Implementation visibility – Part III

In the first article of this series, I presented the concept of “implementation visibility”. Every requirement can be expressed in source code on a scale of how prominent the implementation will be. There are at least five stages (or levels) on the scale:

  • level 0: Inline
    • level 0+: Inline with comment
    • level 0++: Inline with apologetic comment
  • level 1: separate method
  • level 2: separate class
    • level 2+: new type in domain model
  • level 3: separate aggregate
  • level 4: separate package or module
  • level 5: separate application or service

We examined a simple code example in both preceding articles. The level 0, 0+ and 0++ were covered in the first article, while the second article talked about level 1, 2 and 2+. You might want to read them first if you want to follow the progression through the ranks. In this article, we look at the example at level 3, have a short outlook on further levels and then recap the concept.

A quick reminder

Our example is a webshop that lacks brutto prices. The original code of our shopping cart renderer might looked like this:


public class ShowShoppingCart {
  public ShoppingCartRenderModel render(Iterable<Product> inCart) {
    final ShoppingCartRenderModel result = new ShoppingCartRenderModel();
    for (Product each : inCart) {
      result.addProductLine(
            each.description(),
            each.nettoPrice());
    }
    return result;
  }
}

Visibility level 3: Domain drive all the things!

We’ve introduced a new class for our requirement in visibility level 2 and made it a domain type. This is mostly another name for the concept of Entities or Value Objects from Domain Driven Design (DDD). If you aren’t familiar with Domain Driven Design, I recommend you grab the original book or its worthy successor and read about it. It is a way to look at requirements and code that will transform the way you develop software. To give a short spoiler, DDD Entities and DDD Value Objects are named core domain concepts that form the foundation of every DDD application. They are found by learning about the problem domain your software is used in. DDD Entities have an own identity, while DDD Value Objects just exist to indicate a certain value. Every DDD Entity and most DDD Value Objects are part of an DDD Aggregate. To load and store DDD Aggregates, a DDD Repository is put into place. The DDD Repository encapsulates all the technical stuff that has to happen when the application wants to access an DDD Aggregate through its DDD Root Entity. Sorry for all the “DDD” prefixes, but the terms are overloaded with many different meanings in our profession and I want to be clear what I mean when I use the terms “Repository” or “Aggregate”. Be very careful not to mistake the DDD meanings of the terms for any other meaning out there. Please read the books if you are unsure.

So, in Domain Driven Design, our BruttoPrice type is really a DDD Value Object. It represents a certain value in our currency of choice (Euro in our example), but has no life cycle on its own. Two BruttoPrices can be considered “the same” if their values are equal. This raises the question what the DDD Root Entity of the corresponding DDD Aggregate might be. Just imagine what happens in the domain (in real life, on paper) if you calculate a brutto price from a given netto price: You determine the value added tax category of your taxable product, look up its current percentage and multiply your netto price with the percentage. The DDD Root Entity is the value added tax category, as it can be introduced and revoked by your government and therefor has a life cycle on its own. The tax percentage, the netto price and the brutto price are just DDD Value Objects in its vicinity.

To bring DDD into our code and raise the implementation visibility level, we need to introduce a lot of new types with lots of lines of code:

  • NettoPrice is a DDD Value Object representing the concept of a monetary value without taxes.
  • BruttoPrice is a DDD Value Object representing the concept of a monetary value including taxes.
  • ValueAddedTaxCategory is a DDD Root Entity standing for the concept of different VAT percentages for different product groups.
  • ValueAddedTaxPercentage might be a DDD Value Object representing the concept of a percentage being applied to a NettoPrice to get a BruttoPrice. We will omit this explicit concept and let the ValueAddedTaxCategory deal with the calculation internally.
  • ValueAddedTaxRepository is a DDD Repository providing the ability to retrieve a ValueAddedTaxCategory for a known Taxable.
  • Taxable might be a DDD Entity. For us, it will remain an abstraction to decouple our taxes from other concrete types like Product.

The most surprising new class is probably the ValueAddedTaxRepository. It lingered in our code in nearly all previous levels, but wasn’t prominent, not visible enough to be explicit. Remember lines like this?

final BigDecimal taxFactor = <gets the right tax factor from somewhere> 

Now we know where to retrieve our ValueAddedTaxCategory from! And we don’t even know that the VAT is calculated using a percentage or factor anymore. That’s a detail of the ValueAddedTaxCategory given to us from the ValueAddedTaxRepository. If one day, for example at April 1th, 2020, the VAT for bottled water is decreed to be a fixed amount per bottle, we might need to change the internals of our VAT DDD Aggregate, but the netto and brutto prices and the rest of the application won’t even notice.

We’ve given our different reasons of change different places in our code. We have separated our concerns. This separation requires a lot of work to be spelled out. Let’s look at the code of our example at implementation visibility level 3:

public class ShowShoppingCart {
  public ShoppingCartRenderModel render(Iterable<Product> inCart) {
    final ShoppingCartRenderModel result = new ShoppingCartRenderModel();
    final ValueAddedTaxRepository vatProvider = givenVatRepository();
    for (Product each : inCart) {
      final ValueAddedTaxCategory vat = vatProvider.forType(each);
      final BruttoPrice bruttoPrice = vat.applyTo(each.nettoPrice());
      result.addProductLine(
            each.description(),
            each.nettoPrice(),
            bruttoPrice);
    }
    return result;
  }
}

There are now three lines of code responsible for calculating the brutto prices. It gets ridiculous! First we obtain the DDD Repository from somewhere. Somebody probably gave us the reference in the constructor or something. Just to remind you: The class is named ShowShoppingCart and now needs to know about a class that calls itself ValueAddedTaxRepository. Then, we obtain the corresponding ValueAddedTaxCategory for each Product or Taxable in our shopping cart. We apply this VAT to the NettoPrice of the Product/Taxable and pass the resulting BruttoPrice side by side with the NettoPrice in the addProductLine() method. Notice how we changed the signature of the method to differentiate between NettoPrice and BruttoPrice instead of using just to Euro parameters. Those domain types are now our level of abstraction. We don’t really care about Euro anymore. The prices might be expressed in mussle shells or bottle caps and we still could use our code without modification.

The ValueAddedTaxCategory we obtain from the DDD Repository isn’t a class with a concrete implementation. Instead, it is an interface:


/**
* AN-17: Calculates the brutto price (netto price with value added tax (VAT))
* for the given netto price.
*/
public interface ValueAddedTaxCategory {
  public BruttoPrice applyTo(NettoPrice nettoPrice);
}

Now we could nearly get rid of the comment above. It just repeats what the signature of the single method in this type says, too. We keep it for the reference to the requirement (AN-17).

Right now, the interface has only one implementation in the class PercentageValueAddedTaxCategory:


public class PercentageValueAddedTaxCategory implements ValueAddedTaxCategory {
  private final BigDecimal percentage;

  public PercentageValueAddedTaxCategory(final BigDecimal percentage) {
    this.percentage = percentage;
  }

  @Override
  public BruttoPrice applyTo(NettoPrice nettoPrice) {
    final Euro value = nettoPrice.multiplyWith(this.percentage).inEuro();
    return new BruttoPrice(value);
  }
}

You might notice that the concrete code of applyTo still has knowledge about the Euro. As long as we don’t ingrain the relationship between NettoPrice and BruttoPrice in these types, somebody has to do the conversion externally – and needs to know about implementation details of these types. That’s an observation that you should at least note down in your domain crunching documents. It isn’t necessarily bad code, but a spot that will require modification once the currency changes to cola bottle caps.

This is a good moment to reconsider what we’ve done to our ShowShoppingCart class. Let’s refactor the code a bit and move the responsibility for value added taxes where it belongs: in the Product type.


public class ShowShoppingCart {
  public ShoppingCartRenderModel render(Iterable<Product> inCart) {
    final ShoppingCartRenderModel result = new ShoppingCartRenderModel();
    for (Product each : inCart) {
      result.addProductLine(
            each.description(),
            each.nettoPrice(),
            each.bruttoPrice());
    }
    return result;
  }

}

Now we have made a full circle: Our code looks like it began without the brutto prices, but with one additional line that delivers the brutto prices to the product line in the ShoppingCartRenderModel. The whole infrastructure that we’ve built is hidden behind the Product/Taxable type interface. We still use all of the domain types from above, we’ve just changed the location where we use them. The whole concept complex of different price types, value added taxes and tax categories is a top level construct in our application now. It shows up in the domain model and in the vocabulatory of our project. It isn’t a quick fix, it’s the introduction of a whole set of new ideas and our code now reflects that.

The code at implementation visibility level 3 might seem bloated and over-engineered to some. There is probably truth in this judgement. We’ve introduced far more code seams in the form of abstractions and indirections than we can utilize in the moment. We’ve prepared for an uncertain future. That might turn out to be unnecessary and would then be waste.

So let’s look at our journey as an example of what could be done. There is no need to walk all the way all the time. But you should be able to walk it in case it proves necessary.

Visibility level 4 and above: To infinity and beyond!

Remember that there are implementation visibility levels above 3! If you choose such a level, there will be even more code, more classes and types, more indirection and more abstraction. Suddenly, your new code will show up on system architecture diagrams and be deployed independently. Maybe you’ll need a dedicated server for it or scale it all the way up to its own server farm. Our example doesn’t match those criterias, so I stop here and just say that visibility level 3 isn’t the end of the journey. But you probably got the idea and can continue on your own now.

Recap: Rising through the visibility levels

We’ve come a long way since level 0 in terms of implementation visibility. The code still does the same thing, it just accumulates structure (some may call it cruft) and fletches out the relationships between concepts. In doing so, different axis of change emerge in different locations instead of entangled in one place. Our development effort rises, but we hope for a return on investment in the future.

I’ve found it easier to elevate the implementation visibility level of some code later than to decrease it. You might experience it the other way around. In the end, it doesn’t matter which way we choose – we have to match the importance of the requirement in the code. And as the requirements and their importance change, our code has to adjust to it in order to stay relevant. It isn’t the visibility level you choose now that will decide if your code is visible enough, it is the necessary visibility level you cannot reach for one reason or the other that will doom your code. Because it “feels bloated” and gets replaced, because it wasn’t found in time and is duplicated somewhere else, because it fused together with unrelated code and cannot be separated. Because of a plethora of reasons. By choosing and changing the implementation visibility level of your code deliberately, you at least take the responsibility to minimize the effects of those reasons. And that will empower you even if not all your decisions turn out profitable.

Conclusion

With the end of this third part, our series about the concept of implementation visibility comes to an end. I hope you’ve enjoyed the journey and gained some insights. If you happen to identify an example where this concept could help you, I’d love to hear from you! And if you know about a book or some other source where this concept is explained, too – please comment with a link below.

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