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Jun/10

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What is DDD or Domain Driven Design?

Domain Driven Design can be described as a philosophy based on domain modelling. More accurately it may be be described as a very large body of patterns and pattern language in its own right. The term Domain Driven Design or DDD was coined by Eric Evans the author of the book Domain-Driven Design: Tackling Complexity in the Heart of Software also known in the DDD community as the “blue book”.

Understanding the DDD philosophy

The Domain Driven Design philosophy states:

  • Most software projects should focus on business domain
  • Complex domain designs should be based on a model

To understand the meaning of these statements one has to understand the meaning of domain and model.

Domain is a sphere of knowledge, influence or activity. The subject area to which the user applies a program is the domain of the software. In other terms if you work for a bank then banking is your domain.

Model is a system of abstractions that describes selected aspects of a domain and can be used to solve problems related to that domain. For example a map is a model designed to solve a specific problem. A treasure map shows how to find a treasure, a political map shows the borders of countries. A model is a simplification. It is an interpretation of reality that focuses on the problem at hand and ignores the extraneous detail.

Models are designed to be useful to solve domain specific problems. For example in the past the universe was viewed in a geocentric way where the universe revolves around Earth. Heliocentric model is another astronomical model in which the Earth and planets revolve around a stationary Sun at the centre of the universe. Even though geocentric model is not realistic it is a valid model in it’s own right designed to solve a problem – the human desire to be in the centre of everything. It’s not a useful model when it’s used to compare planet movements.

Domain Driven Design advocates designing software systems to reflect the domain model in a very literal way, so that the mapping is obvious, also revising the model continuously and modifying it to be implemented more naturally in software. To tie the implementation to a model well requires tools and languages that support a modelling paradigm, such as object-oriented programming.

A well mapped implementation of a model usually expresses an object model that incorporates both behaviour and data. A decomposed domain model consists of common building blocks: entities, aggregates, value objects, services and factories.

Essential Principles of DDD

The greatest value of a domain model is that it provides a ubiquitous language that ties domain experts and technologists together. Ubiquitous language is a language structured around the domain model and used by all team members to connect all activities of the team with the software. It’s a shared, versatile language between team members and domain experts. A well designed model speaks to the developers through the ubiquitous language. It’s important to understand that a change in the model is a change in the language and vice versa.

When multiple models are in play on a large project it’s beneficial to define bounded contexts where these models apply. A bounded context is a linguistic boundary marking the applicability of distinct models. Usually a subsystem or work owned by another team. For example in a typical e-shop web application a sales reporting application could be defined as a separate bounded context.

Every domain consists of subdomains. For example a very common subdomain is billing. Such a subdomain is usually not the driving part of the domain and therefore not as important. It is harsh reality that not all parts of the design are going to be equally refined therefore priorities must be set. DDD suggests distilling the core domain by distinguishing it from other generic subdomains and applying the top talent to work on it.

Conclusion

DDD helps projects to develop a strong internal language, define clear context boundaries, and focus on the core domain. Domain Driven Design brings structure and cohesion into domain modelling which are much appreciated features of any software project in existence. The blue book has been released six years ago and since then it influenced many developers. Yet I feel it hasn’t reached it’s momentum. One can only hope it will reach widespread adoption.

Update: I’ve added a list of available DDD resources such as papers and video presentations.

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