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Q201014: PRB: Blocking Occurs When You Call a VB ActiveX EXE from ASP

Article: Q201014
Product(s): Internet Information Server
Version(s): 4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbASP kbVBp600 kbGrpDSCom kbDSupport kbiis400
Last Modified: 16-JUL-2001

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The information in this article applies to:

- Microsoft Internet Information Server 4.0 
- Microsoft Active Server Pages 
- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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SYMPTOMS
========

When you instantiate an ActiveX EXE component from Active Server Pages (ASP) in
page scope only, by default, all requests to the component are serialized to a
single thread. This may cause contention and blocking.

CAUSE
=====

Visual Basic 6.0 only supports Single-threaded or Apartment-threaded objects. By
default, ActiveX EXE projects are set to the Apartment threading model. Thus, a
Visual Basic ActiveX EXE is subject to the rules that COM imposes on
single-threaded apartment (STA) objects.

If an object that runs in an STA is called concurrently by multiple clients
(regardless of their threading model), COM synchronizes access to the object by
posting window messages to the component's message queue. As a result, the
object only receives one call each time it retrieves and dispatches a
COM-related message.

Although this interference implies some performance penalty, it allows
applications that support different threading models to work together. Thus, all
possible combinations of client and out-of-process component interoperability
are supported.

RESOLUTION
==========

By default, the Instancing property of a Visual Basic ActiveX EXE is set to
MultiUse. Therefore, only one server process is created using the STA model.
Because Visual Basic only supports Single-threaded and STA objects, you cannot
use either Free-threading or Both-threading to create multi-threaded objects.
However, you can achieve concurrent execution as follows:

- Increase the number of threads in the thread pool.
- In the Visual Basic project properties, set the "Thread by object" option.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

1. Use Performance Monitor (PerfMon) to monitor the following counters:

   - Active Server Pages object

      - Requests Queued

      - Requests Executing

      - Sessions Total

   - Process object (for the ActiveX EXE process)

      - Thread Count

2. If you are using a single computer as your client, make sure that you have
  two instances of Internet Explorer (Iexplore.exe) running. This ensures that
  you have two separate client processes.

3. Create a Visual Basic ActiveX EXE named ThreadWaitProject.exe with one class
  named ThreadTest. Accept the default values of the threading model in the
  project properties, and implement the following function:

  Private Declare Function GetCurrentThreadId Lib "kernel32" () As Long
  Private Declare Sub Sleep Lib "kernel32" (ByVal dwMilliseconds As Long)

  Function ThreadWait(nSeconds As Long) As Long
      Sleep nSeconds * 1000
      ThreadWait = GetCurrentThreadId
  End Function

4. Create an ASP page named Threadwait_vb_exe.asp, and paste the following
  script:

  <%
  Option Explicit
  DIM NSec

  Sub TestEXE
  response.write "<H1>OOP VB EXE Threadwait</H1>"
  Dim objTest
  Set objTest = Server.CreateObject("ThreadWaitProjectEXE.ThreadSleep")
  Response.Write "StartTime: " & Now & "<BR>"
  Response.Write "ThreadID: " & objTest.ThreadWait(10) & "<BR>"
  Response.Write "EndTime: " & Now & "<BR>"
  Response.write "Session ID: " & Session.SessionId & "<BR>"
  Set objTest = Nothing
  End Sub

  TestEXE
  %>

5. From two distinct clients (see step 2), request the ASP page
  Threadwait_vb_exe at the same time. Both requests share the same ThreadID,
  and the start and end times of both requests are serialized (that is, the
  second request starts only after the first request finishes). You can use
  PerfMon (see step 1) to see that one request is queued while the other is
  being executed. When you apply the resolution, the ThreadIDs differ for the
  two requests.

Additional query words:

======================================================================
Keywords          : kbASP kbVBp600 kbGrpDSCom kbDSupport kbiis400 
Technology        : kbVBSearch kbiisSearch kbAudDeveloper kbASPsearch kbZNotKeyword6 kbiis400 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version           : :4.0,5.0,6.0
Issue type        : kbprb

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