Durable Services with Fake Persistence

I’ve been investigating the new WCF/WF integration in .NET Framework 3.5. I want to understand how the new context features work. Unfortunately, there’s not much info out there (that I could find at any rate). You’re pretty much stuck with the samples and Jesus Rodriguez’s overview of durable services. So I sat down to dig a little deeper.

Note, since there are no docs on this stuff as I write this, many of the links below are Reflector code links.

I started by lifting the DurableCalculator sample contract and service implementation and dumping it into a new WCF Service Library project. I did this for two reasons. First, VS08 has added a WCF Service Host much like VS05 added the ASP.NET Development Server. Very cool. But the existing sample is still written to be hosted in IIS, so I wanted to change that. Second, and much more important, I wanted to start with a vanilla config file. I knew it wouldn’t work out of the box, but the point of this exercise was to learn how this works under the covers.

When you fire up the durable service with the vanilla config file, you get an error (as expected). Services marked with the DurableServiceAttribute require a binding that supports the context protocol. WsHttpBinding, the default binding when you create a new service, doesn’t. However, it’s easy to fix by switching to wsHttpContextBinding instead. Via Reflector, we see that wsHttpContextBinding inherits from wsHttpBinding and adds a ContextBindingElement to the binding element collection created by the base class. BasicHttpContextBinding and netTcpContextBinding work the same way.

Even after changing to wsHttpContextBinding, we’re still getting an error on service start. But it’s a new error, so we’re making progress. Now, we’re told that services marked with DurableServiceAttribute need a persistence provider to be specified. If we look in the original sample’s web.config file, we find a persistenceProvider element in the service behavior. This element references the SqlPersistenceProviderFactory type. Obviously, the point here is to persist durable service instances to the database between calls, much as WF can do.

However, merely configuring the existing SQL persistence provider doesn’t really tell you what’s going on under the hood. Besides, often when you’re experimenting, you don’t really want to go thru the headache of setting up a SQL store for persisting instances to. Somewhere along the line, I implemented a fake persistence service for WF that stored the serialized instances in memory. So I decided to do the same for WCF durable services.

Building a WCF Persistence Provider requires building two classes: a factory and the provider itself. Factories inherit from PersistenceProviderFactory, which exposes only one non-CommunicationObject method: CreateProvider. It appears that the service host creates a single persistence provider factory and calls CreateProvider whenever it needs a persistence provider. Providers themselves inherit from PersistenceProvider, which exposes methods to Load, Save and Delete durable service instances.

My FakePersistenceProvider (and factory) are brain dead simple, though the ratio of “real” code to factory and CommunicationObject scaffolding is quite low (about 40 lines out of 258). The factory keeps a dictionary of serialized service instances, keyed by guid. When providers are created, this key guid is passed as a parameter to CreateProvider. The provider instances delegate Load, Save and Delete back to internal methods on the factory class. The methods themselves use the NetDataContractSerializer to serialize the service instance out to a byte array and deserializing it back again. I chose NetDataContractSerializer because that’s what the SQL persistence provider uses under the hood.

PersistenceProvider supports async versions of Load, Save and Delete but I didn’t implement them. Also, there’s a LockingPersistenceProvider abstract class which adds (you guessed it) instance locking semantics. However, my fake provider doesn’t span machines, much less require locking semantics so I skipped it.

So it looks like DurableServiceAttribute, context-supporting bindings and persistence providers are all inter-related. Certainly, you can’t use the attribute with out the binding and persistence provider. As I continue to dig, I’m want to see how context inter-relates with WF as well as it’s possible usage outside of DurableServiceAttribute based scenarios.

If you’re interested in the code, I’ve stuck it up on my SkyDrive. In addition to the FakePersistenceProvider implementation and the simple Durable Calculator service, it includes a simple client to test the service and persistence provider. The WCF Service Host includes a test client, but it doesn’t appear to support the context protocol, so I had to build a simple test app instead. Enjoy.