Building scalable and flexible applications with Domain Driven Design
Domain Driven Design is a software development approach that focuses on understanding and modeling the business domain that the software will support.
The “domain” refers to the specific area of expertise or industry that the software will be used in. For example, if you were building software for a hospital, the domain would be healthcare.
In DDD, developers work closely with domain experts to identify the key concepts, behaviors, and relationships within the domain. These are then used to create a domain model, which serves as a shared language between the development team and the domain experts.
By focusing on the domain, DDD helps ensure that the software being built is aligned with the needs of the business and that it is built in a way that is easy to understand, maintain, and evolve over time.
Eric Evans, a programmer, introduced and popularized Domain-Driven Design in his 2004 book “Domain-Driven Design: Tackling Complexity in the Heart of Software.” Through his book, he explains that DDD is all about building software that connects related pieces of software into an ever-evolving model based on the business domain. This allows developers to create complex applications more easily and effectively, while also ensuring that the software meets the changing needs of the business over time.
When I say building software that connects related pieces of software, I am sure you might be thinking of microservices by now. First, let’s take a look at what the DDD principles are and then see how they benefit when combined with microservices.
Domain-Driven Design has three main principles
Integration of domain-Driven Design (DDD) principles with microservices
Microservices are independent services that are specifically designed to achieve a particular business goal, and they have distinctive configurations and dependencies. The primary objective of microservices is to address organizational issues using Domain-Driven Design (DDD) and DevOps principles. Microservices must have clear interfaces and functions to achieve their business objectives.
Microservices empower teams to work independently on various components of an application, resulting in increased efficiency and reduced coordination time. The combination of DDD and microservices enables developers to create modular and self-contained services that align with the business domain, making it easier to scale and adapt the application to changing needs. This approach results in highly scalable, fault-tolerant applications that allow for continuous delivery and deployment, which enables businesses to quickly adapt to changing market conditions, add new features to their application, and deliver value to their customers faster.
With DDD, developers can model the business domain and ensure that their microservices are specifically designed to fulfill business needs. In contrast, microservices enable the independent deployment and testing of individual services. This combined approach creates a powerful framework for software development that helps businesses stay competitive and deliver value to their customers.
Domain-Driven Design helps solve complexities in software development
Complexity is a problem that Domain-Driven Design can solve.
But what does complexity mean when it comes to software development?
It means things like interconnectedness, many different data sources, different business goals, and so on.
The domain-driven approach is here to help make software development less complex. When a challenge is simple, you can use emergent design. But when your application is complex, the complexity will only grow and cause more problems.
Domain-driven design is based on the business domain. Modern business environments are very complex, and making wrong moves can lead to fatal outcomes. Following the Domain-driven design principles can help solve these complex issues, connecting to the core business concepts.
Important terms you should be knowing when working with Domain-Driven Design (DDD)
Domain Logic
Domain logic refers to the business rules that dictate how data is created, stored, and modified. It is the central purpose of modeling, as it enables developers to create software that meets the specific needs of a business.
Domain Model
A domain model encapsulates all the ideas, knowledge, data, metrics, and goals that are relevant to the problem at hand. It includes the rules and patterns that developers use to manage complex business logic and ensure that software meets the requirements of the business.
Subdomain
A domain is typically made up of multiple subdomains, which each pertain to a specific area of the business logic. For instance, a hospital might have subdomains for patient care, inventory management, and billing, each of which encompasses different functions and rules related to the hospital’s overall operation.
Design Patterns
The goal of design patterns is to reuse code. Whatever the complexity of the problem, someone who has done object-oriented programming has most likely previously devised a pattern that will help you solve it. Breaking your problem down into its basic components will lead you to its answer. Everything you learn through patterns can be applied to any object-oriented language in which you begin to program.
Bounded Context
Bounded context is a key pattern in Domain-Driven Design that limits the application’s complexity. It works with huge models and teams. After you’ve defined the domain and subdomains, you’ll implement the code here.
Bounded contexts are the boundaries within which a specific subdomain is defined and relevant. In this case, the specific subdomain makes sense, although others do not. In different situations, the same entity can have different names. The entire system does not have to change when a subdomain within the confined context changes. That is why developers utilize context adapters.
Ubiquitous Language
The Ubiquitous Language is a methodology that refers to the language that domain experts and developers use when discussing the topic on which they are working. This is crucial since a disturbed language can cause major problems in projects. This occurs because domain experts employ their own vocabulary. Simultaneously, tech professionals utilize their own terminology to discuss the area.
There is a chasm between the words used in everyday conversations and the terms used in the code. That is why a set of terms that everyone uses must be defined. The domain model underpins all of the concepts in the ubiquitous language.
Entities
Entities are objects that consist of both data and behavior, representing specific data points while also possessing the ability to perform actions. They are characterized by their unique identity and attributes, as well as their functionality.
Value Objects
Aggregates
When we work on big projects, the number of objects we have to manage also becomes big. This can be difficult to handle. To make things easier, we can group related objects into something called an “aggregate”.
In complex systems with many entities and value objects, it’s important to group them logically to make them more manageable. These groups are known as aggregates. An aggregate is a collection of related objects that are treated as one unit.
Aggregates are logically related to each other, and they make it easier to manage them as a unit. The objects in an aggregate are governed by a “root” object, which is called the “aggregate root”. This means that if the aggregate root object is deleted, all the other objects associated with it will also be deleted. This ensures that the aggregate is always consistent.
To maintain consistency, we use something called “domain events”. These events are generated whenever something important happens to an object in the aggregate. For example, if the address of a user changes, we can fire a domain event that will update the user’s orders with the new address. This ensures that all the objects in the aggregate are up-to-date and consistent with each other.
Domain Service
A service is essentially any function or type of business logic that does not easily fit within the world of objects. In other words, if any feature is required but cannot be linked to an entity or value object, it is most likely a service.
Factories and Repository
Factories and Repositories are used to handle aggregates. Factories help in managing the start of the lifecycle of an aggregate, while Repositories help in managing the middle and end of the life cycle. Factories are used to create aggregates, while Repositories are used to store them. It’s important to create a repository for each aggregate root, but not for all entities. The repository pattern simplifies the data infrastructure by providing a collection of business entities. This simplification removes the burden of data management from the domain model and separates concerns between layers, which makes the system easier to maintain and more flexible.
An Illustration of Domain-Driven Design
Let’s take an example of an e-commerce app to explain Domain-Driven Design. In this business domain, the main objective is to process an order. The customer browses through the products, selects the desired ones, confirms the order, chooses the shipping type, and pays for it. The app processes this data and generates an order.
e-commerce app can be divided into four layers:
User Interface
This layer displays all the necessary information to the customer for placing an order, such as the products available. It interprets customer actions and presents the data.
Application Layer
This layer does not contain any business logic but is responsible for directing the customer from one UI screen to another. It interacts with application layers of other systems and performs simple validation. It organizes and delegates domain objects to accomplish their task and is the only layer accessible to other limited contexts.
Domain Layer
This layer holds all the concepts related to the business domain. It contains all the information about the business case and the business rules. It has entities like customers and products that combine data and behavior. Entities have a unique identity with a unique key that remains the same even if their attributes change. Value objects represent attributes that multiple entities can share. It also contains services that don’t belong to any domain but are still part of the business domain. The domain layer should be at the center of the business application and separated from the other layers. It shouldn’t depend on the other layers or their frameworks.
Infrastructure Layer
Advantages of Domain-Driven Design (DDD)
Disadvantages of Domain-Driven Design (DDD)
While Domain-Driven Design (DDD) has several advantages, it also has its downsides.
Conclusion
Domain-Driven Design is a way of building software that revolves around the business domain you’re trying to solve. It’s like creating a custom software solution that fits your business perfectly, instead of trying to force a pre-built solution to work.
One of the key aspects of DDD is using a common language between the domain experts and the development team. This helps everyone understand what the other is talking about and leads to better collaboration and understanding of the domain logic.
Of course, there are pros and cons to this approach. One of the biggest advantages is that it leads to simpler communication and a more flexible system. However, it does require domain experts on the development team who truly understand the characteristics of the subject area.
On the other hand, microservices offer some serious advantages over traditional architectures. They allow for scalability, accessibility, and flexibility, and each microservice is focused on a specific area of accountability. This keeps developers focused on a specific task and makes it easier to make changes to the system over time.
Ultimately, the best approach depends on your business needs and the complexity of your application. It’s important to weigh the pros and cons of each approach to make the best decision for your project.