Wcf service waiting for a reply from NServiceBus that will never come

Question

Imagine the following setup: a Silverlight client tunnels a serialized command over the network using a WCF service which in turn deserializes the command and sends it using NServiceBus to a generic host which is responsible for processing the command. The WCF service has - upon sending the command - registered a callback to be invoked. The generic host validates the command and 'returns' an error code (either 0 == success or >0 == failure).

Note: The WCF service is modelled after the built-in WCF service. The difference is that this WCF service receives a 'universal command' (not an IMessage), deserializes it into a real command (which does implement IMessage), and consequently sends the deserialized command off to the bus.

When unexpected exceptions occur, the command gets (after a certain amount of retries) queued in an error queue. At this point, the initiating WCF service sits there idle, unaware of what just happened. At some later point, the Silverlight client will time out according to the WCF client proxy configuration.

Things which are fuzzy in my head:

1. Does NServiceBus handle this scenario in any way? When does the timeout exception get thrown (if at all)? Or is this something exclusive to sagas?
2. Presuming I use [OperationContract(AsyncPattern=true)], are there any WCF related timeout settings that will kill the service operation? Or will the EndXXX method be somehow called? Or will it sit there forever, leaking, waiting for something that will never come?

Ways to proceed:

1. reuse existing timeout mechanisms, provided things don't leak.
2. build my own timeout mechanism between the wcf service and nservicebus.
3. notify the wcf service somehow when the command lands in the error queue.
4. build my own async notifcation mechanism using full blown callback message handler in the WCF service layer.

Things I've done:

1. run the example provided with NServiceBus.
2. spiked the happy case.

Any guidance on how to proceed is welcome, be it blog post, mailing list entries, ...

Some motivations for picking my current approach

• I'm trying to leverage some of the scalability advantages (using distributor in a later phase) of NServiceBus.
• I don't want to host a gazillion WCF services (one for each command), that's why I cooked up a bus-like WCF service.
• Even though this is somewhat request/response style, I'm mostly concerned with gracefully handling a command reply not coming through.

Answer 1

You can develop any sort of message type you desire, IMessage is simply a marker interface. If you inspect the WSDL file that the service mex endpoint provides, there is no reference to IMessage, therefore you can define any command you like in you service. That being the case you should be able to use the provided WCF host.

I was able to reproduce the issue you describe using the built-in WCF hosting option. When an exception is thrown, the entire transaction is rolled back and this includes the Bus.Return, and therefore the service never gets a response.

I found a hack around this that I could provide, but I recommend reconsidering how you are using the service. If you are truly looking to do some expensive operations in a separate process then I would recommend in your WCF endpoint that you do a Bus.Send to a different process altogether. This would ensure to your client that the command was successfully received and that work is in progress. From there it would be up to the server to complete the command(some up front validation would help ensure its success). If the command was not completed successfully this should be made known on another channel(some background polling from the client would do).