Github user tdas commented on a diff in the pull request:

    https://github.com/apache/spark/pull/20096#discussion_r159984700
  
    --- Diff: 
external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaContinuousReader.scala
 ---
    @@ -0,0 +1,224 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one or more
    + * contributor license agreements.  See the NOTICE file distributed with
    + * this work for additional information regarding copyright ownership.
    + * The ASF licenses this file to You under the Apache License, Version 2.0
    + * (the "License"); you may not use this file except in compliance with
    + * the License.  You may obtain a copy of the License at
    + *
    + *    http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.spark.sql.kafka010
    +
    +import org.apache.kafka.clients.consumer.ConsumerRecord
    +import org.apache.kafka.common.TopicPartition
    +
    +import org.apache.spark.internal.Logging
    +import org.apache.spark.sql.SparkSession
    +import org.apache.spark.sql.catalyst.expressions.UnsafeRow
    +import org.apache.spark.sql.catalyst.expressions.codegen.{BufferHolder, 
UnsafeRowWriter}
    +import org.apache.spark.sql.catalyst.util.DateTimeUtils
    +import 
org.apache.spark.sql.kafka010.KafkaSource.{INSTRUCTION_FOR_FAIL_ON_DATA_LOSS_FALSE,
 INSTRUCTION_FOR_FAIL_ON_DATA_LOSS_TRUE}
    +import org.apache.spark.sql.sources.v2.reader._
    +import 
org.apache.spark.sql.sources.v2.streaming.reader.{ContinuousDataReader, 
ContinuousReader, Offset, PartitionOffset}
    +import org.apache.spark.sql.types.StructType
    +import org.apache.spark.unsafe.types.UTF8String
    +
    +/**
    + * A [[ContinuousReader]] for data from kafka.
    + *
    + * @param offsetReader  a reader used to get kafka offsets. Note that the 
actual data will be
    + *                      read by per-task consumers generated later.
    + * @param kafkaParams   String params for per-task Kafka consumers.
    + * @param sourceOptions The 
[[org.apache.spark.sql.sources.v2.DataSourceV2Options]] params which
    + *                      are not Kafka consumer params.
    + * @param metadataPath Path to a directory this reader can use for writing 
metadata.
    + * @param initialOffsets The Kafka offsets to start reading data at.
    + * @param failOnDataLoss Flag indicating whether reading should fail in 
data loss
    + *                       scenarios, where some offsets after the specified 
initial ones can't be
    + *                       properly read.
    + */
    +class KafkaContinuousReader(
    +    offsetReader: KafkaOffsetReader,
    +    kafkaParams: java.util.Map[String, Object],
    +    sourceOptions: Map[String, String],
    +    metadataPath: String,
    +    initialOffsets: KafkaOffsetRangeLimit,
    +    failOnDataLoss: Boolean)
    +  extends ContinuousReader with SupportsScanUnsafeRow with Logging {
    +
    +  private lazy val session = SparkSession.getActiveSession.get
    +  private lazy val sc = session.sparkContext
    +
    +  // Initialized when creating read tasks. If this diverges from the 
partitions at the latest
    +  // offsets, we need to reconfigure.
    +  // Exposed outside this object only for unit tests.
    +  private[sql] var knownPartitions: Set[TopicPartition] = _
    +
    +  override def readSchema: StructType = KafkaOffsetReader.kafkaSchema
    +
    +  private var offset: Offset = _
    +  override def setOffset(start: java.util.Optional[Offset]): Unit = {
    +    offset = start.orElse {
    +      val offsets = initialOffsets match {
    +        case EarliestOffsetRangeLimit => 
KafkaSourceOffset(offsetReader.fetchEarliestOffsets())
    +        case LatestOffsetRangeLimit => 
KafkaSourceOffset(offsetReader.fetchLatestOffsets())
    +        case SpecificOffsetRangeLimit(p) => 
offsetReader.fetchSpecificOffsets(p, reportDataLoss)
    +      }
    +      logInfo(s"Initial offsets: $offsets")
    +      offsets
    +    }
    +  }
    +
    +  override def getStartOffset(): Offset = offset
    +
    +  override def deserializeOffset(json: String): Offset = {
    +    KafkaSourceOffset(JsonUtils.partitionOffsets(json))
    +  }
    +
    +  override def createUnsafeRowReadTasks(): 
java.util.List[ReadTask[UnsafeRow]] = {
    +    import scala.collection.JavaConverters._
    +
    +    val oldStartPartitionOffsets = 
KafkaSourceOffset.getPartitionOffsets(offset)
    +
    +    val newPartitions =
    +      
offsetReader.fetchLatestOffsets().keySet.diff(oldStartPartitionOffsets.keySet)
    +    val newPartitionOffsets = 
offsetReader.fetchEarliestOffsets(newPartitions.toSeq)
    +    val startOffsets = oldStartPartitionOffsets ++ newPartitionOffsets
    +
    +    knownPartitions = startOffsets.keySet
    +
    +    startOffsets.toSeq.map {
    +      case (topicPartition, start) =>
    +        KafkaContinuousReadTask(
    +          topicPartition, start, kafkaParams, failOnDataLoss)
    +          .asInstanceOf[ReadTask[UnsafeRow]]
    +    }.asJava
    +  }
    +
    +  /** Stop this source and free any resources it has allocated. */
    +  def stop(): Unit = synchronized {
    +    offsetReader.close()
    +  }
    +
    +  override def commit(end: Offset): Unit = {}
    +
    +  override def mergeOffsets(offsets: Array[PartitionOffset]): Offset = {
    +    val mergedMap = offsets.map {
    +      case KafkaSourcePartitionOffset(p, o) => Map(p -> o)
    +    }.reduce(_ ++ _)
    +    KafkaSourceOffset(mergedMap)
    +  }
    +
    +  override def needsReconfiguration(): Boolean = {
    +    knownPartitions != null && offsetReader.fetchLatestOffsets().keySet != 
knownPartitions
    +  }
    +
    +  override def toString(): String = s"KafkaSource[$offsetReader]"
    +
    +  /**
    +   * If `failOnDataLoss` is true, this method will throw an 
`IllegalStateException`.
    +   * Otherwise, just log a warning.
    +   */
    +  private def reportDataLoss(message: String): Unit = {
    +    if (failOnDataLoss) {
    +      throw new IllegalStateException(message + s". 
$INSTRUCTION_FOR_FAIL_ON_DATA_LOSS_TRUE")
    +    } else {
    +      logWarning(message + s". $INSTRUCTION_FOR_FAIL_ON_DATA_LOSS_FALSE")
    +    }
    +  }
    +}
    +
    +/**
    + * A read task for continuous Kafka processing. This will be serialized 
and transformed into a
    + * full reader on executors.
    + *
    + * @param topicPartition The (topic, partition) pair this task is 
responsible for.
    + * @param startOffset The offset to start reading from within the 
partition.
    + * @param kafkaParams Kafka consumer params to use.
    + * @param failOnDataLoss Flag indicating whether data reader should fail 
if some offsets
    + *                       are skipped.
    + */
    +case class KafkaContinuousReadTask(
    +    topicPartition: TopicPartition,
    +    startOffset: Long,
    +    kafkaParams: java.util.Map[String, Object],
    +    failOnDataLoss: Boolean) extends ReadTask[UnsafeRow] {
    +  override def createDataReader(): KafkaContinuousDataReader = {
    +    new KafkaContinuousDataReader(topicPartition, startOffset, 
kafkaParams, failOnDataLoss)
    +  }
    +}
    +
    +/**
    + * A per-task data reader for continuous Kafka processing.
    + *
    + * @param topicPartition The (topic, partition) pair this data reader is 
responsible for.
    + * @param startOffset The offset to start reading from within the 
partition.
    + * @param kafkaParams Kafka consumer params to use.
    + * @param failOnDataLoss Flag indicating whether data reader should fail 
if some offsets
    + *                       are skipped.
    + */
    +class KafkaContinuousDataReader(
    +    topicPartition: TopicPartition,
    +    startOffset: Long,
    +    kafkaParams: java.util.Map[String, Object],
    --- End diff --
    
    rename to JMap while importing to avoid full package name in multiple 
places.


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