FlatMapGroupsWithStateExec Unary Physical Operator

FlatMapGroupsWithStateExec is a unary physical operator that represents a FlatMapGroupsWithState logical operator at execution time.

Note
A unary physical operator is a physical operator with a single child physical operator.

FlatMapGroupsWithStateExec is created when FlatMapGroupsWithStateStrategy execution planning strategy is requested to plan a streaming query with FlatMapGroupsWithState logical operators for execution.

FlatMapGroupsWithStateExec is an ObjectProducerExec physical operator with the output object attribute.

FlatMapGroupsWithStateExec is a physical operator that supports streaming watermark.

import java.sql.Timestamp
import org.apache.spark.sql.streaming.GroupState
val stateFunc = (key: Long, values: Iterator[(Timestamp, Long)], state: GroupState[Long]) => {
  Iterator((key, values.size))
}
import java.sql.Timestamp
import org.apache.spark.sql.streaming.{GroupState, GroupStateTimeout, OutputMode}
val rateGroups = spark.
  readStream.
  format("rate").
  load.
  withWatermark(eventTime = "timestamp", delayThreshold = "10 seconds").  // required for EventTimeTimeout
  as[(Timestamp, Long)].  // leave DataFrame for Dataset
  groupByKey { case (time, value) => value % 2 }. // creates two groups
  flatMapGroupsWithState(OutputMode.Update, GroupStateTimeout.EventTimeTimeout)(stateFunc)  // EventTimeTimeout requires watermark (defined above)

// Check out the physical plan with FlatMapGroupsWithStateExec
scala> rateGroups.explain
== Physical Plan ==
*SerializeFromObject [assertnotnull(input[0, scala.Tuple2, true])._1 AS _1#35L, assertnotnull(input[0, scala.Tuple2, true])._2 AS _2#36]
+- FlatMapGroupsWithState <function3>, value#30: bigint, newInstance(class scala.Tuple2), [value#30L], [timestamp#20-T10000ms, value#21L], obj#34: scala.Tuple2, StatefulOperatorStateInfo(<unknown>,63491721-8724-4631-b6bc-3bb1edeb4baf,0,0), class[value[0]: bigint], Update, EventTimeTimeout, 0, 0
   +- *Sort [value#30L ASC NULLS FIRST], false, 0
      +- Exchange hashpartitioning(value#30L, 200)
         +- AppendColumns <function1>, newInstance(class scala.Tuple2), [input[0, bigint, false] AS value#30L]
            +- EventTimeWatermark timestamp#20: timestamp, interval 10 seconds
               +- StreamingRelation rate, [timestamp#20, value#21L]

// Execute the streaming query
import org.apache.spark.sql.streaming.{OutputMode, Trigger}
import scala.concurrent.duration._
val sq = rateGroups.
  writeStream.
  format("console").
  trigger(Trigger.ProcessingTime(10.seconds)).
  outputMode(OutputMode.Update).  // Append is not supported
  start

// Eventually...
sq.stop

FlatMapGroupsWithStateExec uses the performance metrics of StateStoreWriter.

FlatMapGroupsWithStateExec webui query details.png
Figure 1. FlatMapGroupsWithStateExec in web UI (Details for Query)
Note

FlatMapGroupsWithStateStrategy converts FlatMapGroupsWithState unary logical operator to FlatMapGroupsWithStateExec physical operator with undefined StatefulOperatorStateInfo, batchTimestampMs, and eventTimeWatermark.

StatefulOperatorStateInfo, batchTimestampMs, and eventTimeWatermark are defined when IncrementalExecution query execution pipeline is requested to apply the physical plan preparation rules.

When executed, FlatMapGroupsWithStateExec requires that the optional values are properly defined given timeoutConf:

Caution
FIXME Where are the optional values defined?
Table 1. FlatMapGroupsWithStateExec’s Internal Registries and Counters
Name Description

isTimeoutEnabled

stateAttributes

stateDeserializer

stateManager

stateSerializer

timestampTimeoutAttribute

watermarkPresent

Flag that says whether the child physical operator has a watermark attribute (among the output attributes).

Used exclusively when InputProcessor is requested to callFunctionAndUpdateState

Tip

Enable INFO logging level for org.apache.spark.sql.execution.streaming.FlatMapGroupsWithStateExec to see what happens inside.

Add the following line to conf/log4j.properties:

log4j.logger.org.apache.spark.sql.execution.streaming.FlatMapGroupsWithStateExec=INFO

Refer to Logging.

keyExpressions Method

keyExpressions: Seq[Attribute]
Note
keyExpressions is part of the WatermarkSupport Contract to…​FIXME.

keyExpressions simply returns the grouping attributes.

Executing FlatMapGroupsWithStateExec — doExecute Method

doExecute(): RDD[InternalRow]
Note
doExecute is part of the SparkPlan contract to produce the result of a physical operator as an RDD of internal binary rows (i.e. InternalRow).

Internally, doExecute initializes metrics.

doExecute then executes child physical operator and creates a StateStoreRDD with storeUpdateFunction that:

  1. Creates a StateStoreUpdater

  2. Filters out rows from Iterator[InternalRow] that match watermarkPredicateForData (when defined and timeoutConf is EventTimeTimeout)

  3. Generates an output Iterator[InternalRow] with elements from StateStoreUpdater's updateStateForKeysWithData and updateStateForTimedOutKeys

  4. In the end, storeUpdateFunction creates a CompletionIterator that executes a completion function (aka completionFunction) after it has successfully iterated through all the elements (i.e. when a client has consumed all the rows). The completion method requests StateStore to commit followed by updating numTotalStateRows metric with the number of keys in the state store.

Creating FlatMapGroupsWithStateExec Instance

FlatMapGroupsWithStateExec takes the following when created:

  • State function ((Any, Iterator[Any], LogicalGroupState[Any]) ⇒ Iterator[Any])

  • Key deserializer expression

  • Value deserializer expression

  • Grouping attributes (as used for grouping in KeyValueGroupedDataset for mapGroupsWithState or flatMapGroupsWithState operators)

  • Data attributes

  • Output object attribute (that is the reference to the single object field this operator outputs)

  • StatefulOperatorStateInfo

  • State encoder (ExpressionEncoder[Any])

  • State format version

  • OutputMode

  • GroupStateTimeout

  • batchTimestampMs

  • Event time watermark

  • Child physical operator

FlatMapGroupsWithStateExec initializes the internal registries and counters.

shouldRunAnotherBatch Method

shouldRunAnotherBatch(newMetadata: OffsetSeqMetadata): Boolean
Note
shouldRunAnotherBatch is part of the StateStoreWriter Contract to…​FIXME.

shouldRunAnotherBatch…​FIXME

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