In DDD, an aggregate root is an entity that represents the root node of an aggregate (Aggregate). An aggregate is a collection of related domain objects that together form an organic whole. An aggregate root is a special entity inside an aggregate through which we can access and manipulate the entire aggregate.
aggregate root description
When we design and develop software, we often deal with multiple related objects. Sometimes these objects have a close relationship, and together they perform a task or represent a whole. In Domain-Driven Design (DDD), we organize these related objects into an aggregate.
The aggregate root is the core object of this aggregate, and it is the only entrance for us to interact with the outside world. The aggregate root acts as a leader, which represents the entire aggregate and is responsible for managing and coordinating other objects within the aggregate.
Imagine you are developing software for an online store. The aggregate root could be an "Order", while other objects within the aggregate could be "Order Items", "Payments", "Shipping Info", etc. Through the aggregate root, we can access and manipulate these objects.
Aggregate roots have several important characteristics
First, it defines the boundaries of the aggregation, which groups a group of related objects together. This allows for better management of them and avoids confusion and inconsistencies.
Second, the aggregate root is the root entity within the aggregate, which has a unique identifier, like an ID card. Through this identifier, we can find and manipulate other objects within the entire aggregate.
Finally, the aggregate root is responsible for maintaining consistency and integrity within the aggregate. When we perform a change operation, such as creating an order, adding an order item, or modifying the order status, we only need to complete it through the aggregate root. The aggregate root will ensure that all objects inside the aggregate comply with predetermined business rules when performing these operations.
All in all, the aggregate root is an important concept in DDD, which helps us organize and manage related objects and provides a consistent boundary. It simplifies the software design and development process and improves code maintainability and scalability.
Relationship Between Aggregate Roots and Anemia Model and Congestion Model
The aggregate root is neither an anemic model (Anemic Domain Model) nor a hyperemic model (Rich Domain Model). Anemic models are data-only entities that lack behavior and business rules, while hyperemic models are entities that have behavior and business rules.
Aggregate root is closer to the congestion model as it is a stateful entity with behavior and business rules. The aggregate root contains not only attributes (data), but also methods (behavior) and implementation of business rules. Through the aggregate root, we can define and execute related operations in the domain model to ensure the consistency and integrity of the aggregate.
Inside the aggregate, the aggregate root is the only entry point for accessing and manipulating other objects. The aggregate root is responsible for protecting the consistency of the aggregate and restricting external objects from directly accessing other objects within the aggregate. External objects can only operate and obtain objects inside the aggregate through the aggregate root.
To sum up, an aggregate root is a stateful, behavioral, and business-rule entity that represents an aggregate root node. It plays a role in organizing and managing the internal objects of the aggregate in DDD, and is responsible for maintaining the consistency and integrity of the aggregate. It is closer to a hyperemic model than an anemic one.
DDD aggregate root data persistence
When the updated data of the DDD aggregate root needs to be persisted to the database, the following steps can be followed for data conversion and persistence:
1. Update the aggregate root object: First, update the aggregate root object in the application, modify the object's attributes or perform related business operations according to business requirements. This may involve adding, deleting, and modifying objects inside the aggregate root.
2. Data conversion: Next, convert the updated aggregate root object into a form that the database can recognize and store. This can be achieved through data transfer objects (Data Transfer Object, DTO) or domain events.
- Data Transfer Object (DTO): Create a DTO object that contains properties corresponding to the aggregate root and copy the updated data into the DTO object. A DTO object is usually a simple POJO (Plain Plain Java Object) or data structure that contains no business logic.
- Domain event: If you use the domain event mechanism in your DDD practice, you can package the updated data as a domain event object, and pass and process it through the event bus or publish-subscribe mode. In this way, the conversion and persistence of data can be done in the event handler.
3. Data persistence: persist the converted data to the database. This can be achieved by using ORM frameworks (such as Mybatis, Entity Framework) and database access technologies.
- ORM framework: Use the API or persistence method provided by the ORM framework to pass the converted data to the ORM framework, and let it be responsible for mapping the data to the database table and performing the corresponding persistence operations.
- Database access technology: If you don't use an ORM framework, you can use database access technology (such as JDBC) to write your own persistence logic, and write the converted data directly to the database.
4. Transaction management: In the persistence process, in order to ensure data consistency, it is usually necessary to use a transaction management mechanism. Through the transaction management method provided by the ORM framework or programming language, start, commit or rollback the transaction to ensure the atomicity and consistency of the operation.