Worked on the spring-projects/spring-pulsar repository to address a reliability issue affecting batch acknowledgments for partitioned Apache Pulsar topics. Focused on backend development using Java and the Spring Framework, the work involved fixing a bug in AckMode.BATCH where only the last message was acknowledged, which previously compromised at-least-once delivery guarantees. The solution introduced per-topic message tracking and cumulative acknowledgments for each message across all partitions, ensuring that every message is properly acknowledged. This change improved message reliability and reduced the need for manual retries, particularly for Exclusive subscriptions, while enhancing maintainability through clear commit references and traceability.