Hi, I think this part of the documentation is talking about KafkaProducer, and you are reading in the source code of KafkaConsumer.
Best, Stefan > Am 20.09.2018 um 10:48 schrieb 徐涛 <happydexu...@gmail.com>: > > Hi All, > In document of Flink 1.6, it says that "Before 0.9 Kafka did not > provide any mechanisms to guarantee at-least-once or exactly-once semantics” > I read the source code of FlinkKafkaConsumer08, and the comment says: > “Please note that Flink snapshots the offsets internally as part of its > distributed checkpoints. The offsets > * committed to Kafka / ZooKeeper are only to bring the outside view of > progress in sync with Flink's view > * of the progress. That way, monitoring and other jobs can get a view of how > far the Flink Kafka consumer > * has consumed a topic" > Obviously, the kafka partition offsets are checkpointed periodically. > And when some error happens, the data are read from kafka, continued from the > checkpointed offset. Then source and other operator states restart from the > same checkpoint. Then why does the document say “Before 0.9 Kafka did not > provide any mechanisms to guarantee at-least-once or exactly-once semantics” ? > > Thanks a lot. > > > Best > Henry
