c# asp.net waiting for state in db to change - asp.net

Question

I am building a web application where a job submitted by user is handled by a background service and the communication between web app and background service is done on database. So when a search starts, web application inserts a record into db and waits till the status field of the record changes. As I understand, if I implement this on the request thread, I am blocking one of the pool threads unnecessarily, but cannot get my head around to do this asynchronously. What is the best practice in this case?

Answer 1

SQLDependency is very useful, but on the server side it provided only half the solution. I overcame the other half by using SignalR to signal the client that the query is over. In the end it was quite an efficient and elegant solution. Thanks for the info!
public class KYHub : Hub
{
void OnChange(object sender, SqlNotificationEventArgs e)
{
SqlDependency dependency = sender as SqlDependency;
dependency.OnChange -= OnChange;
if (e.Info != SqlNotificationInfo.Error && e.Info != SqlNotificationInfo.Delete)
{
string connstr = System.Configuration.ConfigurationManager.ConnectionStrings["cnnStr"].ConnectionString;
SqlConnection conn = new SqlConnection(connstr);
conn.Open();
SearchResult result= SearchResult.Parse(conn, Caller.TaskID);
conn.Close();
Caller.endsearch(result);
}
}
public void Search(SearchParam search)
{
string connstr = System.Configuration.ConfigurationManager.ConnectionStrings["cnnStr"].ConnectionString;
SqlConnection conn = new SqlConnection(connstr);
conn.Open();
search.ClientID = Caller.id;
int QueryID = search.Save(conn);
Caller.TaskID = QueryID;
SqlCommand cmd = new SqlCommand(String.Format("SELECT Completed FROM dbo.Tasks WHERE TaskID={0}", QueryID), conn);
SqlDependency dep = new SqlDependency(cmd);
dep.OnChange += OnChange;
cmd.ExecuteReader();
conn.Close();
}
}

Answer 2

If you are using SQL Server as your DB then you can use a ADO.NET feature called SQLDependency,
What this dose when setup correctly is, when ever there is a change in table (which you will configure) a C# event will be raised.
Here is a article that expains it
Note: You will have to setup your SQL server to enable this.
http://www.codeproject.com/Articles/12335/Using-SqlDependency-for-data-change-events
And this one expains how to refresh page after data in table has changed
http://msdn.microsoft.com/en-us/library/e3w8402y%28v=vs.80%29.aspx