DDD Part 1: Strategic Domain-Driven Design

This is the first part of our in-depth series on Domain-Driven Design (DDD). Part two discusses tactical DDD, whereas, in part three, you'll learn how to apply DDD to working software using Java and Vaadin. Updated for 2023.

Domain-Driven Design (DDD) has been around since Eric Evans published his book about the subject in 2003. I myself came into contact with DDD some years ago when I joined a project that suffered from data consistency problems. Duplicates showed up in the database, some information was not saved at all, and you could run into optimistic locking errors anywhere and anytime. We managed to solve this by using the building blocks of tactical domain-driven design.

Since then, I have learned more about domain-driven design and tried to use it in my projects wherever appropriate. However, during the past years, when I have talked to other developers, many of them have heard about the term domain-driven design, but they do not know what it means.

In this article series, I am going to give a brief introduction to domain-driven design as I see and understand it. The content is very much based on the books Domain-Driven Design: Tackling Complexity in the Heart of Software by Eric Evans and Implementing Domain-Driven Design by Vaughn Vernon. However, I have tried to explain everything in my own words and also inject my own thoughts, opinions, and experiences.

You will not become an expert in domain-driven design by reading my article series, but I hope it inspires you to read more about it elsewhere. I also highly encourage you to read the books by Evans and Vernon.

Now let us start with the first subject, strategic domain-driven design.

If I look up the word domain in the Dictionary app on my MacBook, I get the following definition:

[A]n area of territory owned or controlled by a particular ruler or government…​

• a specified sphere of activity or knowledge…​

In the case of domain-driven design, it is the second part of the definition that we are interested in. In this case, activity is whatever an organization does, and knowledge is how the organization does it. We are also going to add the environment in which the organization conducts its activities to the domain concept.

Subdomains

Example

Let’s say we are building an EMR (Electronic Medical Records) system for smaller clinics. We have identified the following subdomains:

1. Patient Records for managing patient medical records (personal information, medical history, etc.).

2. Lab for ordering lab tests and managing test results.

3. Scheduling for scheduling appointments.

4. File Archive for storing and managing files that are attached to the patient records (such as different documents, X-ray pictures, and scanned paper documents).

5. Identity Management for making sure the right people have access to the right information.

Now, how would we classify these subdomains? The most obvious ones are file archive and identity management, which are clearly generic subdomains. But what about the others? That depends on what is making this particular EMR system stand out among the others in the market.

Since we are building an EMR system, it is pretty safe to assume that patient records are a core domain. Suppose we are going to take the market by making a system that makes all the clinics work more efficiently through clever and innovative scheduling. In that case, scheduling is probably also a core domain. Otherwise, it is a supporting subdomain, maybe built on top of some existing scheduling engine. The same reasoning can be applied to the lab subdomain: if a significant part of our business case is a seamless integration between patient records and the lab, then the lab is most likely a core domain. Otherwise, it is a supporting subdomain.

You sometimes find the domain referred to as the "problem domain". This comes from the fact that the domain defines problems that the software is going to solve (after all, there is a reason why the software is being made in the first place). Vaughn Vernon splits a domain into a problem and solution space. The problem space concentrates on what business problems we are trying to solve. The subdomains belong to this space.

The solution space concentrates on how the problems in the problem space are going to be solved. It is more concrete, more technical, and contains more details. So how are we going to transform our problems into solutions?

To be able to create software for a domain, you need a way of describing the domain. Having a relational data model or something similar is not enough. You need to be able to describe things and their relations and the dynamics, such as events, processes, business invariants, how things change over time, and so on. You need to be able to discuss and reason about the domain with your fellow developers and domain experts. What you need is a ubiquitous language.

The ubiquitous language is a language that both domain experts and developers consistently use to describe and discuss the domain. Apart from the code itself, this language is the most important deliverable of a domain-driven design process. A big part of the language is domain terminology already being used by domain experts. Still, you may also need to invent new concepts and processes in cooperation with domain experts. Because of this, active participation from domain experts is absolutely essential for domain-driven design to succeed. Suppose the customer is not interested in putting in the time and effort to teach you their domain and help you create a ubiquitous language. In that case, you should either try to convince the customer to change their mind or pick another design method.

You can document the ubiquitous language in various ways. A good starting point is to create a glossary of terms. Business processes can be described graphically using, e.g., swimlane diagrams and flow charts. UML can be used to describe the relationship between things and state diagrams to describe how state changes as different things move through different processes. The subdomains are also a part of the ubiquitous language, and you may even need to define different "dialects" of the language for different subdomains. This embodiment of the ubiquitous language is the domain model, and it will eventually be transformed into working code. In other words, the domain model is not the same as a data model or a UML class diagram.

The ubiquitous language has a nice feature: it tells you whether you are on the right track or not. If you can easily explain a business concept or process using the language, it means you are on the right track. If you, on the other hand, struggle to explain something, you are most likely missing something from the language and, thereby, from your domain model. When this happens, you should grab a domain expert and go looking for the missing pieces. You may even stumble upon a revelation that turns your existing model completely upside-down and results in a far superior domain model than the one you had before.

In a perfect world, only one ubiquitous language and one model would explain everything about a single domain. Unfortunately, this is not the case, save for very small and simple domains. Business processes may overlap or even conflict. The same word may mean different things or different words may mean the same thing in different contexts. Depending on how you view it, there may be (and often are) more than one way to solve a problem in the problem space.

Instead of trying to find the Big Unified Model, we accept the facts and introduce something called bounded contexts. A bounded context is a distinct part of the domain in which a particular subset or dialect of the ubiquitous language is consistent at all times. In other words, we are applying divide and conquer and splitting the domain model up into smaller, more or less independent models with clearly defined boundaries. Every bounded context has its name, which is a part of the ubiquitous language.

There is not necessarily a one-to-one mapping between bounded contexts and subdomains. Since a bounded context belongs to the solution space and a subdomain to the problem space, you should think about the bounded context as one alternative solution among many possible solutions. Thus a single subdomain can contain multiple bounded contexts. You may also find yourself in a situation where a single bounded context spans multiple subdomains. There is no rule against this, but it is an indication that you may need to rethink your subdomains or context boundaries.

Personally, I like to think about bounded contexts as separate systems (e.g., separate executable JARs or deployable WARs in the Java world). A perfect real-world example of this is micro-services, where each micro-service can be considered its own bounded context. However, this does not mean you have to implement all your bounded contexts as micro-services. A bounded context could also be a separate subsystem inside a single monolithic system.

Example

In a non-trivial system, very few (if any) bounded contexts are entirely independent. Most contexts will have some kind of relationship with other contexts. Identifying these relationships is of importance not only technically (how will the systems technically communicate with each other) but also to how they are developed (how will the teams that develop the systems communicate with each other).

The simplest way to identify relationships between bounded contexts is to classify the contexts as upstream contexts and downstream contexts. Think of the contexts as cities next to a river. The cities upstream dump stuff into the river, which reaches the cities downstream. Some of the stuff is essential to the downstream cities, and so they retrieve it from the river. Other stuff is harmful and can do direct damage to the downstream cities ("sh*t rolls downhill").

Being upstream or downstream has its pros and cons. An upstream context does not depend on any other context, making it free to evolve in any direction. However, the consequences of any changes may be severe in downstream contexts, and this may, in turn, impose restrictions on the upstream context. A downstream context is restricted by its dependency on an upstream context. Still, it does not need to worry about breaking other contexts further downstream, which in a way, gives the developers of the downstream context freer hands than the developers of the upstream context.

You can describe the relationships graphically using a dependency diagram where arrows point from the downstream contexts to the upstream contexts or using the U and D roles.

Finally, keep in mind that a context can be both an upstream context and a downstream context at the same time, depending on where you stand.

Once we know what our contexts are and how they are related, we have to decide how to integrate them. This involves several important questions:

1. Where are the context boundaries?

2. How are the contexts going to communicate technically?

3. How are we going to map between the contexts' domain models (i.e., how are we translating from one ubiquitous language to another)?

4. How are we going to guard against unwanted or problematic changes occurring upstream?

5. How are we going to avoid causing trouble for downstream contexts?

The answers to these questions will be compiled into a context map. The context map can be documented graphically like this:

To make creating the context map easier, a set of ready-made integration patterns work for most use cases. Depending on which integration pattern you pick, you may have to add additional information to the context map to make it really useful.

Partnership

Shared Kernel

Customer-Supplier

Conformist

Anticorruption Layer

Open Host Service

Published Language

Separate Ways

I believe strategic domain-driven design was initially meant for larger projects. Still, I think you can benefit from it also in smaller projects - even if you end up not using any other parts of DDD in the project.

For me personally, the major takeaways from strategic domain-driven design are the following:

1. It introduces boundaries. Scope creep is a constant factor in all of my hobby projects. Eventually, they become more exhaustive than fun to work on or entirely unrealistic for finishing alone. When working on customer projects, I have to work hard not to cause technical scope creep by overthinking or overengineering things. Boundaries - wherever they are - help me to divide the project into smaller parts and focus on the right ones at the right time.

2. I do not need to find a super-model that works in all cases. It recognizes that there are often many smaller models in more or less clearly defined contexts in the real world. Instead of breaking these models, it embraces them.

3. It helps you think about the value your system will bring and where you should put the most of your efforts to get the biggest value. I have personal experience from projects where correctly identifying and then concentrating on the core domain would have made an enormous difference. Unfortunately, I had not heard about strategic DDD, and time and money were wasted.

I also know myself well enough to identify risks with this approach: finding subdomains and bounded contexts for the sake of finding subdomains and bounded contexts. When I learn something new that I like, I very much want to try it out in the real world. That may sometimes mean I go looking for things that are not there. I suggest always starting with one core domain and one bounded context. I do the domain modeling carefully. Additional subdomains and bounded contexts will eventually reveal themselves if they exist.