Book Image

Hands-On Domain-Driven Design with .NET Core

By : Alexey Zimarev
5 (1)
Book Image

Hands-On Domain-Driven Design with .NET Core

5 (1)
By: Alexey Zimarev

Overview of this book

Developers across the world are rapidly adopting DDD principles to deliver powerful results when writing software that deals with complex business requirements. This book will guide you in involving business stakeholders when choosing the software you are planning to build for them. By figuring out the temporal nature of behavior-driven domain models, you will be able to build leaner, more agile, and modular systems. You’ll begin by uncovering domain complexity and learn how to capture the behavioral aspects of the domain language. You will then learn about EventStorming and advance to creating a new project in .NET Core 2.1; you’ll also and write some code to transfer your events from sticky notes to C#. The book will show you how to use aggregates to handle commands and produce events. As you progress, you’ll get to grips with Bounded Contexts, Context Map, Event Sourcing, and CQRS. After translating domain models into executable C# code, you will create a frontend for your application using Vue.js. In addition to this, you’ll learn how to refactor your code and cover event versioning and migration essentials. By the end of this DDD book, you will have gained the confidence to implement the DDD approach in your organization and be able to explore new techniques that complement what you’ve learned from the book.
Table of Contents (14 chapters)

Understanding the problem

We rarely write software to just write some code. Of course, we can create a pet project for fun and to learn new technologies, but professionally we build software to help other people to do their work better, faster, and more efficiently. Otherwise, there is no point in writing any software in the first place. It means that we need to have a problem that we intend to solve. Cognitive psychology defines the issue as a restriction between the current state and the desired state.

Problem space and solution space

In their book Human Problem Solving, Allen Newell and Herbert Simon outlined the problem space theory. The theory states that humans solve problems by searching for a solution in the problem space. The problem space describes the initial and desired states, as well as possible intermediate states. It can also contain specific constraints and rules that define the context of the problem. In the software industry, people operating in the problem space are usually customers and users.

Each real problem demands a solution, and if we search properly in the problem space, we can outline which steps we need to take to move from the initial state to the desired state. This outline and all the details about the solution form a solution space.

The classic story of problem and solution spaces, which get completely detached from each other during the implementation, is the story of writing in space. The story goes that in the 1960s, space-exploring nations realized that the usual ballpoint pens wouldn't work in space due to the lack of gravity. NASA then spent a million dollars to develop a pen that would work in space, and the Soviets decided to use the good old pencil, which costs almost nothing.

This story is so compelling that it is still circulating, and was even used in the TV show The West Wing, with Martin Sheen playing the US president. It is so easy to believe, not only because we are used to wasteful spending by government-funded bodies, but mostly because we have seen so many examples of inefficiency and misinterpretation of real-world issues, adding enormous unnecessary complexity to their proposed solutions and solving problems that don't exist.

This story is a myth. NASA also tried using pencils but decided to get rid of them due to the production of microdust, tips breaking, and the potential flammability of wooden pencils. A private company called Fisher developed what is now known as a space pen. Later, NASA tested the pen and decided to use it. The company also got an order from the Soviet Union, and pens were sold across the world. The price was the same for everyone, $2.39 per pen.

Here you can see the other part of the problem space/solution space issue. Although the problem itself appeared to be simple, additional constraints, which we could also call non-functional requirements, or, to be more precise, operational requirements, made it more complicated than it looked at first glance.

Jumping to a solution is very easy, and since most of us have a rather rich experience of solving everyday problems, we can find solutions for many issues almost immediately. However, as Bart Barthelemy and Candace Dalmagne-Rouge suggest in their article When You're Innovating, Resist Looking for Solutions (2013, Harvard Business Review https://hbr.org/2013/09/when-youre-innovating-resist-l), thinking about solutions prevents our brain from thinking about the problem. Instead, we start going deeper into the solution that first came to our mind, adding more levels of detail and making it the most ideal solution for a given problem.

There's one more aspect to consider when searching for a solution to a given problem. There is a danger of fixating all your attention on one particular solution, which might not be the best one at all but it was the first to come to mind, based on your previous experiences, your current understanding of the problem, and other factors:

Refinement versus exploration

The exploratory approach to find and choose solutions involves quite a lot of work to try out a few different things, instead of concentrating on the iterative improvement of the original good idea. However, the answer that is found during this type of exploration will most probably be much more precise and valuable. We will discuss fixation on the first possible solution later in this chapter.

What went wrong with requirements

We are all familiar with the idea of requirements for software. Developers rarely have direct contact with whoever wants to solve a problem. Usually, some dedicated people, such as requirements analysts, business analysts, or product managers, talk to customers and generalize the outcomes of these conversations in the form of functional requirements.

Requirements can have different forms, from large documents called a requirements specification to more agile means such as user stories. Let's have a look at this example:

"Every day, the system shall generate, for each hotel, a list of guests expected to check in and check out on that day."

As you can see, this statement only describes the solution. We cannot possibly know what the user is doing and what problem our system will be solving. Additional requirements might be specified, further refining the solution, but a description of the problem is never included in functional requirements.

In contrast, with user stories, we have more insight into what our user wants. Let's review this real-life user story: "As a warehouse manager, I need to be able to print a stock-level report so that I can order items when they are out of stock." In this case, we have an insight into what our user wants. However, this user story already dictates what the developers need to do. It is describing the solution. The real problem is probably that the customer needs a more efficient procurement process, so they never run out of stock. Or, they need an advanced purchase forecasting system, so they can improve throughput without stockpiling additional inventory in their warehouse.

We should not think that the requirements are a waste of time. There are many excellent analysts out there who produce high-quality requirements specifications. However, it is vital to understand that these requirements almost always represent the understanding of the actual problem from the point of view of the person who wrote them. A misconception that spending more and more time and money on writing higher-quality requirements prevails in the industry.

However, lean and agile methodologies embrace more direct communication between developers and end users. Understanding the problem by all stakeholders, from end users to developers and testers, finding solutions together, eliminating assumptions, building prototypes for end users to evaluate—all these things are being adopted by successful teams, and as we will see later in the book, they are also closely related to DDD.