https://medium.com/@ygnhmt/a-soft-introduction-to-domain-driven-design-from-theory-to-java-code-implementation-part-2-5aa7e1cfef65

This article provides an accessible introduction to the concept of Domain-Driven Design (DDD) and demonstrates its practical implementation in Java. It begins by offering a brief overview of the previous article, establishing a real-world context for the Java code implementation that follows.

DDD, as explained, is a design approach that focuses on aligning software systems with the underlying domain or business it serves. It emphasizes the creation of well-defined domain models, clear boundaries between domains, and the use of events for effective domain communication.

The article then introduces a fictional visa application system set in the fictional country of Krakozhia. Three main domains—VisaApplication, Security, and VisaIssue—are defined, each responsible for specific aspects of the visa application process. Interactions between these domains are orchestrated using Spring events and JMS messaging.

VisaApplication serves as the central domain, encompassing entities like Applicant and PassportInformation. Validation and event publication are key components of this domain. SecurityCheck, the second domain, handles security compliance checks by sending JMS messages to NIA, Homeland, and Interpol. It maintains similar structural and behavioral patterns to the VisaApplication domain. VisaIssue, the third domain, comes into play after successful security checks, updating the visa issuance status.

Throughout the implementation, the article emphasizes the importance of adhering to DDD principles, including using domain events for communication and placing repository interfaces in the Domain layer while their implementations reside in the Infrastructure layer.

The article underscores the significance of maintaining clear boundaries between layers—Domain, Application, and Infrastructure—and the need for distributed transactions due to multi-source data interactions (JPA and JMS).

In conclusion, this article provides a practical, hands-on demonstration of how DDD principles can be applied in Java to solve complex real-world problems. It showcases the power of DDD in aligning software systems with business domains, ultimately leading to more robust and maintainable applications.