COM+ Services Support
This section describes how the OLE DB provider interacts with Microsoft’s COM+ Services.
What is COM+ Services?
COM+ services (Component Object Model) is a Microsoft specific technology used to create business objects in a multi-threaded context. COM+ is primarily designed for, but not limited to, Visual Basic programmers. COM+ allows for rapid creation of multi-tier applications and includes many benefits that would normally have to be implemented by the developer. The benefits of COM+ include
•
contexts
•
concurrency
•
added security
•
object pooling
•
just-in-time activation
•
queued components
•
events
For further information about COM+, see Microsoft’s documentation.
COM+ services are an extension to the benefits provided by MTS (Microsoft Transaction Server). MTS is Microsoft’s previous business object server implementation. In general, references (in Microsoft and PSQL documentation) to MTS can be substituted with COM+ services.
The OLE DB provider is supported in COM+ services. ADO calls made within COM+ services behave like any ADO client call.
Example of COM+ Services for Visual Basic Programmers
As a Visual Basic programmer, you must be aware of the MTSTransactionMode property. Setting this to anything other than NoTransactions will invoke Microsoft transactions. Please refer to COM+ services documentation for the complete reference of this feature.
The following example demonstrates use of Microsoft Transactions. In order for the calls to GetObjectContext to succeed, you must set the MTSTransactionMode property to something other than NoTransactions. Using Microsoft Transactions will allow Microsoft’s Transaction Coordinator to do a two-phase commit.
Public Function UpdatePayroll(employeeID As Integer, salary As Currency)
On Error GoTo ErrHandler
Dim rs As New ADODB.Recordset
rs.Index = "employeeID"
rs.Open "PayrollTable", "Provider=PervasiveOLEDB;Data Source=CompanyDB", adOpenDynamic, adLockOptimistic, adCmdTableDirect
rs.Seek employeeID, adSeekFirstEQ
 
If rs.EOF = True Then
GetObjectContext.SetAbort
Else
rs!salary = salary
End If
rs.Update
rs.Close
rs = Nothing
GetObjectContext.SetComplete
Exit Function
ErrHandler:
GetObjectContext.SetAbort
If Not IsNull(rs) Then
If rs.State = adStateOpen Then
rs.Close
End If
End If
rs = Nothing
End Function
 
However, you could rewrite this business object using ADO transactions (with a connection object). This would allow you to set the MTSTransactionMode property to NoTransactions. Without Microsoft Transactions, you no longer have the overhead of the two-phase commit. Also, objects that do not support transactions are allowed to stay resident in memory, whereas those that do are constructed and destroyed on each reference.
Public Function UpdatePayroll(employeeID As Integer, salary As Currency)
On Error GoTo ErrHandler
Dim cn As New Connection
Dim rs As New ADODB.Recordset
cn.Open "Provider=PervasiveOLEDB;Data Source=CompanyDB"
cn.BeginTrans
rs.Index = "employeeID"
rs.Open "PayrollTable", cn, adOpenDynamic, adLockOptimistic, adCmdTableDirect
rs.Seek employeeID, adSeekFirstEQ
If rs.EOF = True Then
cn.RollbackTrans
Else
rs!salary = salary
End If
rs.Update
rs.Close
cn.CommitTrans
Exit Function
ErrHandler:
cn.RollbackTrans
If Not IsNull(rs) Then
If rs.State = adStateOpen Then
rs.Close
End If
End If
End Function