C# Task Parallel Library and NHibernate/Spring.NET

Question

I have been using Spring.NET and NHibernate for some years and I am very satisfied. However, I was always playing around with multi threading, Reactive Extensions and eventually Task Parallel Library which is a great framework. Unfortunately all kind of multithreading approaches fail because of NHiberntate's session which is not thread safe.

I am asking you how can I benefit from parallel programming and still utilising NHibernate.

For instance: I have a CustomerRegistrationService class which method Register performs several tasks:

ICustumer customer = this.CreateCustomerAndAdresses(params);
this.CreateMembership(customer);
this.CreateGeoLookups(customer.Address);
this.SendWelcomeMail(customer);

The last two methods would be ideal candidates to run parallel, CreateGeoLookups calls some web services to determine geo locations of the customer's address and creates some new entities as well as updates the customer itself. SendWelcomMail does what it says.

Because CreateGeoLookups does use NHibernate (although through repository objects so NHibernate is acutally hidden via Interfaces/Dependency Inection) it won't work with Task.Factory.StarNew(...) or other Threading mechanisms.

My question is not to solve this very issue I have described but I would like to hear from you about NHibenrate, Spring.NET and parallel approaches.

Thank you very much Max

Answer 1

In NH its the ISession that isn't thread-safe but the ISessionFactory is entirely thread-safe, easily supporting what it seems you are after. If you have designed your session-lifecycle-management (and the repositories that depend upon it) such that you assume one single consistent ISession across calls, then, yes, you will have this kind of trouble. But if you have designed your session-handling pattern to only assume a single ISessionFactory but not to make assumptions about ISession, then there is nothing inherently preventing you from interacting with NH in parallel.

Although you don't specifically mention your use case as being for the web, its important to take note that in web-centric use-cases (e.g., what is a pretty common case for Spring.NET users as well as many other NH-managing-frameworks), the often-used 'Session-Per-Request' pattern of ISession management (often referred to in Spring.NET as 'Open Session In View' or just 'OSIV') will NOT work and you will need to switch to a different duration of your ISession lifecycle. This is because (as the name suggests) the session-per-request/OSIV pattern makes the (now incorrect in your case) assumption that there is only a single ISession instance for the duration of each HttpRequest (and presumably you would want to be spawning these parallel NH calls all within the context of a single HttpRequest in the web use case).

Obviously in the non-web case where there's rarely a similar concept to session-per-request you wouldn't be as likely to run into this issue as session-lifecycle management is rarely as fine-grained/short-lived as it in web-based applications.

Hope this helps.

-Steve B.