KafkaWriter Helper Object — Writing Structured Queries to Kafka

KafkaWriter is a Scala object that is used to write the rows of a batch (or a streaming) structured query to Apache Kafka.

spark sql KafkaWriter write webui.png
Figure 1. KafkaWriter (write) in web UI

KafkaWriter validates the schema of a structured query that it contains the following columns (output schema attributes):

  • Either topic of type StringType or the topic option are defined

  • Optional key of type StringType or BinaryType

  • Required value of type StringType or BinaryType

// KafkaWriter is a private `kafka010` package object
// and so the code to use it should also be in the same package
// BEGIN: Use `:paste -raw` in spark-shell
package org.apache.spark.sql.kafka010

object PublicKafkaWriter {
  import org.apache.spark.sql.execution.QueryExecution
  def validateQuery(
      queryExecution: QueryExecution,
      kafkaParameters: Map[String, Object],
      topic: Option[String] = None): Unit = {
    import scala.collection.JavaConversions.mapAsJavaMap
    KafkaWriter.validateQuery(queryExecution, kafkaParameters, topic)
// END

import org.apache.spark.sql.kafka010.{PublicKafkaWriter => PKW}

val spark: SparkSession = ...
val q = spark.range(1).select('id)
scala> PKW.validateQuery(
  queryExecution = q.queryExecution,
  kafkaParameters = Map.empty[String, Object])
org.apache.spark.sql.AnalysisException: topic option required when no 'topic' attribute is present. Use the topic option for setting a topic.;
  at org.apache.spark.sql.kafka010.KafkaWriter$$anonfun$2.apply(KafkaWriter.scala:53)
  at org.apache.spark.sql.kafka010.KafkaWriter$$anonfun$2.apply(KafkaWriter.scala:52)
  at scala.Option.getOrElse(Option.scala:121)
  at org.apache.spark.sql.kafka010.KafkaWriter$.validateQuery(KafkaWriter.scala:51)
  at org.apache.spark.sql.kafka010.PublicKafkaWriter$.validateQuery(<pastie>:10)
  ... 50 elided

Writing Rows of Structured Query to Kafka Topic — write Method

  sparkSession: SparkSession,
  queryExecution: QueryExecution,
  kafkaParameters: ju.Map[String, Object],
  topic: Option[String] = None): Unit

write gets the output schema of the analyzed logical plan of the input QueryExecution.

In the end, write requests the QueryExecution for RDD[InternalRow] (that represents the structured query as an RDD) and executes the following function on every partition of the RDD (using RDD.foreachPartition operation):

  1. Creates a KafkaWriteTask (for the input kafkaParameters, the schema and the input topic)

  2. Requests the KafkaWriteTask to write the rows (of the partition) to Kafka topic

  3. Requests the KafkaWriteTask to close


write is used when:

Validating Schema (Attributes) of Structured Query and Topic Option Availability — validateQuery Method

  schema: Seq[Attribute],
  kafkaParameters: ju.Map[String, Object],
  topic: Option[String] = None): Unit

validateQuery makes sure that the following attributes are in the input schema (or their alternatives) and of the right data types:

  • Either topic attribute of type StringType or the topic option are defined

  • If key attribute is defined it is of type StringType or BinaryType

  • value attribute is of type StringType or BinaryType

If any of the requirements are not met, validateQuery throws an AnalysisException.


validateQuery is used when:

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