Major Description
According to the JavaDoc (https://kafka.apache.org/0101/javadoc/org/apache/kafka/clients/producer/ProducerInterceptor.html) " This is called from KafkaProducer.send(ProducerRecord) and KafkaProducer.send(ProducerRecord, Callback) methods, before key and value get serialized and partition is assigned (if partition is not specified in ProducerRecord)".
Although when using this with Kafka Streams (StreamsConfig.producerPrefix(ProducerConfig.INTERCEPTOR_CLASSES_CONFIG)) the key and value contained in the record object are already serialized.
As you can see from the screenshot, the serialization is performed inside RecordCollectionImpl.send(ProducerRecord<K, V> record, Serializer<K> keySerializer, Serializer<V> valueSerializer,
StreamPartitioner<K, V> partitioner), effectively before calling the send method of the producer which will trigger the interceptor.
This makes it unable to perform any kind of operation involving the key or value of the message, unless at least performing an additional deserialization.
