When creating a Kafka producer with exactly-once semantics using the Kafka API, two properties have to be set: transactional.id has to be set to a transactional id, and enable.idempotence has to be set to true.
In Apache Flink, a FlinkKafkaProducer can be configured with a parameter for the desired semantics of the producer, in particular with the value Semantics.EXACTLY_ONCE for exactly once semantics.
Looking at the source code of the FlinkKafkaProducer, transactional ids are automatically generated and maintained. However, I did not found any place where enable.idempotence is set to true for the ProducerConfig of the underlying KafkaProducer.
Does the property enable.idempotence have to be provided in the ProducerConfig given to the FlinkKafkaProducer, or is there something I have overseen?
No it doesn't need to be set as stated in the Kafka documentation:
If the transactional.id is set, idempotence is automatically enabled along with the producer configs which idempotence depends on.
And as you noticed, after selecting Semantics.EXACTLY_ONCE Flink is taking care of setting and controlling transactional.id property.
However there are a couple of other properties that you might be interested mentioned here. I'm not sure how is it now, but as I checked last time default Kafka brokers/clients configuration was actually allowing for data loss to happen in some scenarios.