Domain Driven Design Quotes

The heart of software is its ability to solve domain-related problems for its user.

It takes fastidiousness to write code that doesn’t just do the right thing but also says the right thing.

A model is a selectively simplified and consciously structured form of knowledge.

The vital detail about the design is captured in the code. A well-written implementation should be transparent, revealing the model underlying it.

When you can, choose Value Objects to model integrations. This advice is applicable in many cases when consuming remote Standard Types.

Knowledge crunching is an exploration, and you can’t know where you will end up.

there should be some learning when a domain model is discussed.

It is more desirable for Entities to have equals() and hashCode() methods that are based on the object’s unique identity rather than other attributes.

To communicate effectively, the code must be based on the same language used to write the requirements—the same language that the developers speak with each other and with domain experts.

Developers are too wrapped up with technology and trying to solve problems using technology rather than careful thought and design. This leads developers to constantly chase after new “shiny objects,” which are the latest fads in technology.

Listen to the language the domain experts use. Are there terms that succinctly state something complicated? Are they correcting your word choice (perhaps diplomatically)? Do the puzzled looks on their faces go away when you use a particular phrase? These are hints of a concept that might benefit the model.

Developing software that delivers true business value is not the same thing as developing ordinary business software. Software that delivers true business value aligns with the business strategic initiatives and bears solutions with clearly identifiable competitive advantage—software that is not about technology, but about the business.

One of the patterns from domain-driven design is called bounded context. Bounded contexts are used to set the logical boundaries of a domain’s solution space for better managing complexity. It’s important that teams understand which aspects, including data, they can change on their own and which are shared dependencies for which they need to coordinate with other teams to avoid breaking things. Setting boundaries helps teams and developers manage the dependencies more efficiently. The logical boundaries are typically explicit and enforced on areas with clear and higher cohesion. These domain dependencies can sit on different levels, such as specific parts of the application, processes, associated database designs, etc. The bounded context, we can conclude, is polymorphic and can be applied to many different viewpoints. Polymorphic means that the bounded context size and shape can vary based on viewpoint and surroundings. This also means you need to be explicit when using a bounded context; otherwise it remains pretty vague.

outside the boundaries of its own skin—these things had driven the species to the edge of destruction. Worse, this one organism threatened the survival of all life on Earth. The Silencer’s makers did not have to look far for a solution. The answer lay in another species that had conquered the entirety of its domain, ruling it with unquestioned authority for millions of years. Beyond their immaculate design, the reason sharks rule the ocean is their complete indifference to everything except feeding, procreation, and defending their territory. The shark does not love. It feels no empathy. It trusts nothing. It lives in perfect harmony with its environment because it has no aspirations or desires. And no pity. A shark feels no sorrow, no remorse, hopes for nothing, dreams of nothing, has no illusions about itself or anything beyond itself. Once a human named Evan Walker had a dream—a dream it can no longer remember—and in that dream there was a tent in the woods and in that tent there was a girl who called herself humanity, and the girl was worth more to it than its own life. No longer. When it finds her,

When Events are delivered to interested parties, in either local or foreign systems, they are generally used to facilitate eventual consistency.

Architecture is important, but architectural influences come and go. Remember to prioritize correctly, placing more emphasis on the domain model, which has greater business value and will be more enduring.

Prefer references to external Aggregates only by their globally unique identity, not by holding a direct object reference (or “pointer”).

Success comes in an emerging set of abstract concepts that makes sense of all the detail. This distillation is a rigorous expression of the particular knowledge that has been found most relevant.

This often happens due to the pressure to deliver software releases on a relentless schedule, where management uses Scrum to primarily control timelines rather than allow for one of Scrum’s most important tenets: knowledge acquisition.

Whatever course you decide upon, there is always someone to tell you that you are wrong. There are always difficulties arising which tempt you to believe that your critics are right. To map out a course of action and follow it to an end requires courage. —Ralph Waldo Emerson

In general, don’t fight your frameworks. Seek ways to keep the fundamentals of domain-driven design and let go of the specifics when the framework is antagonistic.

Tightly relating the code to an underlying model gives the code meaning and makes the model relevant.

Most talented developers do not have much interest in learning about the specific domain in which they are working, much less making a major commitment to expand their domain-modeling skills. Technical people enjoy quantifiable problems that exercise their technical skills.

Simple, informal UML diagrams can anchor a discussion.

Any technical person contributing to the model must spend some time touching the code, whatever primary role he or she plays on the project. Anyone responsible for changing code must learn to express a model through the code. Every developer must be involved in some level of discussion about the model and have contact with domain experts. Those who contribute in different ways must consciously engage those who touch the code in a dynamic exchange of model ideas through the UBIQUITOUS LANGUAGE

But, while bidirectional associations between ENTITIES may be hard to maintain, bidirectional associations between two VALUE OBJECTS just make no sense. Without identity, it is meaningless to say that an object points back to the same VALUE OBJECT that points to it. The most you could say is that it points to an object that is equal to the one pointing to it, but you would have to enforce that invariant somewhere.

When a significant process or transformation in the domain is not a natural responsibility of an ENTITY or VALUE OBJECT, add an operation to the model as a standalone interface declared as a SERVICE. Define the interface in terms of the language of the model and make sure the operation name is part of the UBIQUITOUS LAN- GUAGE. Make the SERVICE stateless.

One of the worst disconnects of a business software development effort is seen in the gap between domain experts and software developers. Generally speaking, true domain experts are focused on delivering business value. On the other hand, software developers are typically drawn to technology and technical solutions to business problems. It’s not that software developers have wrong motivations; it’s just what tends to grab their attention. Even when software developers engage with domain experts, the collaboration is largely at a surface level, and the software that gets developed often results in a translation/mapping between how the business thinks and operates and how the software developer interprets that. The resulting software generally does not reflect a recognizable realization of the mental model of the domain experts, or perhaps it does so only partially. Over time this disconnect becomes costly. The translation of domain knowledge into software is lost as developers transition to other projects or leave the company. A different, yet related problem is when one or more domain experts do not agree with each other. This tends to happen because each expert has more or less experience in the specific domain being modeled, or they are simply experts in related but different areas. It’s also common for multiple “domain experts” to have no expertise in a given domain, where they are more of a business analyst, yet they are expected to bring insightful direction to discussions. When this situation goes unchecked, it results in blurred rather than crisp mental models, which lead to conflicting software models. Worse still is when the technical approach to software development actually wrongly changes the way the business functions. While a different scenario, it is well known that enterprise resource planning (ERP) software will often change the overall business operations of an organization to fit the way the ERP functions. The total cost of owning the ERP cannot be fully calculated in terms of license and maintenance fees. The reorganization and disruption to the business can be far more costly than either of those two tangible factors. A similar dynamic is at play as your software development teams interpret what the business needs into what the newly developed software actually does. This can be both costly and disruptive to the business, its customers, and its partners. Furthermore, this technical interpretation is both unnecessary and avoidable with the use of proven software development techniques. The solution is a key investment.

thorough.

An AGGREGATE is a cluster of associated objects that we treat as a unit for the purpose of data changes.

Extreme Programming recognizes the importance of design decisions, but it strongly resists upfront design. Instead, it puts an admirable effort into communication and improving the project’s ability to change course rapidly. With that ability to react, developers can use the “simplest thing that could work” at any stage of a project and then continuously refactor, making many small design improvements, ultimately arriving at a design that fits the customer’s true needs.

A model is a simplification. It is an interpretation of reality that abstracts the aspects relevant to solving the problem at hand and ignores extraneous detail.

Extreme Programming concentrates exclusively on the active elements of a program and executable tests. Even comments added to the code do not affect program behavior, so they always fall out of sync with the active code and its driving model. External documents and diagrams do not affect the behavior of the program, so they fall out of sync. On the other hand, spoken communication and ephemeral diagrams on whiteboards do not linger to create confusion. This dependence on the code as communication medium motivates developers to keep the code clean and transparent.

It takes fastidiousness to write code that doesn’t just do the right thing but also says the right thing.

Across this linguistic divide, the domain experts vaguely describe what they want. Developers, struggling to understand a domain new to them, vaguely understand. A few members of the team manage to become bilingual, but they become bottlenecks of information flow, and their translations are inexact.

There are two patterns for decomposition: Decompose by business capability, which has its origins in business architecture Decompose by subdomain, based on concepts from domain-driven design

From that perspective, solutions that apply DDD naturally tend to be layered as well.

Some amount of redundancy in the layering may be unavoidable for the purposes of standardization.

In the layer cake anti-pattern, each layer merely proxies the call to the layer beneath it without adding any value.

Another variation of the layer cake we see commonly is one where layers refuse to share input and output types in the name of higher isolation and looser coupling.

If the objects being translated are more or less structurally identical, we have an anemic translation.

While such translations may be necessary across bounded contexts, teams should strive to avoid the need for variations in names and structures of the same concept within a single bounded context. The intentional use of ubiquitous language helps avoid such scenarios.

In such scenarios, it may be worth considering consciously allowing one or more layers to be bypassed.

Consistent use of the common, ubiquitous language helps reinforce a shared understanding and produce solutions that better reflect the mental model of the domain experts.

Fundamentally, an aggregate has the following qualities: Is an entity usually composed of other child entities and value objects Encapsulates access to child entities by exposing behavior (usually referred to as commands) Is a boundary that is used to enforce business invariants (rules) consistently Is an entry point to get things done within a bounded context

It may be prudent to make use of domain events to reduce the amount of coupling between bounded contexts, making it a critical building block of DDD.

Executives and business experts should read this book so that they can articulate their vision and the core concepts that justify the need for the solution. Techniques will allow them to do this in an expedient manner and also gain empathy toward what it takes to implement changes quickly and reliably.

are those aspects of the system that do not affect functionality directly but have a profound effect on the efficacy of those using and maintaining these systems.

they almost always expect these features to be part of any system they use.

We cannot solve our problems with the same level of thinking that created them. — Albert Einstein

evolve mature software solutions that will stand the test of time.

It may be necessary to further break down subdomains where the subdomain is still too complex.

we are opposed to creating additional tiers merely as a means to compensate for poorly factored core domain APIs.

software development is a learning process; working code is a side effect. A software project’s success depends on the effectiveness of knowledge sharing between domain experts and software engineers. We have to understand the problem in order to solve it.

The domain experts had learned more and had clarified the goal of the application.

Translation muddles model concepts, which leads to destructive refactoring of code.

That shallowness of knowledge produces software that does a basic job but lacks a deep connection to the domain expert’s way of thinking.

A lack of focus on a shared language and knowledge of the problem domain results in a codebase that works but does not reveal the intent of the business.

As your product evolves, adding new features becomes as easy as it was in the good old days of greenfield development.

The effort of translation prevents the interplay of knowledge and ideas that lead to deep model insights.

In the old waterfall method, the business experts talk to the analysts, and analysts digest and abstract and pass the result along to the programmers, who code the software. This approach fails because it completely lacks feedback.

Knowledge trickles in one direction, but does not accumulate.

the model is not the diagram.

The behavior of running code is unambiguous.

documenting exclusively through code has some of the same basic problems as using comprehensive UML diagrams.

Diagrams are a means of communication and explanation, and they facilitate brainstorming. They serve these ends best if they are minimal.

crucial discoveries always emerge during the design/implementation effort.

Business activities and rules are as central to a domain as are the entities involved; any domain will have various categories of concepts.

The indirectness of communication conceals the formation of schisms—different team members use terms differently but don’t realize it.

Scrum, for example, was never meant to stand in place of design. No matter how many project and product managers would like to keep you marching on a relentless path of continuous delivery, Scrum was not meant only as a means to keep Gantt chart enthusiasts happy. Yet, it has become that in so many cases.

The kind of knowledge captured in a model such as the PCB example goes beyond “find the nouns.

Written documents should complement the code and the talking.

you may hear the UBIQUITOUS LANGUAGE changing naturally while a document is being left behind.

There is no need for explanatory models to be object models, and it is generally best if they are not.

The astrolabe was a mechanical implementation of an object-oriented model of the sky.

Every software program relates to some activity or interest of its user.

When we set out to write software, we never know enough.

By using the model-based language pervasively and not being satisfied until it flows, we approach a model that is complete and comprehensible, made up of simple elements that combine to express complex ideas.

They show design constraints, but they are not design specifications in every detail. They represent the skeletons of ideas.

A document shouldn’t try to do what the code already does well.

If the design, or some central part of it, does not map to the domain model, that model is of little value, and the correctness of the software is suspect.

This does not mean that effort is spent on modeling the “real world.” Rather, DDD delivers a model that is the most useful to the business. Sometimes useful and realistic models happen to intersect, but to the degree that they diverge, DDD chooses useful. With

Bounded Context is a specific solution, a realization view,

This makes Subdomains a very useful tool in assessing the problem space.

The heart of software is its ability to solve domain-related problems for its user. All other features, vital though they may be, support this basic purpose. When the domain is complex, this is a difficult task, calling for the concentrated effort of talented and skilled people. Developers have to steep themselves in the domain to build up knowledge of the business. They must hone their modeling skills and master domain design. Yet these are not the priorities on most software projects. Most talented developers do not have much interest in learning about the specific domain in which they are working, much less making a major commitment to expand their domain-modeling skills. Technical people enjoy quantifiable problems that exercise their technical skills. Domain work is messy and demands a lot of complicated new knowledge that doesn’t seem to add to a computer scientist’s capabilities. Instead, the technical talent goes to work on elaborate frameworks, trying to solve domain problems with technology. Learning about and modeling the domain is left to others. Complexity in the heart of software has to be tackled head-on. To do otherwise is to risk irrelevance.

Well, you either learn or you fail.

Simply stated, practicing DDD-Lite leads to the construction of inferior domain models. That’s because the Ubiquitous Language, Bounded Context, and Context Mapping have so much to offer. You get more than a team lingo. The Language of a team in an explicit Bounded Context expressed as a domain model adds true business value and gives us certainty that we are implementing the correct software. Even from a technical standpoint, it helps us create better models, ones with more potent behaviors, that are pure and less error prone.

when coupled with data-binding constructs, makes for code that is much more readable, maintainable, and testable.

As Fred Brooks laid out in his widely popular paper, “No Silver Bullet—Essence and Accident in Software Engineering”, there are two types of complexity when building software systems. First, we have the essential complexity that is essential and inherent to the problem space. This is business and domain complexity. Second, there is accidental complexity: the complexity that we—engineers, architects, and designers—create in our solutions. Accidental complexity can and should be reduced.

From a consumer’s perspective, the robustness principle (Postel’s law) applies: be conservative in what you send, be liberal in what you accept.

CAP theorem. The theorem postulates that distributed systems can either be highly available or consistent in the event of a network partition, not both at the same time. Given the three characteristics, consistency, availability, and partition tolerance, the theorem postulates that distributed systems can either be highly available or consistent in the event of a network partition, not both at the same time.

Leaking of domain logic: In an ideal world, the aggregate will remain the custodian of all domain logic. Given that the orchestrator is also stateful, business logic may inadvertently shift to the orchestrator. Care should be taken to ensure that the orchestrator only has flow control logic while business invariants remain within the confines of the aggregate.

It also helps if these teams are co-located and/or have a significant working time overlap.

Important Note This may feel contrary to the notion of centralizing business validations on the backend. While this may be a noble attempt at implementing the Don’t Repeat Yourself (DRY) principle, in reality, it poses a lot of practical problems. Distributed systems expert Udi Dahan has a very interesting take on why this may not be such a virtuous thing to pursue. Ted Neward also talks about this in his blog, entitled The Fallacies of Enterprise Computing.

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Transactional outbox: None of the preceding methods are completely foolproof in the sense that there still exists a window of opportunity where the database and the event bus can become inconsistent (this is true even with two-phase commits). One way to circumvent this problem is by completely eliminating the dual-write problem. In this solution, the command processor writes to its database and the intended event to an outbox table in a local transaction. A separate poller component polls the outbox table and writes to the event bus. Polling can be computationally intensive and could lead back to the dual write problem again because the poller has to keep track of the last written event.

Partnership relationships between teams can lead to a situation where individual team responsibilities become very unclear,

If the implementer has state that is used in the computation, then the implementer is effectively an argument of the operation,

Factoring the constraint into its own method allows us to give it an intention-revealing name that makes the constraint explicit in our design. It is now a named thing we can discuss.

The complexity of a highly detailed interaction ends up being handled in the application layer, allowing domain knowledge to creep into the application or user interface code, where it is lost from the domain layer.