Worked on modernizing the persistence layer in the openmrs-core repository by migrating critical domain models from legacy XML and Hibernate mappings to JPA annotations. Focused on PatientIdentifier and ConceptDescription, the work involved enforcing non-null constraints and updating both the API and Liquibase configuration to align with new schema requirements. Using Java, Hibernate, and JPA, the developer improved data integrity and maintainability by refactoring mappings and resolving build issues related to outdated configurations. These changes established a more robust foundation for future migrations, enabling safer data access, smoother deployments, and easier ongoing maintenance of the persistence layer within the project.