About twenty years ago, I remember looking at a typical business application and most of the code was basically about massaging data to and from the database. The situation has changed, but even the most sophisticated of applications today spent an inordinate amount of time just shuffling data around. It may require lot less code, but CRUD still makes the most of the application codebase. On the one hand, that is pretty boring code to write, but on the other hand, that is boring code to write. That is an excellent property. Boring code is predictable, it will work without issues, it is easy to understand and modify and in general it lacks surprises. I love surprises when it comes to birthday parties and refunds from the IRS, I like them a lot less in my production code.
Here is a typical example of such code:
As you can see, we are using a command handling pattern and here we can choose one of a few architectural options:
- Write the code and behavior directly inside the command handlers.
- The command handlers are mostly about orchestration and we’ll write the business logic inside the business entities (such as the example above).
There is another aspect to the code here that is interesting, however. Take a look at the first line of code. We define there a record, a data class, that we use to note that an event happened.
You might be familiar with the notion of event sourcing, where we are recording the incoming events to the system so we’ll be able to replay them if our logic changes. In this case, that is the exact inverse of that, our code emits business events that can be processed by other pieces of the system.
The nice thing in the code above is that the business event in this case is simply writing the data record to the database. In this manner, we can participate in the overall transaction and seamlessly integrate into the overarching architecture. There isn’t much to do here, after all. You can utilize this pattern to emit whenever something that is interesting or potentially interesting happens in your application.
Aside from holding up some disk space, why exactly would you want to do this?
Well, now that you have the business events in the database, you can start operating on them. For example, we can create a report based on the paid policies by policy types and month. Of far greater interest, however, is the ability to handle such events in code. You can do that using RavenDB subscriptions.
That gives us a very important channel for extending the behavior for the system. Given the code above, let’s say that we want to add a function that would send a note to the user if their policy isn’t paid in full. I can handle that by writing the following subscription:
And then we can write a script to process that:
I intentionally show the example here as a Python script, because that doesn’t have to be a core part of the system. That can be just something that you add, either directly or as part of the system customization by an integrator.
The point is that this isn’t something that was thought of and envision by the core development team. Instead, we are emitting business events and using RavenDB subscriptions to respond to them and enhance the system with additional behavior.
One end result of this kind of system is that we are going to have two tiers of the system. One, where most of the action happens, is focused almost solely on the data management and the core pieces of the system. All the other behavior in the system is done elsewhere, in a far more dynamic manner. That gives you a lot of flexibility. It also means that there is a lot less hidden behavior, you can track and monitor all the parts much more easily, since everything is done in the open.