NHibernate + WCF + Windows Service and WcfOperationSessionContext class

Question

• I have a Windows Service Application in which i create WCF services in it.
• One of the services is data services: add, delete, read , updatte data via WCF.
• WCF use NHibernate for data manipulation

So my guestions are:

• Any advice (best practice) for session management for Hibernate using with WCF?

• Anybody knows anything about

WcfOperationSessionContext (hibernate 3.0) class?

how to use it with WCF?

Well to make it concrete :

Suppose that i have WCF Service called DataServices

class WCFDataService .....
{

   void SaveMyEntity(MyEntity entity)
    {



         .....................?? // How to do? Best Way

         // Should i take one session  and use it all times
         // Should i take session and dipsose when operation finished then get 
         //new session for new operations?
         // If many clients call my WCF service function at the same time?
         // what may go wrong?
         // etc....


     }


 }

And I need a NHibernateServiceProvider class

class NHibernateServiceProvider ....
{

    // How to get Session ?? Best way

     ISession GetCurrentSession(){.... }
     DisposeSession(){ ....}
}

Best Wishes

PS: I have read similiar entries here and other web pages. But can not see "concrete" answers.

Answer 1

Here is a post describing, in detail, all the steps for registering and using the WcfOperationSessionContext. It also includes instructions for using it with the agatha-rrsl project.

Answer 2

The WcfOperationSessionContext, similar to ThreadStaticSessionContext and WebRequestSessionContext is an implementation for a session context. The session context is used to bind (associate) a ISession instance to a particular context.

The session in the current context can be retrieved by calling ISessionFactory.GetCurrentSession().

You can find more information about session context here.

The WcfOperationSessionContext represents a context that spans for the entire duration of a WCF operation. You still need to handle the binding of the session in the begining of the operation and the unbinding/commiting/disposal of the session at the end of the operation.

To get access to the begin/end actions in the wcf pipeline you need to implement a IDispatchMessageInspector. You can see a sample here.

Also regarding WCF integration: if you use ThreadStatic session context it will appear to work on development, but you will hit the wall in production when various components (ex: authorization, authentication ) from the wcf pipeline are executed on different threads.

As for best practices you almost nailed it: Use WcfOperationSessionContext to store the current session and the IDispatchMessageInspector to begin/complete your unit of work.

EDIT - to address the details you added: If you configured WcfOperationSessionContext and do the binding/unbinding as i explained above, all you have to do to is inject the ISessionFactory into your service and just use factory.GetCurrentSession(). I'll post a sample prj if time permits.

Here is the sample project

Answer 3

The model we use for managing NHibernate sessions with WCF is as follows:

1) We have our own ServiceHost class that inherits from System.ServiceModel.ServiceHost which also implements ICallContextInitializer. We add the service host instance to each of the operations in our service as follows:

protected override void InitializeRuntime()
{
    base.InitializeRuntime();
    foreach (ChannelDispatcher cd in this.ChannelDispatchers)
    {
        foreach (EndpointDispatcher ed in cd.Endpoints)
        {
            foreach (DispatchOperation op in ed.DispatchRuntime.Operations)
            {
                op.CallContextInitializers.Add(this);
             }
        }
    }
}

public void AfterInvoke(object correlationState)
{
    // We don't do anything after the invoke
}

public object BeforeInvoke(InstanceContext instanceContext, IClientChannel channel, Message message)
{
    OperationContext.Current.Extensions.Add(new SessionOperationContext());
    return null;
}

The BeforeInvoke simply makes sure that the OperationContext for each WCF call has it's own session. We have found problems with IDispatchMessageInspector where the session is not available during response serialisation - a problem if you use lazy loading.

2) Our SessionOperationContext will then be called to attach itself and we use the OperationCompleted event to remove ourselves. This way we can be sure the session will be available for response serialisation.

public class SessionOperationContext : IExtension<OperationContext>
{

    public ISession Session { get; private set; }

    public static SessionOperationContext Current
    {
        get
        {
            OperationContext oc = OperationContext.Current;
            if (oc == null) throw new InvalidOperationException("Must be in an operation context.");
            return oc.Extensions.Find<SessionOperationContext>();
        }
    }

    public void Attach(OperationContext owner)
    {
        // Create the session and do anything else you required
        this.Session = ... // Whatever instantiation method you use

        // Hook into the OperationCompleted event which will be raised
        // after the operation has completed and the response serialised.
        owner.OperationCompleted += new EventHandler(OperationCompleted);
    }

    void OperationCompleted(object sender, EventArgs e)
    {
        // Tell WCF this extension is done
        ((OperationContext)sender).Extensions.Remove(this);
    }

    public void Detach(OperationContext owner)
    {
        // Close our session, do any cleanup, even auto commit 
        // transactions if required.
        this.Session.Dispose();
        this.Session = null;
    }
}

We've used the above pattern successfully in high-load applications and it seems to work well.

In summary this is similar to what the new WcfOperationSessionContext does (it wasn't around when we figured out the pattern above;-)) but also overcomes issues surrounding lazy loading.

Regarding the additional questions asked: If you use the model outlined above you would simply do the following:

void SaveMyEntity(MyEntity entity)
{
    SessionOperationContext.Current.Session.Save(entity);
}

You are guaranteed that the session is always there and that it will be disposed once the WCF operation is completed. You can use transactions if required in the normal way.

Answer 4

Ok, after few days of reading internet posts etc. all approaches shown in the internets seems to be wrong. When we are using UnitOfWork pattern with NH 3^ with nhibernate transaction this all aprochaes are producing exceptions. To test it and proof that we need to create test enviroment with MSMQ transaction queue, special interface with OneWay operation contract with transaction required set on it. This approach should works like this: 1. We put transactionally message in queue. 2. Service is getting transactionally messege from queue. 3. Everything works queue is empty.

In some cases not so obious with internet approaches this does not work properly. So here are expamples which we tested that are wrong and why:

1. Fabio Maulo approach: Use ICallContextInitializer - open NH session/transaction on BeforeCall, after that WCF is executing service method, on AfterCall in context initializer we call session.Flush + transaction.commit. Automaticly session will be saved when transaction scope will commit operation. In situation when on calling transaction.Complete exception will be thrown WCF service will shutdown! Question can be ok, so take transaction.Complete in try/catch clausule - great! - NO wrong! Then transaction scope will commit transaction and message will be taken from queue but data will not be saved !
2. Another approach is to use IDispatchMessageInspector - yesterday I thought this is best approach. Here we need to open session/transaction in method AfterReceiveRequest, after WCF invoke service operation on message dispatcher inspector BeforeSendReply is called. In this method we have info about [reply] which in OneWay operation is null, but filled with fault information if it occured on invoking service method. Great I thought - this is this ! but NOT! Problem is that at this point in WCF processing pipe we have no transaction ! So if transaction.Complete throw error or session.Flush will throw it we will have not data saved in database and message will not come back to queue what is wrong.

What is the solution?

IOperationInvoker and only this!

You need to implement this interface as a decorator pattern on default invoker. In method Invoke before call we are openning session/transaction open then we call invoke default invoker and after that call transaction.complete in finally clausule we call session.flush. What types of problem this solves: 1. We have transaction scope on this level so when complete throws exception message will go back to queue and WCF will not shutdown. 2. When invocation will throw exception transaction.complete will not be called what will not change database state

I hope this will clear everyones missinformation.

In some free time I will try to write some example.