PRB: QueryTimeout Event Is Not Available (190606)

MORE INFORMATION

The Remote Data Objects (RDO) 2.0 rdoConnection objects expose a QueryTimeout event. For long-running queries, this event fires after QueryTimeout seconds and allows you to cancel the query or to continue for another QueryTimeout seconds, when the event fires again.

ADO does not expose a QueryTimeout event. This article lists some techniques to get similar functionality.

Detecting if a Timeout has Occurred

If you want to detect that a query has timed-out, you can either trap for the run-time error -2147217871 (0x80040E31) in-line in your code (Synchronous queries only), or you can add code to the Connection's ExecuteComplete event and check for adStatus to have a value of two (adStatusErrorsOccurred) and pError.Number of -2147217871 (0x80040E31).

The following code is an example of this:

   If adStatus = adStatusErrorsOccurred Then

     If pError.Number = -2147217871 Then
       Debug.Print "Execute timed-out"
     End If

   End If
				

Cancelling a Long-Running Query

This involves running the query asynchronously with no time-out and using a timer event to simulate the QueryTimeout event. You can then prompt the user and set the Cancel property to TRUE to cancel the query. This does have a limitation over RDO of requiring the Timer event to know about your Recordset object.

Example

WARNING: ANY USE BY YOU OF THE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

NOTE: Since a MsgBox prevents the Execute_Complete message from being received, you have to use a modal form instead to query the user whether to cancel or retry. This is implemented via Form2 and encapsulated by a call to the SafeMsgBox() function.

1. Create a new Visual Basic project and add a reference to the following:

   Microsoft ActiveX Data Objects Library

2. Add a new form (Form2) with a text box and two command buttons:

   Textbox
   Name: txtMessage
   Make this Textbox large enough to display a reasonable message
   
   Command1
   Name: cmdRetry
   Caption: Retry
   
   Command2
   Name: cmdCancel
   Caption: Cancel

3. Add the following code to the code window of Form2:

         Option Explicit
   
         Public fCancel As Boolean
   
         Private Sub cmdCancel_Click ()
           fCancel = True
           Me.Visible = False
         End Sub
   
         Private Sub cmdRetry_Click ()
           fCancel = False
           Me.Visible = False
         End Sub
   					

4. On the default form (Form1) add two command buttons and a Timer control:

   Command1
   Name: cmdDetect
   Caption: Detect Timeout
   
   Command2
   Name: cmdChoose
   Caption: Time-out?

5. Add the following code to Form1's Code Window. Modify the Connection Strings to connect to your SQL Server:

         Option Explicit
   
         Dim WithEvents cn As ADODB.Connection, rs As ADODB.Recordset
   
         Private Sub cmdChoose_Click()
         Dim SQL As String
           Set cn = New ADODB.Connection
           Set rs = New ADODB.Recordset
           cn.Open "dsn=mydsn;database=pubs"  ' *** change connect string ***
           'CommandTimeout is optional; default is 30 seconds.
           cn.CommandTimeout = 15
           '
           ' This query must exceed the Timer1.Interval in order to test.
           '
           SQL = "SELECT authors.* FROM authors, titles a, titles b"
           rs.Open SQL, cn, adOpenKeyset, adLockOptimistic, adAsyncExecute
           Timer1.Interval = 2000
         End Sub
   
         Private Sub cn_ExecuteComplete(ByVal RecordsAffected As Long, _
                                        ByVal pError As ADODB.Error, _
                                        adStatus As ADODB.EventStatusEnum, _
                                        ByVal pCommand As ADODB.Command, _
                                        ByVal pRecordset As ADODB.Recordset, _
                                        ByVal pConnection As ADODB.Connection)
           If adStatus = adStatusErrorsOccurred Then
             If pError.Number = -2147217871 Then
               Debug.Print "Execute timed-out"
             End If
           End If
           Timer1.Interval = 0   ' turn off timer for async code
           if adStatus = adStatusOK Then
             If pRecordset.State = adStateOpen Then
               '
               ' Execute code now async query has completed.
               '
               Debug.Print "Query Complete."
             End If
           End If
         End Sub
   
         Private Sub cmdDetect_Click()
         Dim SQL As String
           Set cn = New ADODB.Connection
           Set rs = New ADODB.Recordset
           cn.Open "dsn=mydsn;database=pubs"  ' *** change connect string ***
           'The below is set low for demonstration purposes, it is optional.
           cn.CommandTimeout = 2
           SQL = "SELECT authors.* FROM authors, titles a, titles b"
           rs.Open SQL, cn, adOpenKeyset, adLockOptimistic, adAsyncExecute
         End Sub
   
         Private Sub Timer1_Timer()
           Select Case rs.State
   
             Case adStateConnecting, adStateExecuting, adStateFetching
               If SafeMsgBox("Query has timed-out.") = vbCancel Then
                 rs.Cancel
                 Timer1.Interval = 0
               End If
   
             Case Else
               Timer1.Interval = 0 ' catch-all
   
           End Select
         End Sub
   
         Private Function SafeMsgBox(ByVal Message As String) As Long
           Load Form2
           Form2.txtMessage = Message
           Form2.Show vbModal
           SafeMsgBox = IIf(Form2.fCancel, vbCancel, vbRetry)
           Unload Form2
         End Function
   					

6. Run the project and click each of the two buttons.

RESULTS: The cmdDetect code should print a message that the query has timed-out. The cmdChoose code should present you with at least one Cancel/Retry dialog box. NOTE: You may have to substitute a longer-running query depending on your data provider, query complexity, table size, machine speed, and network.