Before diving in, we should take a moment to consider why NServiceBus might be a tool worth adding to your repertoire. If you're eager to get started, feel free to skip this section and come back later.
So what is NServiceBus? It's a powerful, extensible framework that will help you to leverage the principles of Service-oriented architecture (SOA) to create distributed systems that are more reliable, more extensible, more scalable, and easier to update.
That's all well and good, but if you're just picking up this book for the first time, why should you care? What problems does it solve? How will it make your life better?
Ask yourself whether any of the following situations describes you:
My code updates values in several tables in a transaction, which acquires locks on those tables, so it frequently runs into deadlocks under load. I've optimized all the queries that I can. The transaction keeps the database consistent but the user gets an ugly exception and has to retry what they were doing, which doesn't make them very happy.
Our order processing system sometimes fails on the third of three database calls. The transaction rolls back and we log the error, but we're losing money because the end user doesn't know whether their order went through or not, and they're not willing to retry for fear of being charged twice, so we're losing business to our competitor.
We built a system to process images for our clients. It worked fine for a while, but now we've become a victim of our own success. We made it multithreaded, which was a challenge. Then we had to replace the server it was running on because it maxed out. We're adding more clients and it's only a matter of time until we max out the new one too! We need to scale it out to run on multiple servers but have no idea how to do it.
We have a solution that integrates with a third-party web service, but when we call the web service, we also need to update data in a local database. Sometimes, the web service times out, so our database transaction rolls back, but sometimes, the web service call does actually complete at the remote end, so now our local data and our third-party provider's data are out of sync.
We're sending emails as part of a complex business process. It is designed to be retried in the event of a failure, but now customers are complaining that they're receiving duplicate emails, sometimes dozens of them. A failure occurs after the email is sent, the process is retried, and the email is sent over and over until the failure no longer occurs.
I have a long-running process that kicks off from a web application. The website sits on an interstitial page while the backend process runs, similar to what you would see on a travel site when you search for plane tickets. This process is difficult to set up and fairly brittle. Sometimes, the backend process fails to start and the web page just keeps loading forever.
We have a batch job that runs every night during off hours, but it's taking so long to run that it's intruding on regular business hours. Plus waiting for the batch job is a headache. It needs to be more real-time.
We don't want to keep investing in on-premises infrastructure to deal with potential spikes in traffic. We need to figure out how to transition some of our business processes to run in the cloud.
If any of these situations has you nodding your head in agreement, I invite you to read on.
NServiceBus will help you to make multiple transactional updates utilizing the principle of eventual consistency to reduce database locking and blocking and make deadlocks easy to deal with in a reliable way. It will ensure that valuable customer order data is not lost in the deep dark depths of a multi-megabyte logfile.
By the end of the book, you'll be able to build systems that can easily scale out as well as up. You'll also be able to reliably perform non-transactional tasks such as calling web services and sending emails. You will be able to easily start up long-running processes in an application server layer, leaving your web application free to process incoming requests and you'll be able to unravel your spaghetti codebases into a logical system of commands, events, and handlers that will enable you to more easily add new features and version the existing ones.
You could try to do this all on your own by rolling your own messaging, but that would be really dangerous and wasteful. It is a far better strategy to take advantage of all of the industry-leading expertise that has been applied in NServiceBus in the last several years, and concentrate on what your business does best. NServiceBus is the easiest and most capable solution to solve the aforementioned problems without having to expend too much effort to get it right, allowing you to put your focus on your business concerns, where it belongs.
So if you're ready, let's get started creating an NServiceBus solution.