Limitations of DAO, DAO SDK in NT Service or with Threads

ID: Q156138


The information in this article applies to:


SUMMARY

With the release of MFC Open Database Connectivity (ODBC) in Visual C++ 4.2, you can use both MFC and the MFC ODBC classes within the multithreaded environment of a Windows NT Service. Neither the MFC Data Access Objects (DAO) classes nor the DAO SDK are thread-safe because the underlying Jet engine is not thread-safe and cannot be used in a Windows NT Service.

The techniques presented here are valid for any multithreaded environment with the MFC ODBC classes, not just a Windows NT Service.

The remainder of this article explores the following three areas relating to using MFC-based database classes within a Windows NT Service:


MORE INFORMATION

How a Windows NT Service Works

A Windows NT Service consists of code that is roughly similar to the following:

   VOID WINAPI Main( VOID )
   {
       ...
       if (!StartServiceCtrlDispatcher(...))
           AddToMessageLog(TEXT("StartServiceCtrlDispatcher failed."));
       ...
   } 
When a service is started, the Service Control Manager waits for the primary thread of the service to invoke a call to StartServiceCtrlDispatcher(). This call triggers the creation of a named pipe whose purpose is to provide a communication channel between the service and the Service Control Manager. The primary thread acts as a control dispatcher for the service. The service receives control requests from the Service Control Manager via the control handler (callback function). In addition, the StartServiceCtrlDispatcher() invokes a secondary thread that contains the entry point of the service as specified by the SERVICE_TABLE_ENTRY structure. If the StartServceCtrlDispatcher() call succeeds, the StartServiceCtrlDispatcher() call in the primary thread does not return until all running services in the process have terminated.

Any attempt to use database classes within a running service must not only be thread-safe, but must also be able to operate in the secondary thread.

How to Use MFC ODBC in a Multithreaded Environment

With the release of Visual C++ 4.2, both MFC and the MFC ODBC database code is safe to use in a multithreaded environment. However, you should keep the following points in mind:

Why MFC DAO or DAO SDK Cannot Be Used in a Multithreaded Environment

DAO version 3.x is a single-threaded in-process server and was originally written for Access 1.x and 2.0 and Visual Basic 3.0, both of which are single-threaded. This means that clients (MFC DAO or DAO SDK) can use DAO via custom interfaces only from the first thread in the process that initialized COM, that is, the "primary" thread.

If the client has already initialized COM in the primary thread and then calls CoCreateInstance in the secondary thread asking for the IDAODBEngine interface, CoCreateInstance fails. This fails because COM sees that DAO is not marked as apartment or free threaded in the registry, calls DllGetClassObject from the primary thread, gets the IDAODBEngine interface, tries to marshal it back to the secondary thread, and fails.

See the Knowledge Base article listed in the REFERENCES section of this article for more information.


REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

Q169395 PRB: Thread Safety for DAO/Jet 3.5

System Data Sources

Q136481 INF: Running ODBC Applications as Windows NT Services

Impersonating a Valid Windows NT User

Q96005 Validating User Accounts (Impersonation)

Using MFC Database Classes in a Console Application

Q152696 HOWTO: Using the MFC Database Classes in Console Applications

Why DAO Cannot Be Used in a Multithreaded Environment

Q151407 PRB: DAO Must Be Used in Primary Thread

Using MFC ODBC with an ISAPI DLL

Q160906 HOWTO: Use MFC 4.2 ODBC Classes in an ISAPI DLL


Keywords          : kbcode kbDAO kbDatabase kbMFC kbODBC kbVC 
Version           : Winnt:4.0,4.1,4.2
Platform          : winnt 
Issue type        : kbhowto 

Last Reviewed: July 9, 1999