How do you balance consistency and flexibility when defining policy-driven aggregates and value objects?

1 What are policy-driven aggregates and value objects?

Policy-driven aggregates and value objects are those that are influenced by some business rules or policies that define their behavior and constraints. For example, a policy might dictate how to calculate the price of a product, how to validate an order, or how to handle a refund. Policy-driven aggregates and value objects encapsulate these policies in their methods and attributes, and ensure that they are enforced and applied consistently throughout the system.

3 How to achieve consistency with policy-driven aggregates and value objects?

One way to achieve consistency with policy-driven aggregates and value objects is to use the principle of cohesion, which means that the elements of a design unit should be related and work together for a common purpose. For example, you can group the objects that share the same policy into an aggregate, and use the aggregate root as the single point of access and control for the policy. You can also define value objects that represent the concepts and values that are relevant for the policy, and use them as parameters and return types for the aggregate methods. This way, you can ensure that the policy is applied consistently and correctly within the aggregate boundary, and that the value objects are valid and meaningful.

4 How to achieve flexibility with policy-driven aggregates and value objects?

Another way to achieve flexibility with policy-driven aggregates and value objects is to use the principle of coupling, which means that the degree of dependency and interaction between design units should be minimized. For example, you can decouple the policy logic from the aggregate or value object implementation, and use interfaces, abstractions, or strategies to inject or select the policy at runtime. You can also decouple the policy from the context, and use factories, repositories, or services to create or retrieve the appropriate aggregate or value object for a given scenario. This way, you can allow for variations and changes in the policy, and avoid hard-coding or duplicating the policy logic across the system.

5 What are some trade-offs to consider when balancing consistency and flexibility?

When balancing consistency and flexibility with policy-driven aggregates and value objects, it is important to consider the trade-offs involved, such as complexity versus simplicity, performance versus modifiability, and standardization versus customization. These trade-offs depend on the nature and complexity of the domain and the policy. For example, more abstraction or decoupling layers might increase flexibility but also lead to increased complexity. Injecting or selecting the policy at runtime might improve modifiability but affect performance and scalability. Additionally, defining a common policy for all aggregates or value objects might enhance consistency but limit customization. Therefore, it is essential to weigh the pros and cons of each design option to choose the one that best suits your domain and policy needs.

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?