HOWTO: Reusing ADO Connections Within MTS Transactions

ID: Q234218

The information in this article applies to:

Microsoft Transaction Server 2.0
Microsoft Visual Studio 6.0 sp1, sp2, sp3
Microsoft Data Access Components version 2.1

SUMMARY

When you use a single ADODB Connection object multiple times within an MTS transaction, other ADO objects must be cleaned up after use. Also, client cursors should be used when possible. If ADO objects are not cleaned up properly ADO may rise an "Unspecified Error" [-2147467259 / 80004005] when executing another operations against the database.

MORE INFORMATION

When within a transaction in MTS, using an open ADO Connection object on multiple operations it is recommends the following:

All open recordsets be either disconnected; or closed and set to nothing before executing other operations,

All Command objects that are not used are set to nothing.

If these steps are not followed ADO might display an "Unspecified Error" or open secondary connections to complete the operations.

This situation arises more frequently when within an MTS transaction.
A simple example of code that FAILS when it is run within an MTS transaction as follows:

This is FAILING code:


   Public Sub DoStuff()
   On Error Goto ErrHandler

    Dim oConn As New ADODB.Connection
    Dim oCmd As New ADODB.Command
    Dim oRS As ADODB.Recordset

    oConn.Open sConnectionString

    Set oCmd.ActiveConnection = oConn
    oCmd.CommandText = "SELECT * FROM Authors"
    oCmd.CommandType = adCmdText
    Set oRS= oCmd.Execute
    '...Operate on Recordset...

    'This FAILS if executed within an MTS transaction with ADO 'Unspecified    Error' message:
    oConn.Execute "INSERT INTO Jobs (job_desc, min_lvl, max_lvl) values (    'edjez',25,100 )"

    GetObjectContext.SetComplete

   Exit Sub
   ErrHandler:
    GetObjectContext.SetAbort
    Err.Raise Err.Number, Err.Source, Err.Description, Err.HelpFile,    Err.HelpContext
   End Sub

The preceding code should be optimized and organized as follows:


   Public Sub DoStuff()
   On Error Goto ErrHandler
    Dim oConn As ADODB.Connection
    Dim oCmd As ADODB.Command
    Dim oRS As ADODB.Recordset

    Set oConn = New ADODB.Connection
    oConn.Open sConnectionString
    'Here we specify cursor locations to adUseClient (3)
    'because we will be reusing the connection afterwards

    Set oCmd = New ADODB.Command
    Set oCmd.ActiveConnection = oConn
    oCmd.CommandText = "SELECT * FROM Authors"
    oCmd.CommandType = adCmdText

    Set oRS= oCmd.Execute
    '...Operate on Recordset...
    '...and we won't be needing it anymore so let's clean up
    oRS.Close
    Set oRS = Nothing
    Set oCmd = Nothing

    oConn.Execute "INSERT INTO Jobs (job_desc, min_lvl, max_lvl) values (    'edjez',25,100 )"

    oConn.Close
    Set oConn = Nothing

    GetObjectContext.SetComplete
   Exit Sub
   ErrHandler:
    GetObjectContext.SetAbort
    Err.Raise Err.Number, Err.Source, Err.Description, Err.HelpFile,    Err.HelpContext
   End Sub

Please note that the error is raised to the caller within the error handler, and that the ADO objects are not created using ObjectContext.CreateInstance but rather the New operators - this is because ADO objects do not need to be created using ObjectContext.CreateInstance to participate in the current transaction (if any). If you are using a custom wrapper for database access (which in turn could be using ADO) you would need to create this wrapper using ObjectContext.CreateInstance for it to participate in the MTS transactions.

REFERENCES

For additional information about , please see the following article(s) in the Microsoft Knowledge Base:

Q186342 HOWTO: Create a 3-Tier App using VB, MTS and SQL Server

Additional query words:

Keywords : kbMTS200 kbVS600 kbVS600sp2 kbVS600sp3 kbGrpCom
Version : WINDOWS:2.1,6.0 sp1, sp2, sp3; winnt:2.0
Platform : WINDOWS winnt
Issue type : kbhowto