OptimisticConcurrencyException with System.Web.Providers

Question

I'm using the new Universal Providers from Microsoft for session in SQL Server. The old implementation of session on SQL Server required a job (running every minute) to clear expired sessions. The new one does this check and clear on every request. Since I'm actually running in SQL Azure, I don't have SQL Agent to schedule jobs, so this sounds like a reasonable way to go about it (no, I don't want to pay for Azure Cache for session).

The problem is when multiple users access the site at the same time, they're both trying to clear the same expired sessions at the same time and the second gets an optimistic concurrency exception.

System.Data.OptimisticConcurrencyException: Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries.
   at System.Data.Mapping.Update.Internal.UpdateTranslator.Update(IEntityStateManager stateManager, IEntityAdapter adapter)
   at System.Data.Objects.ObjectContext.SaveChanges(SaveOptions options)
   at System.Web.Providers.DefaultSessionStateProvider.PurgeExpiredSessions()
   at System.Web.Providers.DefaultSessionStateProvider.PurgeIfNeeded()
   at System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData)
   at System.Web.HttpApplication.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

I'm using ELMAH for error logging, and this is showing up with some frequency. Before I try to configure ELMAH to ignore the errors, does anyone know of a way to stop the errors from happening in the first place? Is there some configuration with the new providers that I'm missing?

Answer 1

This package has a what I would describe as a bug. The premise of out of process session state is that multiple instances might be managing session state clean up. In this case all instances are running this method...

private void PurgeExpiredSessions()
{
    using (SessionEntities entities = ModelHelper.CreateSessionEntities(this.ConnectionString))
    {
        foreach (SessionEntity entity in QueryHelper.GetExpiredSessions(entities))
        {
            entities.DeleteObject(entity);
        }
        entities.SaveChanges();
        this.LastSessionPurgeTicks = DateTime.UtcNow.Ticks;
    }
}

The problem is that one instance deletes the entities before the other(s) and entities throws the error described in the post. I asked the package authors to release the source code or fix this.. My untested text editor fix would be to add public virtual, so one could override the method or just change it too..

private void PurgeExpiredSessions()
{
    using (SessionEntities entities = ModelHelper.CreateSessionEntities(this.ConnectionString))
    {   
        var sqlCommand = @"DELETE dbo.Sessions WHERE Expires < GETUTCDATE()";
        entities.ExecuteStoreCommand(sqlCommand);
        this.LastSessionPurgeTicks = DateTime.UtcNow.Ticks;
    }
}

The package authors are really fast with a response (answered while posting this!) and today they stated that they are working on releasing the code but that they might be able to just fix this. I requested an ETA and will try to follow up here if I get one.

Great package, it just needs a little maintenance.

Proper Answer: Wait for source code release or a fix update. Or De-compile and fix it yourself (If that is consistent with the license!)

*Update the package owners are considering fixing it this week. Yeah! **Update.SOLVED!!! They evidently fixed it a while ago and I was installing the wrong package.. I was using http://nuget.org/packages/System.Web.Providers and I should have been using http://nuget.org/packages/Microsoft.AspNet.Providers/ .. It was not obvious to me which one was legacy and included in another package. They wrapped it in an empty catch..

private void PurgeExpiredSessions()
{
    try
    {
        using (SessionEntities entities = ModelHelper.CreateSessionEntities(this.ConnectionString))
        {
            foreach (SessionEntity entity in QueryHelper.GetExpiredSessions(entities))
            {
                entities.DeleteObject(entity);
            }
            entities.SaveChanges();
            this.LastSessionPurgeTicks = DateTime.UtcNow.Ticks;
        }
    }
    catch
    {
    }
}

Thank you to the package team for such quick responses and great support!!!

Answer 2

Please download the newer version of the providers which can be found here: http://www.nuget.org/packages/System.Web.Providers. We have updated the way that we clean up expired sessions to run asynchronously and to handle error conditions. Please let us know if it does not correct your issues.

Answer 3

I posted a question on this at the NuGet(http://www.nuget.org/packages/System.Web.Providers), and got a very quick response from the owners. After a bit of backwards and forwards, turns out they do have a fix for this, but is going out in the next update.

There was a suggestion here, that Microsoft isnt too keen on supporting this, but my experience has been otherwise, and have always received good support.