What is Domain-Driven Design?

What is Domain-Driven Design

Domain-Driven Design or DDD is a business cum technical term that might be new to you. A domain-driven-design comes with several benefits, but its usage can only be found in complex applications. If you are creating a mobile application, such as an FPS shooter game or a sketch application, you might not need to use the DDD approach. On the other hand, if you look at today’s enterprise applications, you can see they are pretty sophisticated and run smoothly without prompting errors. Also, these heavy applications rely on specialized technologies to perform the task for which they have been developed.

As a developer, one needs to understand the requirements of the project and work accordingly. The DDD concept aims to look at the core of the application, focusing on its complexity, which is intrinsic to the business domain on which the project is based. In DDD, we also try to find what makes the business unique and based on the research, try to find specific solutions.

Today we will be looking at the patterns which the Domain-Driven Design uses for building massive enterprise-level applications. If you are working in the software development industry, you might have already encountered some of these patterns. When you are using the DDD approach, the developed application will meet the various aspects of the business requirements, no matter how small or big they are in terms of their usage.

What is Domain in Domain-Driven Design?

The DDD methodology is the practice of designing and implementing the project’s development cycle on the basics of the domain of the project. The bigger the project is, the harder it is to keep a close eye on its development work. A better approach would be to break the project into sub-parts according to the business rules. These subparts are known as domains. In the field of software engineering, domains play a vital role in deciding whether the developer has enough skill and experience to work on the project or not.

Understanding the Importance of Domain-Driven Design

DDD is a software development approach that simplifies the complexity of the initial phase of application development. This decrease in complexity is done by connecting various implementations to an already evolving development model.

Explanation of Domain-Driven Design’s Working

Let’s take a food recipe with four ingredients (tomatoes, cottage cheese, onion and garlic), and these four ingredients are different from each other. But once you put them in the bowl with other essential ingredients like salt, pepper, turmeric, red chilly, etc., and cook them on the stove, all the elements come together and make a mouth-watering dish.

Now, let’s have a breakfast experience as our second example. Here again, you have four different dishes (toasted bread, hot chocolate, coleslaw, and omelet), and each of them is present in their separate plates and containers. Even if one of the dishes is not present, the whole experience will be less wholesome, but still, it is a breakfast, and you can eat it.

So from above these two examples, which one do you think follows the concept of Domain-Driven Design? The answer is the second example, where all four breakfast dishes were completely independent of each other. But we can still have them together and separately and still call it breakfast.

In the same way, the DDD approach of developing an application has different layers which are being used one after the other. To complete the complex development process in a much more understandable way so that even the rookies will be able to identify the individual core concepts of the application along with their working with other parts of the application.

Core Usage

Now think about replacing one of the four dishes (toasted bread, hot chocolate, coleslaw, and omelet) from our second example even if we replace one of the four dishes from the breakfast and cook something else like boiled eggs. It can still be a part of the breakfast. Like that, the addition and removal of the different aspects of the applications are how a DDD functions. As a result, the DDD approach becomes exceptionally crucial for business owners and developers to create enterprise-level applications, adding new features after its official release.

As a developer, one is always excited to start a new project. To test their coding skills and creativity for building a code that is both efficient and reliable. However, there is no need to grab your laptop to start coding before you completely understand the client’s requirements from the application.

DDD’s approach puts a lot of emphasis on comprehending the client’s demands and making sure that the development team works as a partner with the client throughout the entire run of the project. The end goal in DDD is not just to write code to get paid for it, but it is to solve the problem of the client in the most efficient way possible.

Building Blocks of Domain-Driven Design

Given below, we have explained each building block of the DDD approach so that it becomes easier for software engineers to implement it.

1. Entities

An entity is more like a plain object that is given an identity, and if there’s no use to it, we can mute it. Every single entity present in the project is assigned an ID for easier identification.

Two entities could be considered equal to each other if they have the same ID even though they contain different attributes inside them. As a result, the state of the entity can be changed anytime during the development cycle. But as long as two entities have the same ID, they will be considered equal regardless of their different attributes.

2. Value Objects

Just from the name of it, you can figure out that these objects are not mutable. Also, they don’t have IDs like entities. Two value objects will only be considered equal if they have the same type and the same attributes.

The value objects are mainly used for passing the message. Besides this, the value objects come in pretty handy in the API layer, which is present within the onion architecture to expose the domain concepts without exposing the immutable aspects of the domain.

Some of the benefits of having a value object in DDD methodology are as follows:

  • Value objects as a whole can swallow a lot of the computational complexity of the project. As a result, the entities which are responsible for the complexity can be removed.
  • It helps in increasing the extensibility, especially for carrying out tests on concurrency issues, if they are deployed correctly.

3. Aggregate Roots

The Aggregate Roots is also an entity that binds itself with other entities. In addition to this, aggregate roots are a part of aggregates which are a collection/cluster of different closely associated objects. The aggregate roots are all treated as a single unit for data changes. Thus, if you think about it, all the aggregates have their aggregate root along with a boundary.

Let’s take an example to explain the working of aggregate roots. Here we have two aggregates by the name Order and OrderLineItem. Both of these aggregates are present in SalesOrderDomain, which is also considered as an aggregate. In SalesOrderDomain, the Order works as an aggregate root while the OrderLineItem is the child of Order present within the SalesOrder boundary.

It would be best if you keep in mind that all the operations within the domain have to go through an aggregate root whenever it’s possible for them. Some of the exceptions are factories, repositories, and services. But during the implementation of DDD, it is better to pass the operation through an aggregate root to keep all things intact and aligned with each other.

4. Repositories

We all know repositories are the places to store data, they are considered to be essential components of the DDD method. With the use of repositories in DDD the concerns of keeping all the data in one place and keeping it secure vanishes away.

The implementation of a repository could be both file-based storage or a database that uses SQL as an input language.

A repository must not be confused as a date hoarding place. The primary function of the repository is to store aggregate roots. Moreover, it has to do one of the tedious tasks of talking to multiple storage locations to construct the aggregates. As a result, a single aggregate root can be taken out from the REST API, along with its files and database. The repository is an added layer of abstraction on top of all the different data stores.

5. Factories

Factories in DDD are used for the abstraction for the object construction in the project. So, it has the potential to return an aggregate root or an entity or even a value object when needed. In many cases, we have seen that when you need to perform the factory method for an aggregate root, it will be dropped into the repository. This makes the repository implement the finder create method.

Most factories are deployed as an interface within the domain and domain services layer. With the infrastructure layer being defined by the implementation logic.

6. Services

All the operations that can’t find their place in the aggregate root find a shelter in the home called services in DDD methodology. For example, if there’s an operation used by multiple aggregate roots, it becomes confusing to decide where to put that operation in a specific aggregate root. In such scenarios, you can add that operation or logic to the service.

On the other hand, be cautious and don’t put everything in the services. First, you need to analyze and see if the given operation fits an existing aggregate root or not. If there’s no aggregate root that can be used to house the operation, it is essential to take a step back and check if you have missed any aggregate roots.

These six different components are the building blocks of the Domain-Driven Design approach for application development. Each one of them is equally important to the other, and all of them have to be created with utmost precision, detail and research.

Benefits Of Domain-Driven Design

Now you know about DDD methodology and its six main pillars, let’s talk about some of the benefits which you get by using it:

  • Better Communication

DDD is a common model which every team working on the project can easily understand. The teams that are handling the business aspects of the project and the developer team responsible for the coding part can have the common ground to discuss the various requirements of the project, such as entities, data, and process models and interface.

  • Flexibility

DDD completely turns the tables for Object-Oriented design. It simply means that everything in the domain model is based on an object, and therefore it will be in the form of a module and encapsulated. As a result, it will give the system the freedom to be continuously evolved and open to regular updates.

  • Easy to Understand Patterns

DDD allows software developers to use patterns and principles which are easy to understand. These patterns are used to solve complex development problems and sometimes even help in solving business-related challenges.

  • Reduce Risk Of Misinterpretation

Any project which a company takes, no matter how big or small it is, will be explained from the perspective of the domain which the project will be using. Moreover, it becomes easier to conceptualize the software systems in terms of the business domain to reduce the confusion of non-technical teams. Also, it will reduce the time which gets wasted due to the back-and-forth explanation of concepts.

  • Better Coordination

As different teams are working with the same terminologies. As a result, there will be less bottlenecking during the development of the application. Thus, members of various groups can ideally work with each other in a more coherent project atmosphere. Leading to better results in a much shorter time.

  • Agility Becomes Your Standard

DDD is all about the agile approach, which is both iterative and incremental. This also makes it easier for every team member to have a clear picture of the project model and how it is going to develop.

  • Balanced Application

With the DDD methodology, you and your team members are going to build the application around the domain. Also, you will be taking notes of what the experts of the domain are advising. As a result, the developed application will function while meeting all the domains’ needs, which helps create a more balanced product.

  • Keeps Workforce Focused

When you can clearly see what will be the end result and the path to achieving it, you and your team can stay calm and focused.

Example Of Domain-Driven Design

So we are done with the explanation of DDD and its importance along with benefits. Now let’s move to the real-world example, where DDD has already been in use and is helping both developers and clients to collaborate and create a more cohesive application.

E-commerce App

Any big e-commerce app is a near-perfect example of DDD methodology. The business domain of this project is processing the Order. When a customer is looking to place an order, they first need to select the product from the list of products. After choosing the product, they need to confirm it and add their address. Once the shipping address form has been filled, they need to make the payment. Once the payment is made, the app will provide the confirmation, and the request will be sent to the e-commerce company to find the product from their store and send it to the customer.

So the app will consist of the following layers according to the DDD approach:

  • User Interface

This is where a customer will find all the information needed to place an order. In the case of e-commerce applications, this is the layer where the developer needs to work on the user experience as this layer will house all information that is to be displayed to a customer.

  • Application Layer

This layer will work without any business logic, and it’s the part that makes the user go from one UI screen to another. In addition to this, it will interact with the different layers which are present in the application. There is no domain-related logic to be found here, but it does contain simple validation tasks. The primary purpose of the application layer is to organize and delegate domain objects so they can finish the required job effectively. This is the only layer in the whole project which is accessible to other bounded contexts.

  • Domain Layer

The domain layer resides in the main concepts of the business domain. All the information about the business case and its rules can be found here. This is where you can find different entities and defined operational behavior, which doesn’t have to be a part of the domain. In other words, the domain layer is said to be the center of the business application. As a result, it needs to be separated from the rest of the layers and not dependent on other layers or frameworks.

  • Infrastructure Layer

This layer manages and controls all the communication which happens between the different layers, and it does contain supporting libraries for the UI layer of the application.

Wrapping Up

Domain-Driven Design is one of the best software development approaches. This methodology has been developed to make the workflow more transparent and understandable for complex projects, which require multiple teams to work together. It helps in exploring the client’s needs and the problems that the application aims to solve. On the other hand, for developers, DDD allows them to break a large project into small parts so that different teams can work on various sections of the application.

While DDD is an excellent method to develop a project, one needs to have the proper knowledge regarding its implementation. Otherwise, DDD could quickly become a mess and increase the complexity of a project.


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