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From koeninger <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-4964] [Streaming] Exactly-once semantic...
Date Wed, 28 Jan 2015 21:50:35 GMT
Github user koeninger commented on a diff in the pull request:

    https://github.com/apache/spark/pull/3798#discussion_r23726952
  
    --- Diff: external/kafka/src/main/scala/org/apache/spark/streaming/kafka/KafkaUtils.scala
---
    @@ -144,4 +150,116 @@ object KafkaUtils {
         createStream[K, V, U, T](
           jssc.ssc, kafkaParams.toMap, Map(topics.mapValues(_.intValue()).toSeq: _*), storageLevel)
       }
    +
    +  /** A batch-oriented interface for consuming from Kafka.
    +   * Starting and ending offsets are specified in advance,
    +   * so that you can control exactly-once semantics.
    +   * @param sc SparkContext object
    +   * @param kafkaParams Kafka <a href="http://kafka.apache.org/documentation.html#configuration">
    +   * configuration parameters</a>.
    +   *   Requires "metadata.broker.list" or "bootstrap.servers" to be set with Kafka broker(s),
    +   *   NOT zookeeper servers, specified in host1:port1,host2:port2 form.
    +   * @param batch Each OffsetRange in the batch corresponds to a
    +   *   range of offsets for a given Kafka topic/partition
    +   * @param messageHandler function for translating each message into the desired type
    +   */
    +  def createRDD[
    +    K: ClassTag,
    +    V: ClassTag,
    +    U <: Decoder[_]: ClassTag,
    +    T <: Decoder[_]: ClassTag,
    +    R: ClassTag] (
    +      sc: SparkContext,
    +      kafkaParams: Map[String, String],
    +      batch: Array[OffsetRange],
    +      messageHandler: MessageAndMetadata[K, V] => R
    +  ): RDD[R] = {
    +    val parts = batch.zipWithIndex.map { case (o, i) =>
    +        new KafkaRDDPartition(i, o.topic, o.partition, o.fromOffset, o.untilOffset, o.host,
o.port)
    +    }.toArray
    +    new KafkaRDD[K, V, U, T, R](sc, kafkaParams, parts, messageHandler)
    +  }
    +
    +  /**
    +   * This DOES NOT guarantee that side-effects of an action will see each message exactly
once.
    --- End diff --
    
    I mentioned this in the design docs, but I'm not comfortable with naming something "exactlyOnce"
when it doesn't actually guarantee that for the usual meaning of exactly once in a messaging
system.  Hence the allcaps warning.
    
    I think a better option would be to rename the method, suggested a couple of possibilities
in the design doc.


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