Thread safety with Jet has been achieved with the release of DAO 3.5/3.6 in Visual C++ (169395)
The information in this article applies to:
- Microsoft Visual C++, 32-bit Enterprise Edition 5.0
- Microsoft Visual C++, 32-bit Enterprise Edition 6.0
- Microsoft Visual C++, 32-bit Professional Edition 5.0
- Microsoft Visual C++, 32-bit Professional Edition 6.0
- Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q169395 SYMPTOMS Thread safety with Jet has been achieved with the release
of DAO 3.5/3.6, but only for applications built with the _UNICODE switch. To
use DAO 3.5/3.6 in a multi-threaded environment you need to use a class library
(or other access mechanism) that is also thread-safe. Visual C++ offers two
class libraries that use DAO: MFC DAO and the DAO SDK. Only the DAO SDK is
itself thread-safe and therefore can use DAO 3.5/3.6 in a multi-threaded
environment.
NOTE: The DAO SDK is no longer available or supported. It is
recommended that new code not use DAO but instead use Microsoft ActiveX Data
Objects (ADO) for multithreaded applications. CAUSE The Jet engine was originally written as a stand-alone
desktop application, and originally conceived before thread-safety was as
important as it is today. Jet 3.5/4.0 itself is not thread-safe. However DAO
3.5/3.6 can implement thread-safety using Jet 3.5/4.0 within an apartment
model, but only if your application is compiled with the _UNICODE switch and
using the UNICODE version of the DAO SDK.
Even with the DAO SDK and
a UNICODE build, you need to create and terminate your DAO objects within the
same thread. You must do this because Jet itself uses Visual Basic for
Applications, which is also not thread-safe. You will encounter unexpected
behavior if you try to pass DAO objects between threads.
DAO 3.5/3.6
offers thread safety (under UNICODE builds) because it creates a separate
instance of Jet for each thread. RESOLUTION If your application does not use _UNICODE (for example, it
is built using ANSI), it will not be thread-safe and Jet will not behave
correctly in a multi-threaded environment.
In order for your
application to use Jet correctly, the application and any mechanism using Jet
must be thread-safe. There are two class libraries provided by Visual C++ for
accessing DAO/Jet 3.5, but only one is thread- safe for _UNICODE builds.
The DAO SDK library for Jet 3.5/4.0 is thread-safe, mainly because
it has no internal state variables. The DAO SDK is just a very thin wrapper
around the DAO/Jet Automation Server.
MFC DAO, while modified to use
Jet 3.5 if present on your machine, has not been modified to be thread-safe. It
has a number of internal "state" variables that are not synchronized. Further,
calls to critical operations such as database open and close have not been
wrapped in any kind of critical section or lock to prevent re-entrance from
other threads before completed. MFC ODBC, when rendered thread-safe, had a
number of such critical sections "locked" in order to ensure synchronization by
multiple threads. MFC DAO has no such implementation. STATUS This behavior is by design. REFERENCES For more information about creating multi-threaded
applications using DAO 3.5, read the section "DAO and the Apartment-threading
Model" in DAO SDK 3.5 online documentation.
| Modification Type: | Major | Last Reviewed: | 6/3/2005 |
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| Keywords: | kbtshoot kbcode kbDatabase kbprb KB169395 kbAudDeveloper |
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