    {
        _sessionFactoryBuilder = sessionFactoryBuilder;
        Logger = NullLogger.Instance;
        IsolationLevel = IsolationLevel.ReadCommitted;
    }

    public ILogger Logger { get; set; }
    public IsolationLevel IsolationLevel { get; set; }

    public ISession GetSession()
    {
        ((ICustomTransactionManager)this).Demand();
        return _session;
    }

    public void Demand()
    {
        EnsureSession();

        if (_transaction == null)
        {
            _transaction = _session.BeginTransaction(IsolationLevel);
        }
    }

    public void RequireNew()
    {
        RequireNew(IsolationLevel);
    }

    public void RequireNew(IsolationLevel level)
    {
        EnsureSession();

        if (_cancelled)
        {
            if (_transaction != null)
            {
                _transaction.Rollback();
                _transaction.Dispose();
                _transaction = null;
            }

            _cancelled = false;
        }
        else
        {
            if (_transaction != null)
            {
                _transaction.Commit();
            }
        }

        _transaction = _session.BeginTransaction(level);
    }

    public void Cancel()
    {
        _cancelled = true;
    }

    public void Dispose()
    {
        if (_transaction != null)
        {
            try
            {
                if (!_cancelled)
                {
                    _transaction.Commit();
                }
                else
                {
                    _transaction.Rollback();
                }
            }
            catch (Exception e)
            {
                Logger.Error(e, "Error while disposing the transaction.");
            }
            finally
            {
                _transaction.Dispose();

                _transaction = null;
                _cancelled = false;
            }
        }

        if (_session != null)
        {
            _session.Dispose();
            _session = null;
        }

    }

    private void EnsureSession()
    {
        if (_session != null)
        {
            return;
        }

        _session = _sessionFactoryBuilder.SessionFactory.OpenSession();
    }
}