The ultimate gist of DDD

Domain-driven design (DDD) has little to do with actual coding, but it is an effective method for exploring and understanding the internal structure of the business domain. Dino Esposito explains what DDD is all about in this sample chapter from Clean Architecture with .NET.

Get your facts first, and then you can distort them as much as you please.

Mark Twain

Domain-driven design (DDD) is a 20-year-old methodology. Over the years, there have been several books, learning paths, and conferences dedicated to it, and every day, various social networks archive hundreds of posts and comments about it. Still, although the essence of DDD remains surprisingly simple to grasp, it is much less simple to adopt.

Today more than ever, software adds value only if it helps streamline and automate business processes. For this to happen, the software must be able to faithfully model segments of the real world. These segments are commonly referred to as business domains.

For a few decades, client/server, database-centric applications have provided an effective way to mirror segments of the real world—at least as those segments were perceived at the time. Now, though, working representations of segments of the real world must become much more precise to be useful. As a result, a database with just some code around is often no longer sufficient. Faithfully mirroring real-world behaviors and processes requires an extensive analysis.

What does this have to do with DDD? Ultimately, DDD has little to do with actual coding. It relates to methods and practices for exploring the internals of the business domain. The impact of DDD on coding and on the representation of the real world depends on the results of the analysis.

DDD is not strictly required per se, but it is an effective method for exploring and understanding the internal structure of the business domain. What really matters is getting an accurate analysis of the domain and careful coding to reflect it. DDD systematizes consolidated practices to produce an architectural representation of the business domain, ready for implementation.

Design driven by the domain

Conceptually, DDD is about design rather than coding. It rests on two pillars: one strategic and one tactical. The original authors of DDD outlined the strategy pillar and suggested tactics to achieve it. Today, however, I believe strategic analysis is the beating heart of DDD.

Strategic analysis

Any world-class software application is built around a business domain. Sometimes, that business domain is large, complex, and intricate. It is not a natural law, however, that an application must represent an intricate business domain to be broken down into pieces with numerous and interconnected function points. The strategic analysis can easily return the same monolithic business domain you started from.

Top-level architecture

The ultimate goal of the DDD strategic analysis is to express the top-level architecture of the business domain. If the business domain is large enough, then it makes sense to break it down into pieces, and DDD provides effective tools for the job. Tools like ubiquitous language (UL) and bounded contexts may help identify subdomains to work on separately. Although these subdomains may potentially overlap in some way, they remain constituent parts of the same larger ecosystem.

Figure 2-1 illustrates the conceptual breakdown of a large business domain into smaller pieces, each of which ultimately results in a deployed application. The schema—overly simplified for the purposes of this book—is adapted from a real project in sport-tech. The original business domain—a data-collection platform—is what stakeholders attempted to describe and wanted to produce. The team conducted a thorough analysis and split the original domain into five blocks. Three of these blocks were then further broken into smaller pieces. The result is 10 applications, each independent from the other in terms of technology stack and hosting model, but still able to communicate via API and in some cases sharing the same database.


FIGURE 2.1 Breakdown of a business domain.

Business domain breakdown

Nobody really needs DDD (or any other specific methodology) to move from the dashed circle on the left of Figure 2-1 to the final list of 10 bold squares on the right. As hinted at earlier, DDD doesn’t push new revolutionary practices; rather, it systematizes consolidated practices. With knowledge of the business and years of practice in software architecture, a senior architect might easily design a similar diagram without using DDD, instead relying on the momentum of experience and technical common sense. Still, although deep knowledge of a business domain might enable you to envision a practical way to break up the domain without the explicit use of an analytical method, DDD does provide a step-by-step procedure and guidance.

Subdomains versus features

Recall the block labeled “Management” in Figure 2-1. This refers to a piece of functionality whose cardinality is not obvious. That is, whereas all the other blocks in Figure 2-1 reasonably map to a single leaf-level application, this one doesn’t. Within the Management block, you could identify the functions shown in Figure 2-2.


FIGURE 2.2 Further functional split of the Management module.

The question is, are these functions just features in a monolithic application or independent services? Should this block be broken down further?

Determining the ideal size of building blocks is beyond DDD. That task requires the expertise and sensitivity of the architect. In the actual project on which this example is based, we treated the Management module as a whole and treated the smaller blocks shown in Figure 2-2 as features rather than subdomains. Ultimately, the DDD breakdown of subdomains hinges on the invisible border of local functions. All the blocks in Figure 2-2 are objectively local to the Management module and not impactful or reusable within the global, top-level architecture. Hence, in the actual project we treated them as features.

The confusing role of microservices

These days, at this point of the domain breakdown, one inevitably considers microservices. I’ll return to microservices in Chapter 3, “Laying the ground for modularity,” and in Chapter 9 “Microservices versus modular monoliths.” Here, however, I would like to make a clear statement about microservices and DDD: DDD refers only to top-level architecture and breaks the business domain in modules known as bounded contexts. A bounded context is an abstract element of the architectural design. It has its own implementation, and it can be based on microservices, but microservices are on a different level of abstraction than bounded context and DDD.

With reference to Figure 2-2, the question whether blocks are features of a domain or subdomains relates to top-level architecture. Once it is ascertained that the Management block is a leaf subdomain—namely, a bounded context—its recognized features in the implementation can be treated as in-process class libraries, functional areas, lambda functions, or even autonomous microservices. The abstraction level, though, is different.

The actual scale of DDD solutions

Many articles and blog posts that discuss DDD and bounded contexts presume that the entire enterprise back end is the domain that needs to be decomposed. So, they identify, say, Sales, Marketing, IT, Finance, and other departments as bounded contexts on which to focus. Such a large-scale scenario is fairly uncommon, however; companies rarely plan a big rewrite of the entire back end. But should this happen, the number of architects involved at the top level of the design, as large as that may be, would be relatively small.

DDD is a design approach primarily used for designing and organizing the architecture of software systems. It’s not tied to a specific scale in terms of the size of the system. Instead, it focuses on the organization of domains and subdomains within the software. Since the beginning, it has been pushed as a method dealing with enterprise-scale applications, but it is also applicable and effective at a medium- and small-scale level.

Tactical design

In general terms, strategy sets out what you want to achieve; tactics define how you intend to achieve it. Strategically, DDD provides tools to partition the business domain in smaller bounded contexts. Tactically, DDD suggests a default architecture to give life to each bounded context.

The default supporting architecture

Chapter 1 presented the highlights of the default DDD supporting architecture—the layered architecture, whose inspiring principles are now at the foundation of clean architecture. The layered architecture evolved from the multi-tier architecture in vogue when DDD was first devised.

The DDD reference architecture, monolithic and OOP-friendly, is just one suggestion. It was ideal in 2004 but sufficiently abstract and universal to retain great value even now. Today, though, other options and variations exist—for example, command/query responsibility segregation (CQRS), event sourcing, and non-layered patterns such as event-driven patterns and microservices. The key point is that for a long time, with respect to DDD, applying the layered architecture and some of its side class modeling patterns has been the way to go, putting domain decomposition in the background.

What’s a software model, anyway?

Beyond the preliminary strategic analysis, DDD is about building a software model that works in compliance with identified business needs. In his book Domain-Driven Design: Tackling Complexity at the Heart of Software (2003), author Eric Evans, uses the object-oriented programming (OOP) paradigm to illustrate building the software model for the business domain, and calls the resulting software model the domain model.

At the same time, another prominent person in the software industry, Martin Fowler—who wrote the foreword for Evans’ book—was using the same term (domain model) to indicate a design pattern for organizing the business logic. In Fowler’s definition, the domain model design pattern is a graph of interconnected objects that fully represent the domain of the problem. Everything in the model is an object and is expected to hold data and expose a behavior.

In a nutshell, in the context of DDD, the domain model is a software model. As such, it can be realized in many ways, such as OOP, functional, or CRUD. In contrast, the domain model design pattern as defined by Martin Fowler is just one possible way to implement such a software model.

DDD misconceptions

The name conflict with Fowler’s design pattern—quite paradoxical in a methodology in which unambiguous language is key—sparked several misconceptions around DDD.

The relevance of coding rules

The DDD definition details certain characteristics of the classes that participate in an object-oriented domain model: aggregates, value types, factories, behaviors, private setters, and so on. Having an object-oriented model, though, is neither mandatory nor crucial. To be crystal-clear, it’s not the extensive use of factory methods in lieu of unnamed constructors, or using carefully crafted value objects instead of loose primitive values, that makes a software project run on time and budget.

Put another way, blind observation of the coding rules set out in the DDD tactics guarantees nothing, and without a preliminary strategic design and vision, may generate more technical issues and debt than it prevents. For example, using a functional approach in the design of the domain model is neither prohibited nor patently out of place. You’re still doing DDD effectively even if you code a collection of functions or build an anemic object model with stored procedures doing the persistence work.

The value of coding rules

When it comes to DDD coding rules, though, there’s a flip side of the coin. Those rules—value types over primitive types, semantic methods over plain setters, factory methods over constructors, aggregates to better handle persistence—exist for a clear and valid reason. They enable you to build a software representation of the business model that is much more likely to be coherent with the language spoken in the business. If you don’t first identify the language of the business (the ubiquitous language) and the context in which that language is spoken, the blind application of coding rules just creates unnecessary complexity with no added value.

Database agnosticism

When you examine DDD, it’s easy to conclude that the domain model should be agnostic of the persistence layer—the actual database. This is great in theory. In practice, though, no domain model is truly agnostic from the persistence.

Note, though, that the preceding sentence is not meant to encourage you to mix persistence and business logic. A clear boundary between business and persistence is necessary. (More on this in the next chapter.) The point of DDD is that when building an object-oriented software model to represent the business domain, persistence should not be your primary concern, period.

That said, however, be aware that at some point the same object model you may have crafted ignoring persistence concerns will be persisted. When this happens, the database and the API you may use to go to the database—for example, Entity Framework (EF) Core, Dapper, and so on—are a constraint and can’t always be blissfully ignored. More precisely, blissfully ignoring the nature of the persistence layer—although a legitimate option—comes at a cost.

If you really want to keep the domain model fully agnostic of database concerns, then you should aim at having two distinct models—a domain model and a persistence model—and use adapters to switch between the two for each operation. This is extra work, whose real value must be evaluated case by case. My two cents are that a pinch of sane pragmatism is not bad at times.

Language is not simply about naming conventions

DDD puts a lot of emphasis on how entities are named. As you’ll soon see, the term ubiquitous language (UL) simply refers to a shared vocabulary of business-related terms that is ideally reflected in the conventions used to name classes and members. Hence, the emphasis on names descends from the need for code to reflect the vocabulary used in the real world. It’s not a mere matter of choosing arbitrary descriptive names; quite the reverse. It’s about applying the common language rules discovered in the strategic analysis and thoughtfully choosing descriptive names.

Tools for strategic design

I’ve touched on the tools that DDD defines to explore and describe the business domain. Now let’s look at them more closely.

You use three tools to conduct an analysis of a business model to build a conceptual view of its entities, services, and behavior:

  • Ubiquitous language

  • Bounded context

  • Context mapping

By detecting the business language spoken in a given area, you identify subdomains and label them as bounded context of the final architecture. Bounded contexts are then connected using different types of logical relationships to form the final context map.