Spark Streaming + Kafka: how to check name of topic from kafka message - scala

Question

I am using Spark Streaming to read from a list of Kafka Topics.
I am following the official API at this link. The method I am using is:
val kafkaParams = Map("metadata.broker.list" -> configuration.getKafkaBrokersList(), "auto.offset.reset" -> "largest")
val topics = Set(configuration.getKafkaInputTopic())
val stream = KafkaUtils.createDirectStream[String, String, StringDecoder, StringDecoder](
ssc, kafkaParams, topics)
I am wondering how will the executor read from the message from the list of topics ? What will be their policy? Will they read a topic and then when they finish the messages pass to the other topics?
And most importantly, how can I, after calling this method, check what is the topic of a message in the RDD?
stream.foreachRDD(rdd => rdd.map(t => {
val key = t._1
val json = t._2
val topic = ???
})

Answer 1

I am wondering how will the executor read from the message from the
list of topics ? What will be their policy? Will they read a topic and
then when they finish the messages pass to the other topics?
In the direct streaming approach, the driver is responsible for reading the offsets into the Kafka topics you want to consume. What it does it create a mapping between topics, partitions and the offsets that need to be read. After that happens, the driver assigns each worker a range to read into a specific Kafka topic. This means that if a single worker can run 2 tasks simultaneously (just for the sake of the example, it usually can run many more), then it can potentially read from two separate topics of Kafka concurrently.
how can I, after calling this method, check what is the topic of a
message in the RDD?
You can use the overload of createDirectStream which takes a MessageHandler[K, V]:
val topicsToPartitions: Map[TopicAndPartition, Long] = ???
val stream: DStream[(String, String)] =
KafkaUtils.createDirectStream[String, String, StringDecoder, StringDecoder](
ssc,
kafkaParams,
topicsToPartitions,
mam: MessageAndMetadata[String, String]) => (mam.topic(), mam.message())