CategoryDesign Patterns

Using Decorators to handle cross-cutting concerns — Part 2 : a practical example

In my previous article I discussed a bit about how to use the Decorator pattern to implement cross-cutting concerns and reduce clutter in your codebase.

Today it’s going to be a bit more practical: we’ll be looking at a small demo I published on Github that makes use of Decorators as well as some other interesting things like .NET Attributes, CQRS and Dependency Injection.

I’m not going to deep dive into the details of CQRS as it would obviously take too much time and it’s outside the scope of this article. I’m using it here because query/command handlers usually expose just one method so there is no need to implement a big interface. Also, I like the pattern a lot 🙂

So let’s go straight to the code! The repository is available here: https://github.com/mizrael/cross-cutting-concern-attributes

It’s a very small .NET Core WebAPI application, nothing particularly fancy. No infrastructure of course, there’s no need for this article.

There’s just one API controller, exposing a single GET endpoint to retrieve a list of “values”. I might have called it “stuff” instead of “values”, it’s just an excuse to retrieve some data from the backend.

As you may have noticed, there’s no direct reference to the query handler in the API controller: I prefer to use MediatR to avoid injecting too many things in the constructor. It has become an habit so I’m doing it even when there’s just one dependency.

For those who don’t know it, MediatR acts as a simple in-process message bus, allowing quick dispatch of commands, queries and events. So, basically, it’s a very handy tool when implementing CQRS.

The ValuesArchiveHandler class handles the actual execution of the query. Actually it’s not doing much, apart from returning a fixed list of strings.

What we’re interested into actually is that small attribute, [Instrumentation] . It is just a marker, the real grunt-work will be elsewhere. I could have used an interface as well of course, but there are several reasons why I didn’t.

First of all, I prefer to avoid empty interfaces: an interface is a contract, and an interface without method doesn’t define any contract.

Moreover, attributes can always be configured to not propagate to descendant types automatically, something you cannot do with interfaces.

Now, take a look at the InstrumentationQueryHandlerDecorator class. It’s a query handler Decorator, so it gets an instance of a query handler injected in the constructor, and uses it in the Handle() method.

This decorator is not doing anything particular fancy, it’s just using Stopwatch to track how much time the inner handler is taking to complete.

What we’re interested into is the constructor: there the system is checking if the inner instance has been marked with the [Instrumentation] attribute, flipping a boolean value based on the result. That bool will then be used in the Handle() method to turn the instrumentation on or off. That’s it!

I’m using StructureMap as my IoC container and I’m taking care of the handler registration here . In the same file I also decorate all the query handlers with the InstrumentationQueryHandlerDecorator .

Keep in mind that I could have added some smarts here and check at registration time if a particular handler had been decorated with the [Instrumentation] attribute.

That would probably be a better solution as it would avoid runtime type checks, handling everything during the application bootstrap.

I’ll probably add this to the repository, I left it out to keep things simple 🙂

This article is also available on Medium as part of a series:

Using Decorators to handle cross-cutting concerns

I was actually planning of posting this article here but I was migrating to another server the last week and it took one week for the domain to point to the new DNS. Turns out this gave me the chance to try Medium instead, so published my first article there.

This time I’ll be writing about a very simple but powerful technique to reduce boiler-plate caused by cross-cutting concerns. In this post we’ll explore a simple way to encapsulate them in reusable components using the Decorator pattern.

Let’s first talk a bit about “cross cutting concerns”. On Wikipedia we can find this definition:

Cross-cutting concerns are parts of a program that rely on or must affect many other parts of the system.

In a nutshell, they represent almost everything not completely tied to the domain of the application but that can affect in some way the behaviour of its components.

Examples can be:
– caching
– error handling
– logging
– instrumentation

Instrumentation for instance can lead to a lot of boilerplate code which eventually will create clutter and pollute your codebase. You’ll basically end up with a lot of code like this:

Of course, being IT professionals, you can quickly come up with a decent solution, find the common denominator, extract the functionality, refactor and so on.

So…how would you do it? One option would be to use the Decorator pattern! It’s a very common pattern and quite easy to understand:

Basically you have a Foo class that you need somewhere that implements a well known interface, and you need to wrap it into some cross-cutting concern. All you have to do is:

  1. create a new container class implementing the same interface
  2. inject the “real” instance
  3. write your new logic where you need
  4. call the method on the inner instance
  5. sit back and enjoy!

Very handy. Of course it can be quite awkward in case your interface has a lot of methods, but in that case you might have to reconsider your architecture as it is probably breaking SRP.

One option would be moving to CQSCQRS. In the next post of the series we will see a practical example and discuss why those patterns can be an even more interesting option when combined with Decorators.

Stay tuned!

© 2019 Davide Guida

Theme by Anders NorenUp ↑