Effectively-once semantics is a feature in Apache Pulsar that ensures that messages are processed and delivered only once, even in the face of failures and retries. It guarantees that messages are processed exactly once, avoiding duplicates and inconsistencies in the system. In Apache Pulsar, effectively-once semantics is achieved through a combination of features such as message deduplication which ensires that if a producers sends duplicate messages, that only one copy is retained, combined with transactional producers and consumers. When a message is consumer by a transactional consumer, it is assigned a unique transaction ID that is used to ensure that the message is processed exactly once. If a failure occurs during message processing, the transaction is rolled back along with any corresponding output messages, ensuring that it is not processed multiple times.