Getting an error creating a Query object in SubSonic - subsonic

Question

I am getting the following error in one of our environments. It seems to occur when IIS is restarted, but we haven't narrowed down the specifics to reproduce it.
A DataTable named 'PeoplePassword' already belongs to this DataSet.
at System.Data.DataTableCollection.RegisterName(String name, String tbNamespace)
at System.Data.DataTableCollection.BaseAdd(DataTable table)
at System.Data.DataTableCollection.Add(DataTable table)
at SubSonic.SqlDataProvider.GetTableSchema(String tableName, TableType tableType)
at SubSonic.DataService.GetSchema(String tableName, String providerName, TableType tableType)
at SubSonic.DataService.GetTableSchema(String tableName, String providerName)
at SubSonic.Query..ctor(String tableName)
at Wad.Elbert.Data.Enrollment.FetchByUserId(Int32 userId)
Based on the stacktrace, I believe the error is happening on the second line of the method while creating the query object.
Please let me know if anyone else has this problem.
Thanks!
The code for the function is:
public static List<Enrollment> FetchByUserId(int userId)
{
List<Enrollment> enrollments = new List<Enrollment>();
SubSonic.Query query = new SubSonic.Query("Enrollment");
query.SelectList = "userid, prompt, response, validationRegex, validationMessage, responseType, enrollmentSource";
query.QueryType = SubSonic.QueryType.Select;
query.AddWhere("userId", userId);
DataSet dataset = query.ExecuteDataSet();
if (dataset != null &&
dataset.Tables.Count > 0)
{
foreach (DataRow dr in dataset.Tables[0].Rows)
{
enrollments.Add(new Enrollment((int)dr["userId"], dr["prompt"].ToString(), dr["response"].ToString(), dr["validationRegex"] != null ? dr["validationRegex"].ToString() : string.Empty, dr["validationMessage"] != null ? dr["validationMessage"].ToString() : string.Empty, (int)dr["responseType"], (int)dr["enrollmentSource"]));
}
}
return enrollments;
}

Answer 1

This is a threading issue.
Subsonic loads it's schema on the first call of SubSonic.DataService.GetTableSchema(...) but this is not Thread safe.
Let me demonstrate this with a little example
private static Dictionary<string, DriveInfo> drives = new Dictionary<string, DriveInfo>;
private static DriveInfo GetDrive(string name)
{
if (drives.Count == 0)
{
Thread.Sleep(10000); // fake delay
foreach(var drive in DriveInfo.GetDrives)
drives.Add(drive.Name, drive);
}
if (drives.ContainsKey(name))
return drives[name];
return null;
}
this explains well what happens, on the first call to this method the dictionary is empty
If that's the case the method will preload all drives.
For every call the requested drive (or null) is returned.
But what happens if you fire the method two times directly after the start? Then both executions try to load the drives in the Dictionary. The first one to add a drive wins the second will throw an ArgumentException (element already exists).
After the initial preload, everything works fine.
Long story short, you have two choices.
Modify subsonic source to make SubSonic.DataService.GetTableSchema(...) thread safe.
http://msdn.microsoft.com/de-de/library/c5kehkcz(v=vs.80).aspx
"Warmup" subsonic before accepting requests. The technic to achive this depends on your application design. For ASP.NET you have an Application_Start method that is only executed once during your application lifecycle
http://msdn.microsoft.com/en-us/library/ms178473(v=vs.100).aspx
So you can basically put a
var count = new SubSonic.Query("Enrollment").GetRecordCount();
in the method to force subsonic to init the table schema itself.