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:

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!

The importance of setting the boundaries (of your domain models)

First article of the year! I really wanted to start writing this few weeks ago, but honestly I wasn’t inspired enough.
Now that I’ve spent a good portion of the Christmas break reading blogs, books and watching courses on Pluralsight, I still don’t feel inspired enough.

I guess it’s due to how I spent the other portion (eating, sleeping and playing with my kids, mostly), which left me basically without any energy at all.

But as they say, the first step is always the hardest, no?

Lately I’ve been putting some effort into improving my DDD techniques. For those of you who are still living under a rock, please consider taking a copy of the marvelous blue book by Eric Evans.

I still am in the middle of the learning process, even though I probably started this path years ago. Might be a sign of impostorism. Or it might be the fact that in this job, as in many other jobs, you never stop learning.

Not going to discuss about DDD now, or what the benefits are. I shall leave this for another post.

This is going to be just a very quick introduction about boundaries and aggregates instead. What’s an aggregate? An example should make things easier.

We can consider an Order our aggregate and Order Lines compose its internal state.

I said “internal” for a reason: the Order is the “entry-point”. We can’t have Order Lines without an Order that contains them and obviously an Order without Order Lines is pretty useless.

At the same time, you can’t access Order Lines from the outside world without going through the Order.

Clear? Definitely not rocket science.

Why “aggregate” ? Well, because you’re combining things together and building up a structure that mimics your current domain. The Order and the Order Lines are entities and value objects that will eventually form the Aggregate.

I’ll talk more about the distinction between Entities and Value Objects in another post, for now just take that for granted (or lookup on Google!).

For now just think that eventually your system will be a composition of multiple Aggregates acting and interacting together, and the quality of their interaction will be a representation of how good you know your Domain. Communication is the key here!

At this point I would say that the trick is to find the right balance and being able to identify the right boundaries for your Aggregates. Why? Simply because of divide-et-impera.

Our domain might be complex from the beginning, or become extremely complex over time. Everybody has seen this happening at some point. So take a deep breath, talk to your Domain Expert ™ and identify what are the edges and the sets of your entities. That’s it. Compartmentalize.

There’s a lot more to write on this, and I definitely will. When? That’s definitely a good question! A lot is going on in my life these days and weeks and I have the sensation that 2019 is going to be a crucial year for everyone here.


talk @ UniSa – 12/12/2018

Every couple of weeks or so the University of Salerno hosts former students like me to talk a bit about their professional experience. Yesterday was my turn!

I was invited by my old teacher Vittorio Scarano (hi prof!) to talk in front of a big audience of grads and undergrads. I had a lot of fun telling them my story, we discussed about my current Company, my daily job and routine and a little bit about Design Patterns (because, why not).

I got very interesting questions from them at the end and it’s amazing to see how much passion these young students have and how strong is their desire to learn and improve.

So a big, big thank you to everyone who attended! And just FYI:

  1. my team is always hiring, so make sure to check the latest open positions
  2. a very good friend of mine is hiring as well in Salerno, check it out!
  3. don’t forget to register to the DevDay Salerno community!

Testing the boundaries of your Web APIs

How do you make sure an entire software you wrote works? And how would you do that if your system doesn’t have a UI? Well, simply by testing the boundaries of course!

From time to time I like to extract pieces of code from what I’m working on and create small repos just to showcase a single functionality or idea. 

This time I’m putting some efforts on TDD on APIs and after few refactorings I came up with a nice structure that you can use as a starting skeleton for a simple system. You can find all the sources here on GitHub.

The demo is very simple, just a single controller that stores and provides user details. Nothing fancy. The user model class exposes only three properties: id, full name and email.

Few points worth noting though:

  • the class is immutable. I wrote a bit about the concept here.
  • I’m adopting the Special Case (or Null Object) pattern a lot these days. Hence the NullUser static property.

Persistence is done in-memory as it’s obviously outside the scope. Moreover, as you can see the Tests project contains only the end-to-end tests, no unit/integration test to cover the persistence layer.

The testing infrastructure is where things gets interesting, even though it’s actually fairly straightforward. An XUnit Fixture is firing up a TestServer and bootstrapping the application using (possibly) the same settings as the real system.

A shared WebHostBuilderFactory class is indeed responsible of building the required IWebHostBuilder instance.

That’s it!

Ok, just to be honest, I got the idea from Mark Seemann : he has a very interesting course on Pluralsight named “Outside-in TDD“. If you have the chance, I strongly suggest you to watch it.

So, now that we have our nice infrastructure ready, all we have to do is write our tests! Being this a Web API, these might be considered either “functional” or “end to end”.

Honestly I think it’s simply a naming thing and doesn’t change the fact that probably these should be the first tests you would write.

Why? Because (and Mark explains it really well in his course) you’re ensuring from the consumer’s perspective that your APIs do what they’re expected to do.

You’ll be “testing the boundaries”.

But most importantly, you’re validating your acceptance criteria and making sure your system works. 

Everything else is just an implementation detail.

So what are we testing here? The routes of course! Our API is managing users, and being it RESTful, we’re asserting that all the http verbs are doing what we expect to do. 

Most of these tests should derive directly from the acceptance criteria written by your Product Owner. In case you don’t have one but instead rely on some (even vague) specifications, a good starting point is simply testing inputs and outputs. 

Happy testing!

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