Storage for DataMappers in ASP.NET WebApplication

Question

In Martin Fowler's "Patterns of Enterprise Application Architecture" is described approach for organizing DAL like a set of mappers for entities. Each has it's own IdentityMap storing specific entity.

for example in my ASP.NET WebApplication:

//AbstractMapper - superclass for all mappers in DAL
public abstract class AbstractMapper
{
    private readonly string _connectionString;
    protected string ConnectionString
    {
      get { return _connectionString; }  
    }

    private readonly DbProviderFactory _dbFactory;
    protected DbProviderFactory DBFactory
    {
        get { return _dbFactory; }
    } 

    #region LoadedObjects (IdentityMap)
    protected Hashtable LoadedObjects = new Hashtable();

    public void RegisterObject(long id, DomainObject obj)
    {
        LoadedObjects[id] = obj;            
    }

    public void UnregisterObject(long id)
    {
        LoadedObjects.Remove(id);
    }
    #endregion

    public AbstractMapper(string connectionString, DbProviderFactory dbFactory)
    {
        _connectionString = connectionString;
        _dbFactory = dbFactory;
    }

    protected virtual string DBTable
    {
        get
        {
            throw new NotImplementedException("database table is not defined in class " + this.GetType()); 
        }
    }

    protected virtual T Find<T>(long id, IDbTransaction tr = null) where T : DomainObject
    {
        if (id == 0)
            return null;
        T result = (T)LoadedObjects[id];
        if (result != null)
            return result;
        IDbConnection cn = GetConnection(tr);
        IDbCommand cmd = CreateCommand(GetFindStatement(id), cn, tr);
        IDataReader rs = null;
        try
        {
            OpenConnection(cn, tr);
            rs = cmd.ExecuteReader(CommandBehavior.SingleRow);
            result = (rs.Read()) ? Load<T>(rs) : null;                  
        }
        catch (DbException ex)
        {
            throw new DALException("Error while loading an object by id in class " + this.GetType(), ex);
        }
        finally
        {
            CleanUpDBResources(cmd, cn, tr, rs);
        }
        return result;
    }

    protected virtual T Load<T>(IDataReader rs) where T : DomainObject
    {
        long id = GetReaderLong(rs["ID"]);
        T result = (T)LoadedObjects[id];
        if (result != null) 
            return result;

        result = (T)DoLoad(id, rs);
        RegisterObject(id, result);
        return result;
    }

    // another CRUD here ...
}

// Specific Mapper for entity Account
public class AccountMapper : AbstractMapper
{
internal override string DBTable
{
    get { return "Account"; }
}

public AccountMapper(string connectionString, DbProviderFactory dbFactory) : base(connectionString, dbFactory) { }

public Account Find(long id)
{
    return Find<Account>(id);
}

public override DomainObject DoLoad(long id, IDataReader rs)
{
    Account account = new Account(id);
    account.Name = GetReaderString(rs["Name"]);
    account.Value = GetReaderDecimal(rs["Value"]);
    account.CurrencyID = GetReaderLong(rs["CurrencyID"]);
    return account;
}

// ...

}

The question is: where to store these mappers? How system services (entities) should call mappers?

I decided to create MapperRegistry containing all mappers. So services can call mappers like:

    public class AccountService : DomainService
{
    public static Account FindAccount(long accountID)
    {
        if (accountID > 0)
            return MapperRegistry.AccountMapper.Find(accountID);
        return null;
    }
    ...
}

But where can I store MapperRegistry instance? I see following variants, but don't like any of them:

1. MapperRegistry is global for application (Singleton)

   • Not applicable because of necessity of synchronization in multi-thread ASP.NET application (at least Martin says that only mad can choose this variant)

2. MapperRegistry per Session

   • Seems not so good too. All ORMs (NHibernate, LINQ to SQL, EntityFramework) masters advise to use DataContext (NHibernateSession, ObjectContext) per Request and not to store context in Session.
   • Also in my WebApp almost all requests are AJAX-requests to EntityController.asmx (with attribute ScriptService) returning JSON. And session is not allowed.

3. MapperRegistry per Request

   • There are a lot of separate AJAX calls. In this case life cycle of MapperRegistry will be too small. So the data almost always will be retrieved from database, as a result - low performance.

Dear Experts, please help me with architectural solution.