Have Kafka transactions (exactly-once semantics) been used?

Yes, Apache Kafka supports transactions that provide exactly-once semantics when writing messages. This is achieved through a transactional producer that groups multiple records into a single atomic operation. In Kotlin, using the Kafka Producer API, you can configure transactions to ensure that messages are either fully written or not written at all, preventing duplication in case of failures.

Example of initializing a transactional producer in Kotlin:

val props = Properties().apply {
    put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092")
    put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer::class.java.name)
    put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, StringSerializer::class.java.name)
    put(ProducerConfig.ENABLE_IDEMPOTENCE_CONFIG, "true")
    put(ProducerConfig.TRANSACTIONAL_ID_CONFIG, "my-transactional-id")
}

val producer = KafkaProducer<String, String>(props)
producer.initTransactions()

try {
    producer.beginTransaction()
    producer.send(ProducerRecord("topic", "key", "value"))
    // other operations
    producer.commitTransaction()
} catch (e: Exception) {
    producer.abortTransaction()
}