DDD Lightning Tour

Let's take a quick overview of Domain Driven Design.
Illustration from Undraw
Domain Driven Design, DDD for short, was a game changer (and is still somewhat singular) in that it insists on the software not just the engineering part, but also how it logically connects the physical and very real business end to software to accurately represent those ideas. Therefore, DDD was from the start contingent on language as a primary tool to create cohesion and allow for expressive and rich modeling. It also came with many prescriptive ideas divided between the higher-level "strategic DDD" and the implementation patterns part of the "tactical DDD".
Domain Driven Design has grown in the 20-odd years it's been around to be a foundational part of modern software architecture and shaping the methodology with which many work in software. It seems to have been given an enormous upswing after the microservices pattern become more in vogue some 5-10 years ago.
For me personally, reading about Domain Driven Design—first through articles and then through Eric Evans' "blue book" (2004) and Vaughn Vernon's "red book" (2013)—made for an exciting summer some years back: It was really obvious (!) that we need to connect the "business" with the implementation. The explosive thing about the books, however, was that they went well and beyond the platitudes of the statement (as you read it in the last sentence) to actually detailing patterns, strategies, and ways to get there. And boy does their stories consume a lot of paper and reading time!
The three primary books on DDD plus Vlad Khononov's modern, slimmer take on DDD from 2021.
I guess it's fair to say that the "problem", if one can call it that, is that both of the books are big. Like really big. I think they fall squarely into the lap of certain types of folks who still enjoy the intellectual exercise and sometimes relatively abstract way of learning that goes with the territory. Thankfully there are complementary resources to pad out your understanding—though as always with this type of literature, it is wise to understand the source material.
Already Vernon wrote in his first book that sometimes DDD is "first embraced as a technical toolset" (Vernon 2013, p. xxi) saying that some refer to this modus as "DDD-Lite". This will bring forth a number of useful patterns but will miss out on the glue that binds together DDD as a complete concept.

What is DDD?

The Wikipedia definition is actually very good and condensed. I'm going to reference it as-is. It says that...
Domain-driven design (DDD) is a software design approach focusing on modelling software to match a domain according to input from that domain's experts.
In terms of object-oriented programming it means that the structure and language of software code (class names, class methods, class variables) should match the business domain. For example, if a software processes loan applications, it might have classes like LoanApplication and Customer, and methods such as AcceptOffer and Withdraw.
DDD connects the implementation to an evolving model.
Domain-driven design is predicated on the following goals:
  • placing the project's primary focus on the core domain and domain logic;
  • basing complex designs on a model of the domain;
  • initiating a creative collaboration between technical and domain experts to iteratively refine a conceptual model that addresses particular domain problems.
For a software engineer or architect, the above should at least superficially sound clear and reasonable. Two terms, however, should outline themselves as being in repeated use and being somewhat mysterious or vague in meaning: domain and model. These are core to understanding Domain Driven Design.

The "domain"

The domain can be thought of as the principal subject or material of the project. It may be as broad or as narrow as necessary. Eric Evans defines it as:
[...] a sphere of knowledge, influence, or activity. The subject area to which the user applies a program is the domain of the software.
It should be somewhat clear in most circumstances what the implicit domain boundaries are. DDD eschews implicit boundaries and is particular on boundaries being collaboratively and explicitly defined.

The "model"

Eric Evans spends quite some space at the start of his book on the notion of a "model" and what model-driven design means. Being model-driven can be likened to virtually being domain-driven. By having a shared understanding, and respecting that there is a need for zooming in/out, we can condense our knowledge to an efficient and useful model that is possible to share with others without losing meaning in the process.
The intangible domain can be distilled into a tangible and malleable (domain) model, which can act as a vehicle for securing shared understanding. It is the "organized and structured knowledge of the problem". In total, the domain model can exist as one or more individual pieces of documentation (text, diagrams, code...) as long as it adequately represents the problem in a meaningful, truthful, but necessarily simplified meaning.
There is a beautiful Borges quote that you may be familiar with:
“In this empire, the art of cartography was taken to such a peak of perfection that the map of a single province took up an entire city and the map of the empire, an entire province. In time, these oversize maps outlived their usefulness and the college of cartographers drew a map of the empire equal in format to the empire itself, coinciding with it point by point. The following generations, less obsessed with the study of cartography, decided that this overblown map was useless and somewhat impiously abandoned it to the tender mercies of the sun and seasons. There are still some remains of this map in the western desert, though in very poor shape, the abode of beasts and beggars. No other traces of the geographical disciplines are to be seen throughout the land.” — Jorge Luis Borges in A Universal History of Infamy (1946; from https://www.thepolisblog.org/2012/10/jorge-luis-borges-on-empire-and.html)
Besides being the nightmare of enterprise architects and surveyors, grand maps (or schemas) that intend to explain everything can devolve into reprehensible detail. That's where the very human ability to abstract complex knowledge into models comes in. DDD taps into this quality while being completely open to the ways in which a group might do it.
I am seeing in most traditional organizations that a divide is created, problematically, between domain experts and developers. I find it reprehensible that it's so common to believe developers (and sometimes architects) somehow cannot understand the details of the domain.
The better option is, thus, to do whatever it takes to create an amenable, low-threshold environment where all relevant parties (from business to implementation) can work together. Before this, none of the ubiquitous languages or domain models will emanate.

The patterns of DDD

Domain Driven Design is not just something that makes you sound smart, but it's actually a very hands-on toolbox as well. I've reproduced one of Eric Evans' diagrams with colored bubbles to easier see where the tactical and strategic patterns reside. Green (top) is for tactical patterns, more concerning the implementation and code side, and the lilac (bottom) is for strategic ones that deal with the modeling, understanding, and integration of our domains.
How the patterns match up, as presented in Eric Evans's book (2003) and re-drawn by myself.
It may look overwhelming but don't fear!
The central concept is Model-Driven Design, which I am sure to come as no surprise—without that, there is nothing more than either theory or some sound coding advice.
There are of course dedicated sections in this book for strategic and tactical DDD, respectively.

Strategic DDD in short

You need to start with the strategic part of the work. In this work, you will uncover the domain(s), its language and terminology, how things relate to one another, and where responsibilities lay (or should lay!).
The work here is collective and should be done with a broad set of constituents, from business to design to programmers and any domain experts. Expect diagrams, post-it notes, coffee, and arguments!

Tactical DDD in short

The opposite side of the coin is the tactical work, which instead revolves chiefly around the code and any implementation work. The beauty of DDD is that we are expected to express 1:1 the actual business processes and language through the code and its functionality. DDD provides a small set of patterns to use, all of which are mutually complementary.