David Gelhar optimized principal-based session retrieval in the mmoayyed/cas repository by refactoring the MongoDB ticket registry. He replaced an inefficient text index with a direct IDX_PRINCIPAL index lookup for the getSessionsFor(principalId) operation, streamlining queries and reducing resource consumption. David also removed the obsolete JSON text index, simplifying database maintenance and paving the way for future index-driven improvements. To ensure reliability, he added targeted tests for session retrieval and deletion by principal, enhancing regression coverage. His work demonstrated depth in backend development, database management, and Java, resulting in improved performance and maintainability for principal-scoped session operations.