NHibernate and TransactionScope and Web API

###The problem I have a Web API project that uses NHibernate and NServiceBus, and I need them to succeed or fail together, within a business transaction. There is exactly one transaction per web request.

###What I tried To enlist in and coordinate the distributed transaction, I need to wrap each web request in a TransactionScope. I set up an ActionFilter.

Inside the transaction scope, I need to set up the standard NHibernate session-per-request unit of work. I set up another ActionFilter.

#####Action Filter order Action filters don’t get executed in a specific order. I couldn’t ensure the transaction scope would be opened before the NHibernate session. A slightly modified version of this ordered filter provider worked.

#####Per request I’m using StructureMap nested containers to ensure everything gets cleaned up at the end of each request, even if the filter gets jacked up. This matches up really well with the Web API concept of dependency scopes / dependency resolver.

#####Lazy loading I’m using NHibernate lazy loading and LINQ deferred execution in a few responses. I don’t have to do it that way, but it was already written, and I’d rather just make it work. The issue is that when the response is serialized, the LINQ query is executed, which triggers lazy loading. By the time this happens, my filter’s OnActionExecuted has already run. The NHibernate session is already closed.

The solution was to use a message handler instead of filters.

###The (partial) solution

I pass in the factory delegates to make the handler testable.

#####Can’t detect ambient transaction The standard way to detect an ambient transaction didn’t work for me. Transaction.Current was always null. I suspect an issue flowing it across async / await. For now, every request gets a transaction, whether it needs it or not.

#####Still lazy loading issues So far, this works flawlessly when the controller actions return something other than void, HttpResponseMessage or an IHttpActionResult. Check out the Result Conversions box in the lower right corner of the ASP.NET Web API 2 lifecycle poster to understand why.

If I need more control over the responses, I have to return either HttpResponseMessage or IHttpActionResult. Now the service can return more intelligent responses like custom 404 messages, descriptive 409 conflict responses, 302 redirects, etc. The drawback is that these responses aren’t serialized at the same point in the request lifecycle. In fact, they aren’t serialized inside the Web API stack at all. It’s left up to the host.

This means my lazy loading issue continues. To fix it once and for all, I need to serialize the response content before the NHibernate session gets closed. The simplest solution I could come up with required a second message handler.

#####Wiring it up Here’s how to wire up these two new message handlers.

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