How do you design aggregate roots and entities for complex business rules?

1 What are aggregate roots and entities?

An aggregate root is a cluster of related objects that form a consistent and cohesive unit of work in the domain. An aggregate root defines the boundaries and invariants of the aggregate, and acts as the single point of access and control for all the objects within the aggregate. An entity is an object that has a unique identity and a lifecycle in the domain, and that can change its state and behavior over time. An entity can be part of an aggregate, or it can be an aggregate root itself, depending on the context and the business rules.

2 How to identify aggregate roots and entities?

One of the main challenges in designing aggregate roots and entities is to identify them correctly from the domain knowledge and the business rules. A common approach is to use the ubiquitous language, a shared and expressive vocabulary that captures the essential terms and concepts of the domain, and to use it consistently throughout the design and implementation. Another approach is to use domain events, which are meaningful occurrences that affect the state of the domain, and to trace the causal relationships and dependencies between them. By doing so, you can discover the aggregate roots and entities that are involved in the events, and how they collaborate and interact with each other.

3 How to design aggregate roots and entities?

Once you have identified the aggregate roots and entities, it is important to design them in a way that preserves their consistency and integrity, and supports the complex business rules of the domain. To do this, you should define a clear and explicit identity for each aggregate root and entity, using a natural or artificial identifier that is meaningful and stable in the domain. Additionally, each aggregate root and entity should be encapsulated to expose only the necessary operations and properties to the outside world. Furthermore, invariants and constraints of each aggregate root and entity should be enforced to ensure they are always valid and consistent within their boundaries. It is also important to avoid sharing references or mutable state between different aggregate roots and entities; instead, value objects, which are immutable and interchangeable objects that represent attributes or measurements, should be used to communicate or transfer data between them. Finally, the principle of least privilege should be applied to grant only the minimum permissions and access rights to each aggregate root and entity according to their roles and responsibilities in the domain.

4 How to apply common patterns for aggregate roots and entities?

In addition to design principles and guidelines, you can also utilize common patterns and techniques to help design aggregate roots and entities for complex business rules. The factory pattern is a creational pattern that encapsulates the logic and details of creating an aggregate root or an entity, providing a simple and consistent interface for doing so. The repository pattern is a behavioral pattern that abstracts the persistence and retrieval of aggregate roots or entities, offering a collection-like interface for accessing them. The specification pattern is a structural pattern that encapsulates the criteria and logic of a business rule, providing a reusable and composable way of applying it to an aggregate root or an entity. Finally, the domain service pattern is a behavioral pattern that encapsulates the logic and operations that do not belong to a single aggregate root or an entity, but require the collaboration or coordination of multiple aggregate roots or entities.

5 How to test aggregate roots and entities?

Testing aggregate roots and entities is a critical part of guaranteeing the quality and dependability of your domain model, as well as verifying that it complies with the intricate business rules of the domain. You can use several strategies and techniques such as unit testing, integration testing, and behavior-driven development (BDD). Unit testing focuses on examining the functionality and behavior of a single aggregate root or an entity, while integration testing centers on testing the interaction between different aggregate roots or entities or between them and external systems. BDD is a methodology that uses a natural language to describe the expected outcomes and scenarios of a business rule and automates the testing of an aggregate root or an entity based on them.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?