NServiceBus design ideas

Question

Can any developers/architects with experience with NServiceBus offer guidance and help on the following?

We have a requirement in the business (and not a lot of money) to create a robust interface between an externally hosted application and our internal ERP's (yup, more than one).

When certain activities take place in the third party application they will send us the message. i.e. call a web service passing various fields of information in the message etc. We are not in control nor can we change this third party application.

My responsibility is creating this web service and the processing of the messages into each ERP. The third party dictates how the web service will look, but not what its responsible for. We have to accept that if they get a response back of 'success' then we at this point have taken responsibility for that message! i.e. we need to ensure as close to perfect no data loss takes place.

This is where I'm interested in the use of NServiceBus. Use it to store/accept a message at first. At this point I get lost, I can't tell what should happen, i.e. what design follows. Does another machine (process) subscribe and grab the message to process it into an ERP, if so since each ERPs integration logic differs do I make a subscriber per ERP? A message may have two destination ERP targets however, so is it best the message is sent and not subscribed to.

Obviously in the whole design, I need to have some business rules which help determine the destination ERP's and then business rules that determine what actually takes place with in each ERP. So I also have a question on BRE's but this can wait although still may be a driver for what the message has to do.

so:

Third party > web service call > store message (& return success) > determine which ERP is target > process each into ERP > mark message complete

If anything fails along the lines making sure the message does not get lost. p.s. how does MSMQ prevent loss since the whole machine may die ? is this just disk resilience etc?

Many thanks if you've read and even more for any advice.

Answer 1

This sounds like a perfect application for NServiceBus.

Your web service should ONLY parse the request from the third and translate it into an NServiceBus message, which it should Bus.Send(). You don't respond with a 200 status code until that message is on the Bus, at which point, you are responsible for it, and NServiceBus's built-in error/retry and error queue facilities become your best friend.

This message should be received by another endpoint, but it needs to be able to account for duplicate messages or use idempotence so that duplicates aren't a problem. If the third party hits your web service, and the message is successfully placed on the bus, but then some error prevents them from receiving the 200 response code, you will get duplicates from them.

At this point, the endpoint receiving the MessageFromWebServiceCommand message could Bus.Publish() a SomeBusinessEventHappenedEvent that contains the command data.

For each ERP, create an additional endpoint that subscribes to the SomeBusinessEventHappenedEvent and uses your business logic to decide what to do respective to that ERP. In some cases, that "something" may be "nothing". Keep idempotence in mind here too, because if the message fails it will be retried.

All the other things you're worried about (preventing loss of messages, what happened if machines die) will be taken care of thanks to NServiceBus and MSMQ being naturally resilient to such problems.

Here is a blog post, including a sample project, that shows how to receive messages from an external partner via a web service and handle them with NServiceBus, and a link straight to the sample project on GitHub:

• Robust 3rd Party Integrations with NServiceBus
  • Project Source Code on GitHub