DDD: Where to Put Exceptions

One of the challenges faced when utilizing Domain-Driven Design (DDD) is determining where to handle exceptions within bounded contexts. This article explores potential locations and provides guidance for making these decisions, ensuring that your DDD implementation is robust, maintainable, and adheres to best practices.

Bounded Contexts and Responsibility

In DDD, bounded contexts represent distinct areas or domains within your application. They encapsulate related entities, rules, and domain logic, providing a clear separation of concerns. Each bounded context should be responsible for its own exceptions, promoting cohesion and reducing coupling between contexts.

Designated Exception Classes

One approach to handling exceptions is to define designated exception classes within each bounded context. These exceptions should be specific to the domain, reflecting domain-specific error conditions that can occur within that context. For example, an online store might define exceptions such as InventoryUnavailableException or InvalidOrderException.

By handling exceptions within designated classes, you can provide rich information about the error, including contextual details and potentially actionable resolution strategies tailored to the domain. This approach facilitates more effective error handling and minimizes the need for boilerplate code.

Service Layer and Domain layer

Another consideration is the placement of exception handling within the service and domain layers of your application. The service layer often acts as an intermediary between the domain and external clients, such as web services or user interfaces. It's a suitable location for handling exceptions that occur during interactions with external clients, allowing you to provide a consistent and user-friendly response.

Conversely, the domain layer should focus on handling exceptions that originate from within the domain logic itself. These exceptions represent domain-specific constraints, invariants, or business rules that have been violated. By isolating exception handling to the domain layer, you ensure that the domain logic remains cohesive and focused on its core responsibilities.

Propagating Exceptions

When an exception occurs in the domain layer that should be handled at a higher level, the exception can be propagated to the service layer. This can be done by rethrowing the exception, allowing the service layer to respond appropriately. Alternatively, a custom exception can be thrown in the service layer, providing additional information or context specific to that layer.

Conclusion

In DDD, the placement of exception handling is crucial for maintaining the integrity, cohesion, and maintainability of your application. By designating exception classes within each bounded context and carefully considering the location of exception handling in the service and domain layers, you can ensure effective error handling practices that align with DDD principles.

FAQs

1. Why is it important to handle exceptions within bounded contexts?

Handling exceptions within bounded contexts promotes cohesion and reduces coupling between contexts, resulting in a more maintainable and extensible application.

2. What are the advantages of using designated exception classes?

Designated exception classes provide rich information about errors, including contextual details and potentially actionable resolution strategies.

3. Where should exceptions be handled in the service and domain layers?

Exceptions originating from interactions with external clients should be handled in the service layer, while exceptions related to domain logic should be handled in the domain layer.

4. How can exceptions be propagated from the domain layer to the service layer?

Exceptions can be propagated by rethrowing the exception or by throwing a custom exception in the service layer that provides additional information or context.

5. What are some best practices for exception handling in DDD?

Best practices include using designated exception classes, handling exceptions as close to the source of the problem as possible, providing informative error messages, and logging exceptions for analysis and monitoring.