Expanding on our previous article that covered Object-Oriented Analysis and Design (OOAD), today's article will explore domain-driven design (DDD). DDD is a software development approach that uses and builds upon OOAD principles and ideas, so it's the next logical topic for us to dive into.
Throughout this article, we'll examine what domain-driven design is, how it is commonly implemented in modern development life cycles, and consider a few potential advantages and disadvantages of using DDD in your own projects.
What is the Domain?
To define domain-driven design, we should first establish what we mean by domain in this context (and in development in general). The standard dictionary definition of domain is: "A sphere of knowledge or activity." Drilling down a bit from that, the domain in software engineering commonly refers to the subject area on which the application is intended to apply.
In other words, during application development, the domain is the "sphere of knowledge and activity around which the application logic revolves."
Another common term used during software development is the domain layer or domain logic, which may be better known to many developers as business logic. The business logic of an application refers to the higher-level rules for how business objects (see: OOAD) interact with one another to create and modify modeled data.
What is Domain-Driven Design?
Initially introduced and made popular by programmer Eric Evans in his 2004 book Domain-Driven Design: Tackling Complexity in the Heart of Software, domain-driven design is the expansion upon and application of the domain concept as it applies to the development of software. DDD aims to ease the creation of complex applications by connecting the related pieces of software into an ever-evolving model.
DDD focuses on three core principles:
- Focus on the core domain and domain logic.
- Base complex designs on models of the domain.
- Collaboration with domain experts to improve the application model and resolve any emerging domain-related issues.
Evans' Domain-Driven Design further defines a few common terms that are useful when describing and discussing DDD practices:
- Context: The setting in which a word or statement appears determines its meaning. Statements about a model can only be understood in a context.
- Model: A system of abstractions that describes selected aspects of a domain and can be used to solve problems related to that domain.
- Ubiquitous Language: A language structured around the domain model and used by all team members to connect all the activities of the team with the software.
- Bounded Context: A description of a boundary (typically a subsystem or the work of a specific team) within which a particular model is defined and applicable.
Domain-driven design also defines several high-level concepts that can be used in conjunction with one another to create and modify domain models:
- Entity: An object identified by its consistent thread of continuity, as opposed to traditional objects, which are defined by their attributes.
- Value Object: An immutable (unchangeable) object that has attributes but no distinct identity.
- Domain Event: An object that records a discrete event related to model activity within the system. While all events within the system could be tracked, a domain event is only created for event types that the domain experts care about.
- Aggregate: A cluster of entities and value objects with defined boundaries around the group. Rather than allowing every single entity or value object to perform all actions on its own, the collective aggregate of items is assigned a singular aggregate root item. Now, external objects no longer have direct access to every individual entity or value object within the aggregate but instead only have access to the single aggregate root item and use that to pass along instructions to the group as a whole. This practice correlates with many of the actual coding practices we're covering in our design patterns series.
- Service: Essentially, a service is an operation or form of business logic that doesn't naturally fit within the realm of objects. In other words, if some functionality must exist, but it cannot be related to an entity or value object, it's probably a service.
- Repositories: Not to be confused with common version control repositories, the DDD meaning of a repository is a service that uses a global interface to provide access to all entities and value objects within a particular aggregate collection. Methods should be defined to allow for the creation, modification, and deletion of objects within the aggregate. However, by using this repository service to make data queries, the goal is to remove such data query capabilities from within the business logic of object models.
- Factories: As we've discussed through several design patterns articles already, DDD suggests using a factory, which encapsulates the logic of creating complex objects and aggregates, ensuring that the client does not know the inner-workings of object manipulation.
Domain-driven design also heavily emphasizes the ever-more-popular practice of continuous integration, which asks the entire development team to use one shared code repository and push commits to it daily (if not multiple times a day).
An automatic process executes at the end of the work day, which checks the integrity of the entire code base, running automated unit tests, regression tests, and the like to quickly detect any potential problems that may have been introduced in the latest commits.
Advantages of Domain-Driven DesignEases Communication: With an early emphasis on establishing a common and ubiquitous language related to the domain model of the project, teams will often find communication throughout the entire development life cycle to be much easier. Typically, DDD will require less technical jargon when discussing aspects of the application since the ubiquitous language established early on will likely define simpler terms to refer to those more technical aspects.
Improves Flexibility: Since DDD is so heavily based on the concepts of object-oriented analysis and design, nearly everything within the domain model will be based on an object and will, therefore, be modular and encapsulated. This allows for various components, or even the entire system, to be altered and improved on a regular, continuous basis.
Emphasizes Domain Over Interface: Since DDD is the practice of building around the concepts of domain and what the domain experts within the project advise, DDD will often produce applications that are accurately suited for and representative of the domain at hand, as opposed to those applications which emphasize the UI/UX first and foremost. While balance is required, focusing on the domain means that a DDD approach can produce a product that resonates well with the audience associated with that domain.
Disadvantages of Domain-Driven DesignRequires Robust Domain Expertise: Even with the most technically proficient minds working on development, it's all for naught if there isn't at least one domain expert on the team that knows the exact ins and outs of the subject area. In some cases, domain-driven design may require integrating one or more outside team members who can act as domain experts throughout the development life cycle.
Encourages Iterative Practices: While many consider this an advantage, it cannot be denied that DDD practices strongly rely on constant iteration and continuous integration to build a malleable project that can adjust itself when necessary. Some organizations may have trouble with these practices, particularly if their past experience is primarily tied to less-flexible development models, such as the waterfall model.
Ill-Suited for Highly Technical Projects: DDD is not very well-suited for applications with marginal domain complexity but has a great deal of technical complexity. Since DDD so heavily emphasizes the need for (and importance of) domain experts to generate the proper ubiquitous language and then domain model on which the project is based, a project that is incredibly technically complex may be challenging for domain experts to grasp, causing problems down the line, perhaps when all members of the team did not fully understand technical requirements or limitations.
Should you use Domain-Driven Design within your software development life cycle? It wholly depends on what you're building. If you have a domain expert on the team and want a design that promotes communication and flexibility on a project that isn't overly technical, Domain-Driven Design would be a great option for you! But, for those more technical projects, you might want to consider a different software development life cycle, like OOAD or an Iterative Model.