NServiceBus: The Magic of Generic Host

I have done a few posts on NServiceBus and have talked in great details about how it works with messages and MSMQ. However I haven’t talked much about the Generic Host and today’s post is just about that.And hope you will find quite amazing and valuable for your SOA development.

As you had seen in my previous post that it is a means to host your NServiceBus application, whether it’s the client sending a message to the bus or the server reading the message from the queue they both are assemblies and are hosted in this executable called the “Generic Host Process” for NServiceBusIt is this host which takes of wiring up messages, handler and endpoints to MSMQ etc.

When we run the generic host using command line we can see some interesting options as shown below.

This means that you can configure any message endpoint as a Windows Service and Generic host will take care of installation. You don’t have to do a windows service host application, installer etc. All the plumbing and hard work is taken care by the generic host and you have to focus on writing your messages,message handlers and endpoints.

Let’s try installing the server component in the Full-Duplex sample as a windows service.

The Command Line:

NServiceBus.Host.exe /install /serviceName:MyServer.dll 
/displayName:"My Super Duper service" 
/description:"My server installed by NService Magic"

As you can see the command line is very self-explanatory, we have an assembly called MyServer.dll which implements a handler of a specific type and we want it’s display name to be My Super Duper Service and the description to be My server installed by NService Magic.

Lets verify this by opening the Windows Service Management Console and we can see our NServiceBus Server component installed as a windows service. When i did this for the first time it was not less than a magic.

To test whether it is doing what it is supposed to do I am going to run the Client only project from Visual Studio and our message gets picked by the windows service and our handler processes it successfully.

That’s it for now and I hope you guys will find a lot of value in using NServiceBus

NServiceBus Integration Pattern – Part 2

Continuing with my previous post on NServiceBus in this part I am going to implement what we had discuss about integration and how NServiceBus can be used. Based on the previous architecture diagram I am doing to drill into component architecture and for easy illustration I am going to use the FullDuplex example which is part of the samples provided by NServiceBus.

And let’s see how it looks from the visual studio.

And when we look into the project properties you can see that the project type is an output library and it is configured to run with an external program which is the NServiceBus.Host.exe.

So let’s run a typical failover / offline scenario and run the client application in isolation

and as you can see in the above diagram that the client can send messages even if the server is not running.

Let’s run the server application with debugging and mimic as if there is a failure. For example the finance system is down, exchange server in unavailable or any system to system integration failure. So in this scenario when we run the server application we see this exception thrown and seen like this in the console window.

Interesting thing is that if you have a break point in the server application set correctly you will see that the code gets retried 5 times before the exception is shown in the console.
This happens due to the fact that the server application is configured to use transactional queues and in the config file it has been configured to retry 5 times.

Upon close inspection of error queue this is what it looks like

And when we click to open the message you will see this is how the message looks like.

Next thing we have to do is fix the problem and whether the problem was due to hardware, software infrastructure failures let’s pretend those problems are resolved. So now what we have to do is put the message(s) back into where they belong and run our server component and there is exactly a tool called “ReturnedToSourceQueue” in the tools folder.Let’s run the tool.

This first thing you have to enter is the name of your error queue and after that you can specify a specific message with the Guid or all of it. For demonstration sake I’ll specify the keyword ‘all’ and you will see the message gets disappeared from the error queue and reappears in the server input queue.

and the message reappears in the input queue.

And now when we run our server component you will see the message gets processed.

As you can see we can fine tune and design these components to integrated different integration points and have that failover/durability scenario taken care by NServiceBus, we can take the message and transform into different type and let the handler of the new type take care of it.

For example our FinanceMessageHandler talks to the Finance system and processes the order if it fails to do so the order remains in the error. We don’t have to hack into some database or ask user to reenter the order. As soon as we resolved the Finance system failure/downtime the message will be processed successfully.On succesfully processing of this business process we can transform the message into an Email message and puts it into EmailQueue.

Now our EmailMessageHandler picks up the message and tries to process and if it encounters any problem with the mail server the message gets routed to the error queue that’s it. This is like creating a messaging eco-system where each system talks to each other in terms of a message and we are building this orchestration.This is what I meant by “Pass the Parcel (message)” pattern in my previous post.

That’s it for now and in my next post I am going to talk about the Generic host and what wonders it can create for your enterprise.