SqlDependency OnChange event not firing for SignalR - asp.net

Question

I know there are multiple questions on SO with a near identical subject, but unfortunately, after following countless guides and reading several answers, I'm at a loss to answer why this is occurring as a new user to SignalR / SqlDependencies.
I have an ASP.Net WebForms app that uses SignalR to push a realtime figure to the page. The code executes on initial load, and fires the event, but after that, I cannot get the event to fire when the dependency detects a change using the OnChange event.
I can see the Queue and SPs are created on the server just fine, but I can't ever see the queue receives any notifications when data is either added/removed or updated in the table. I think it may be something to do with the OnChange resubscription, but I'm not entirely sure.
What could be causing the event to not fire after initial load or the notification not be received from SQL?
I've posted all code below:
Hub Code
Namespace SignalR.Models
<HubName("notificationHub")>
Public Class NotificationHub
Inherits Hub
Private notifCount As Integer
<HubMethodName("sendNotifications")>
Public Sub SendNotifications()
Using db As SqlConnection = New SqlConnection(System.Web.Configuration.WebConfigurationManager.ConnectionStrings("aspCreate_fetch2").ConnectionString)
Dim query As String = " SELECT IIF(COUNT(l.[id]) > 99, 99, COUNT(l.[id]))
FROM pla.[lv_test] as l
WHERE 1=1
AND l.[lv_int_id] = 419"
Using sp As SqlCommand = New SqlCommand(query, db)
Dim dependency As SqlDependency = New SqlDependency(sp)
AddHandler dependency.OnChange, New OnChangeEventHandler(AddressOf dependency_OnChange)
sp.Notification = Nothing
Dim dt As DataTable = New DataTable()
db.Open()
If db.State = ConnectionState.Closed Then db.Open()
Dim reader = sp.ExecuteReader()
dt.Load(reader)
If dt.Rows.Count > 0 Then
notifCount = Int32.Parse(dt.Rows(0)(0).ToString())
End If
Dim context = GlobalHost.ConnectionManager.GetHubContext(Of NotificationHub)()
context.Clients.All.ReceiveNotification(notifCount)
End Using
End Using
End Sub
Private Sub dependency_OnChange(sender As Object, e As SqlNotificationEventArgs)
If e.Type = SqlNotificationType.Change Then
SendNotifications()
End If
End Sub
End Class
End Namespace
Global ASAX
Sub Application_Start(sender As Object, e As EventArgs)
Dim sConn = System.Web.Configuration.WebConfigurationManager.ConnectionStrings("redacted1").ConnectionString
' Fires when the application is started
RouteConfig.RegisterRoutes(RouteTable.Routes)
BundleConfig.RegisterBundles(BundleTable.Bundles)
SqlDependency.[Stop](sConn)
SqlDependency.Start(sConn)
End Sub
Private Sub Application_End(ByVal sender As Object, ByVal e As EventArgs)
Dim sConn = System.Web.Configuration.WebConfigurationManager.ConnectionStrings("redacted1").ConnectionString
SqlDependency.[Stop](sConn)
End Sub
JavaScript
$(function () {
var nf = $.connection.notificationHub;
nf.client.receiveNotification = function (notifCount) {
console.log("connection started");
$("#notif-badge").text(notifCount);
}
$.connection.hub.start().done(function () {
nf.server.sendNotifications();
}).fail(function (e) {
alert(e);
});
});

Answer 1

I am no VB or Javascript expert but I believe that your subscription to OnChange is removed immediately after exiting SendNotifications().
In your code, you have the following dependencies:
SqlDependency -> SqlCommand -> SqlConnection
and you're getting attached to SqlDependency object. However, because your SqlConnection is disposed at the end of the method, your subscription is just gone.
Declare your SqlConnection as a private property and keep the connection open. Besides, move the event subscription to a separate initialization method or the constructor to do it only once.
EDIT :
Here is more or less what I have in mind (in C# , sorry ^^ ):
public class DemoSqlSubscriber : Hub
{
readonly string connectionString = System.Web.Configuration.WebConfigurationManager.ConnectionStrings("aspCreate_fetch2").ConnectionString;
private SqlDependency dependency;
public void StartListening()
{
SqlDependency.Start(connectionString);
SqlConnection connection = new SqlConnection(connectionString);
connection.Open();
SqlCommand command=new SqlCommand();
command.CommandText= "SELECT [notification_count] FROM pla.[notif_count]";
command.Connection = connection;
command.CommandType = CommandType.Text;
dependency = new SqlDependency(command);
dependency.OnChange += new OnChangeEventHandler(OnCountChanged);
}
private void OnCountChanged(object s,SqlNotificationEventArgs e)
{
if(e.Type == SqlNotificationType.Change)
{
// Publish
IHubContext<NotificationHub> context = GlobalHost.ConnectionManager.GetHubContext<NotificationHub>();
context.Clients.All.ReceiveNotification(notifCount);
}
}
public void StopListening()
{
SqlDependency.Stop(connectionString);
}
}
Could you try to structure your Hub accordingly in VB. NET and let us know ?

Answer 2

The query for notifications has many restrictions as detailed here. One of the limitations is:
The statement must not use any of the following aggregate functions:
AVG, COUNT(*), MAX, MIN, STDEV, STDEVP, VAR, or VARP.
If the query is invalid for notification subscriptions, the OnChange handler will fire immediately with SqlNotificationType.Invalid.
Below are the SqlNotificationEventArgs property values I get when running a query similar to yours (i.e. with a COUNT aggregate function) when OnChange is invoked:
Info=Invalid, Source=Statement, Type=Subscribe
However, your handler code silently ignores the invalid subscription since it checks for only SqlNotificationType.Change.