public class NHibernateMultiTenantSessionSource : ISessionSource {
#region Static Members and Constructor.
static readonly object _FactorySyncRoot = new object();
static readonly ConcurrentDictionary<string, ISessionFactory> _SessionFactories = new ConcurrentDictionary<string, ISessionFactory>();
#endregion
readonly IConfigurationReader _configurationManager;
readonly IAccountAccessor _accountAccessor;
readonly string _connectionStringModel;

  public NHibernateMultiTenantSessionSource(IConfigurationReader configurationManager, IAccountAccessor accountAccessor) {
    _configurationManager = configurationManager;
    _accountAccessor = accountAccessor;
    _connectionStringModel = _configurationManager.GetConnectionString("{Your tenant storage medium connection string template, whatever that is}");
  }

  public ISession CreateSession() {
    var account = _accountAccessor.GetCurrentAccount();
    return account != null ? GetSessionFactory(account).OpenSession() : null;
  }

  private ISessionFactory GetSessionFactory(Account account) {
    ISessionFactory accountSessionFactory;
    lock (_FactorySyncRoot) {
      if (_SessionFactories.TryGetValue(account.GUID, out accountSessionFactory)) {
        return accountSessionFactory;
      }

      var cfg = BuildConfiguration(account);
      accountSessionFactory = cfg.BuildSessionFactory();
      _SessionFactories.TryAdd(account.GUID, accountSessionFactory);
      return accountSessionFactory;
    }
  }

  private Configuration BuildConfiguration(Account account) {
    // build up whatever you need here to connect to your store of choice
    return config;
  }
}