The cost of messaging
Greg Young has a post about the cost of messaging. I fully agree that the cost isn't going to be in the time that you are going to spend actually writing the message body. You are going to have a lot of those, and if you take more than a minute or two to write one, I am offering overpriced speed your typing courses.
The cost of messaging, and a very real one, comes when you need to understand the system. In a system where message exchange is the form of communication, it can be significantly harder to understand what is going on. For tightly coupled system, you can generally just follow the path of the code. But for messages?
When I publish a message, that is all I care about in the view of the current component. But in the view of the system? I sure as hell care about who is consuming it and what it is doing with it.
Usually, the very first feature in a system that I write is login a user. That is a good proof that all the systems are working.
We will ignore the UI and the actual backend for user storage for a second, let us thing about how we would deal with this issue if we had messaging in place? We have the following guidance from Udi about this exact topic. I am going to try to break it down even further.
We have the following components in the process. The user, the browser (distinct from the user), the web server and the authentication service.
We will start looking at how this approach works by seeing how system startup works.
The web server asks the authentication service for the users. The authentication service send the web server all the users he is aware off. The web server then cache them internally. When a user try to login, we can now satisfy that request directly from our cache, without having to talk the the authentication service. This means that we have a fully local authentication story, which would be blazingly fast.
But what happens if we get a user that we don't have in the cache? (Maybe the user just registered and we weren't notified about it yet?).
Then the browser call us again (note that this is transparent to the user), and we have the information that we need, so we can successfully log them in:
There is another scenario here, what happens if the user is not valid. The first part of the scenario is identical, we ask the authentication service to tell us if this is valid or not. When the service reply that this is not a valid user, we cache that. When the browser call back to us, we can tell it that this is not a valid user.
(Just to make things interesting, we also have to ensure that the invalid users cache will expire or has a limited size, because otherwise this is an invitation for DOS attack.)
Finally, we have the process of creating a new user in the application, which work in the following fashion:
Now, I just took three pages to explain something that can be explained very easily using:
Backed by the ACID guarantees of the database, those two stored procedures are much simpler to reason about, explain and in general work with.
We have way more complexity to work with. And this complexity spans all layers, from the back end to the UI! My UI guys needs to know about async messaging!
Isn't this a bit extreme? Isn't this heavy weight? Isn't this utterly ridiculous?
Yes, it is, absolutely. The problem with the two SPs solution is that it would work beautifully for a simple scenario, but it creaks when start talking about the more complex ones.
Authentication is usually a heavy operation. ValidateLogin is not just doing a query. It is also recording stats, updating last login date, etc. It is also something that users will do frequently. It make sense to try to optimize that.
Once we leave the trivial solution area, we are face with a myriad of problems that the messaging solution solve. There is no chance of farm wide locks in the messaging solution, because there is never a lock taking place. There are no waiting threads in the messaging solution, because we never query anything but our own local state.
We can take the authentication service down for maintenance and the only thing that will be affected is new user registration. The entire system is more robust.
Those are the tradeoffs that we have to deal with when we get to high complexity features. It make sense to start crafting them, instead of just assembling a solution.
Just stop and think about what it would require of you to understand how logins work in the messaging system, vs. the two SP system. I don't think that anyone can argue that the messaging system is simpler to understand, and that is where the real cost is.
However, I think that you'll find that after getting used to the new approach, you'll find that it start making sense. Not only that, but it is fairly easy to see how to approach problems once you have started to get a feel for messaging.