Earlier this month I encountered the notion of the Conversation per Business transaction paradigm for NHibernate session management as the overrall successor to the session per request pattern. With my exploration on this topic I found it quite confusing to implement in the only existing examples of it. Which is why I wrote a simple version for myself leveraging inversion of control and PostSharp.
Firstly, everything drives off an IConversation:
public interface IConversation
{
void Start();
void Pause();
void Resume();
void End();
void Abort();
ISession Session { get; }
}
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The conversation is a fairly simple idea, just as in a real life conversation you have a start to it, you can pause the conversation (which is how to commit the conversations), resume the conversation, end the conversation and abort the conversation. Imagine the scenario of a school class and students who have learning commited to their memory. A day could follow a similar context as this:
8:00 Class starts
Math is taught, Science is taught
12:00 Class is paused for recess (all information is committed)
13:00 Class is resumed
Creationism is taught, but realized to be made up and Class is aborted
14:00 Class is started again
More subjects are taught
15:00 Class is ended and all information flushed to students long term memory.
This example is fairly contrived but if you replace students with the database and act of class being taught with the conversation it should be pretty clear a flow that could occur over a single session for a conversation.
Next is the actual implementation of the Conversation class.
public class Conversation : IConversation, IDisposable
{
private readonly INHibernateSessionManager _sessionManager;
public ISession Session { private set; get; }
[InjectionConstructor]
public Conversation(INHibernateSessionManager sessionManager)
{
_sessionManager = sessionManager;
}
/// <summary>
/// Starts this instance.
/// </summary>
public void Start()
{
if (Session != null && (Session.IsOpen || Session.IsConnected))
{
Abort();
}
Session = _sessionManager.GetSession(FlushMode.Never);
Resume();
}
/// <summary>
/// Pauses this instance.
/// </summary>
public void Pause()
{
Commit(Session);
}
/// <summary>
/// Resumes this instance.
/// </summary>
public void Resume()
{
if (Session == null)
Start();
if (Session == null)
throw new AccessViolationException("Session could not be created");
if (!Session.IsConnected)
{
Session.Reconnect();
}
Session.BeginTransaction();
}
/// <summary>
/// Ends this instance.
/// </summary>
public void End()
{
if (Session == null || !Session.IsOpen) return;
FlushAndCommit(Session);
Session.Close();
}
/// <summary>
/// Aborts this instance.
/// </summary>
public void Abort()
{
if (Session == null || !Session.IsOpen) return;
if (Session.Transaction != null && Session.Transaction.IsActive)
{
Session.Transaction.Rollback();
}
Session.Close();
}
/// <summary>
/// Commits the specified session.
/// </summary>
/// <param name="session">The session.</param>
private static void Commit(ISession session)
{
if (session.Transaction != null && session.Transaction.IsActive)
{
session.Transaction.Commit();
}
}
/// <summary>
/// Flushes the and commit.
/// </summary>
/// <param name="session">The session.</param>
private static void FlushAndCommit(ISession session)
{
if (session.Transaction == null || !session.Transaction.IsActive) return;
session.Flush();
session.Transaction.Commit();
}
#region Implementation of IDisposable
public void Dispose()
{
End();
}
#endregion
}
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This class is a little bit long but nothing is that complex and I think every method is very straight forward. I’m going to skip over a few things that are not needed for understanding what this class does. The INHibernateSessionManager is an interface to a class that is a basically a wrapper to NHibernate’s ISessionFactory. For all intensive purposes this class could be ISessionFactory instead and the implementation wouldn’t change other than it would be session = Factory.OpenSession(); session.FlushMode = FlushMode.Never;
The InjectionConstructor attribute is a Microsoft Unity (Inversion of Control framework) specific convention that informs Unity to inject the NHibernate SessionFactory dependency into a Conversation anytime it is created. Unity will store a singleton copy of the SessionFactory for the life of the application.
Next up is the DataProvider class that will actually use the Conversation to talk to the database physically.
public class EmployeeDataProvider : IEmployeeDataProvider
{
private readonly IConversation _conversation;
[InjectionConstructor]
public EmployeeDataProvider(IConversation conversation)
{
_conversation = conversation;
}
IList<Employee> IEmployeeDataProvider.GetEmployees()
{
return _conversation.Session.CreateCriteria(typeof (Employee)).List<Employee>();
}
}
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This is a very basic method that is used with NHiberate, it’s the syntax to get a list of objects from the database. I have some other methods in my provider but the code is less important now onto calling the provider.
[Test]
[BusinessConversation]
public void BusinessConversationTest()
{
const int id = 9;
var employee = _provider.GetEmployee(id);
Assert.IsNotNull(employee);
var employees = _provider.GetEmployees();
Assert.IsTrue(employees.Count > 0);
var territories = _provider.GetTerritories();
Assert.IsTrue(territories.Count > 0);
}
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In the setup call for this test Unity will resolve the _provider and assign it a Conversation which has the SessionFactory in it for opening the session. The same session is used for each of the calls to the database and committed as a single transaction when the method exits. This is done automagically by PostSharp.
PostSharp is an aspect orientated programming framework that allows you to create attributes can trigger methods before entering a method with the attribute and after exiting. The code for the BusinesConversation attribute is
[Serializable]
public sealed class BusinessConversation : OnMethodBoundaryAspect
{
public override void OnEntry(MethodExecutionEventArgs eventArgs)
{
var container = HttpContext.Current != null ?
HttpContext.Current.Application.GetContainer() : UnityTestContainer.Instance;
var conversation = container.Resolve<IConversation>();
conversation.Resume();
}
public override void OnExit(MethodExecutionEventArgs eventArgs)
{
var container = HttpContext.Current != null ?
HttpContext.Current.Application.GetContainer() : UnityTestContainer.Instance;
var conversation = container.Resolve<IConversation>();
conversation.Pause();
}
}
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Now this has some Unity specific code in it where it calls to the container to resolve the Conversation but the important part is that OnEntry to a method with the BusinessConversation attribute the conversation is resumed. This will either resume an existing conversation that’s stored in the session or it will create a new one. When it exits the method the conversation is paused which commits the transaction to the database.
The other code where it does resolution checking on HttpContext or reads from my UntityTestContainer does not really make me happy and eventually will be refactored to be cleaner in one way or another. But this code would be specific to how you interact with your container that would store the Conversations.
The only other issue I have with any of this is exposing the Session from the Conversation, I couldn’t figure out a simplier way for allowing my data providers to actually interact with the session to physically hit the database otherwise. I considered instead of a Conversation containing a session to actually make it implement ISession and assign it to itself during the creation but that didn’t sound right to me even more so.
I’d be grateful for any feedback on my implementation here: good, bad or indifferent. I feel I’ve finally put together a clean simple to understand post on how to correctly manage NHibernate’s session that doesn’t require implementing the UnitOfWork pattern and leverages dependency injection to keep the code seperated and decoupled as much as possible. The full source to my project as always can be found on my Assembla page.
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