How do you design aggregate roots for complex business rules?

3 Define the aggregate boundaries

The third step is to define the aggregate boundaries, which are the limits of the objects and relationships that belong to the aggregate. To ensure consistency and integrity, these boundaries should be as small as possible, but large enough to include all essential objects for the aggregate root's functions and rules. You can use criteria such as including only those objects directly affected by the aggregate root's actions or state changes, excluding those with weak or eventual consistency requirements, and excluding any objects that are part of another aggregate or bounded context. Instead of including these objects, use references or identifiers.

4 Design the aggregate interfaces

Designing aggregate interfaces is the fourth step. These methods and properties expose the aggregate root's behavior and state. When designing these interfaces, use verbs that reflect the domain language and the intent of the commands and events, such as placeOrder, confirmPayment, or shipOrder. Additionally, use value objects to represent the parameters and results of the methods, such as OrderId, PaymentDetails, or ShippingAddress. It's important to avoid exposing the internal state or structure of the aggregate. To do this, use encapsulation and abstraction to hide implementation details. Additionally, avoid exposing any methods or properties that are not relevant for the aggregate's clients by using private or protected modifiers to restrict their access.

5 Implement the aggregate logic

The fifth step is to implement the aggregate logic, which is the code that executes the aggregate root's methods and manipulates its state. To do this, domain events should be used to capture the state changes and outcomes of the methods and publish them to other aggregates or bounded contexts that need to react. Additionally, domain services should be used to encapsulate any logic that involves more than one aggregate or does not belong to any aggregate, such as calculations, validations, or integrations. Repositories should be used to abstract the persistence and retrieval of the aggregates, while factories should be used to create new instances of the aggregates and initialize their state and invariants.

6 Test and refactor the aggregates

The final step is to test and refactor the aggregates, which is the process of verifying and improving the quality and performance of the aggregates. To do this, you should use unit tests to check the correctness and consistency of the aggregate logic, and integration tests to check communication and coordination with other aggregates, domain services, or repositories. Additionally, you should use performance tests to check scalability and efficiency of the aggregates, as well as code analysis tools to check readability and maintainability. You may also need to use caching, batching, or partitioning strategies to optimize them, as well as refactoring patterns to simplify or restructure them.