vb.net polling Sql server once a second causing timeout - sql

Question

I was asked to do a system that would poll one table in the database every second and if it counters a row that meets a criteria start actions to handle that.
I've done this but every now and then I get a time out exception. I have a WPF application where I have a thread that runs in background. This thread has a loop and sleeps for one second at the end of the loop. The connection to the database is opened inside "using" clause.
Below is my thread sub:
Private Sub PollDatabase()
While m_StopThread = False
Try
Dim listOfRows As List(Of DataObject) = db.GetDataObjects()
... Do something with the rows ...
Catch ex As Exception
m_log.WriteLine(ex.ToString())
End Try
Thread.Sleep(1000)
End While
End Sub
And my SQL function looks like this:
Public Function GetDataObjects() As List(Of DataObject)
Dim result As New List(Of DataObject)
Dim sb As New StringBuilder("... the sql query ...")
Using cnn = New SqlConnection(_connectionString)
cnn.Open()
Using cmd = New SqlCommand(sb.ToString(), cnn)
cmd.CommandTimeout = 0
Using DataReader As SqlDataReader = cmd.ExecuteReader()
Do While DataReader.Read()
... read the columns from table
to the dataobject ...
result.Add(DataObject)
Loop
End Using
End Using
End Using
Return result
End Function
Now what seems randomly my log has a time out exception:
System.Data.SqlClient.SqlException (0x80131904): Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.
...
at System.Data.SqlClient.SqlConnection.Open()
My questions are: is this at all save way of doing this? Or am I doing something fundamentally wrong here? And of course if anyone have a suggestion to fix this issue.
EDIT:
I tried a bit different approach with the SQL function. I'm now opening a connection once when my application starts and dumped the "using" clauses. So my function looks something like this now:
Public Function GetDataObjects() As List(Of DataObject)
Dim result As New List(Of DataObject)
Dim sb As New StringBuilder("... the sql query ...")
_sqlCmd.CommandText = sb.ToString()
Using DataReader As SqlDataReader = _sqlCmd.ExecuteReader()
Do While DataReader.Read()
... fill the list with objects ...
Loop
End Using
Return result
End Function
My log is clean form errors. So is there something wrong opening a connection to the server once in a second as I do with the using?
EDIT:
I've done a lot of testing now to identify the problem. What I discovered is that just connecting multiple times to the server doesn't cause any problems. Neither does adding a select statement after the connection. But when I actually implement a function where is the complete reader part and return my results I run into the time out problems. Here is two examples.
This isn't causing issues:
Private Sub Window_Loaded(sender As System.Object, e As System.Windows.RoutedEventArgs)
Me.DataContext = Me
m_Thread = New Thread(AddressOf ConnectionTestFunction)
m_Thread.IsBackground = True
m_Thread.Start()
End Sub
Private Sub ConnectionTestFunction()
While m_stopThread = False
Try
m_log.WriteLine("GetData (" & m_ThreadCounter & ")")
Using cnn As SqlConnection = New SqlConnection("Data Source=server;Initial Catalog=db;Integrated Security=True;MultipleActiveResultSets=True")
cnn.Open()
Using cmd As SqlCommand = New SqlCommand("SELECT * FROM Data", cnn)
Using DataReader As SqlDataReader = cmd.ExecuteReader()
Do While DataReader.Read()
Loop
End Using
End Using
End Using
Catch ex As Exception
m_log.WriteLine(ex.ToString())
End Try
m_ThreadCounter += 1
Thread.Sleep(1000)
End While
End Sub
This is causing timeout errors:
Private Sub Window_Loaded(sender As System.Object, e As System.Windows.RoutedEventArgs)
Me.DataContext = Me
m_Thread = New Thread(AddressOf ConnectionTestFunction)
m_Thread.IsBackground = True
m_Thread.Start()
End Sub
Private Sub ConnectionTestFunction()
While m_stopThread = False
Try
m_log.WriteLine("GetData (" & m_ThreadCounter & ")")
Dim datarows As List(Of Data) = Me.GetData()
Catch ex As Exception
m_log.WriteLine(ex.ToString())
End Try
m_ThreadCounter += 1
Thread.Sleep(1000)
End While
End Sub
Private Function GetData() As List(Of Data)
Dim result As New List(Of Data)
Using cnn As SqlConnection = New SqlConnection("Data Source=server;Initial Catalog=db;Integrated Security=True;MultipleActiveResultSets=True")
cnn.Open()
Using cmd As SqlCommand = New SqlCommand("SELECT * FROM Data", cnn)
Using DataReader As SqlDataReader = cmd.ExecuteReader()
Do While DataReader.Read()
Dim d As New Data()
d.DataId = DataReader("DataId")
... etc fields about 10 of them ...
result.Add(d)
Loop
End Using
End Using
End Using
Return result
End Function
I'm really happy if anyone have any thoughts about this... I have to admit I'm really confused now.

Answer 1

Probably, your code is taking longer to complete than the default time-out value for the connection is. Try specifying the time-out when you create your Sql Connection. Make sure that it's longer than the time your code needs to complete.

Answer 2

This approach doesn't seem very good.. Instead of pooling, why not react when new data comes? You can use a trigger or SqlDependency?
http://dotnet.dzone.com/articles/c-sqldependency-monitoring