FlatMapGroupsWithStateExec Unary Physical Operator¶

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

FlatMapGroupsWithStateExec is an ObjectProducerExec physical operator and so produces a single output object.

Creating Instance¶

FlatMapGroupsWithStateExec takes the following to be created:

• User-defined state function that is applied to every group (of type (Any, Iterator[Any], LogicalGroupState[Any]) => Iterator[Any])
• Deserializer expression for keys
• Deserializer expression for values
• 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)
• Optional StatefulOperatorStateInfo
• State encoder (ExpressionEncoder[Any])
• State format version
• OutputMode
• GroupStateTimeout
• Optional Batch Processing Time
• Optional Event-Time Watermark
• Child physical operator

FlatMapGroupsWithStateExec is created when FlatMapGroupsWithStateStrategy execution planning strategy is executed (and plans a FlatMapGroupsWithState logical operator for execution).

Executing Physical Operator¶

doExecute(): RDD[InternalRow]

doExecute first initializes the metrics (which happens on the driver).

doExecute then requests the child physical operator to execute (and generate an RDD[InternalRow]).

doExecute uses StateStoreOps to create a StateStoreRDD with a storeUpdateFunction that does the following (for a partition):

1. Creates an InputProcessor for a given StateStore

2. (only when the GroupStateTimeout is EventTimeTimeout) Filters out late data based on the event-time watermark, i.e. rows from a given Iterator[InternalRow] that are older than the event-time watermark are excluded from the steps that follow

3. Requests the InputProcessor to create an iterator of a new data processed from the (possibly filtered) iterator

4. Requests the InputProcessor to create an iterator of a timed-out state data

5. Creates an iterator by concatenating the above iterators (with the new data processed first)

6. In the end, creates a CompletionIterator that executes a completion function (completionFunction) after it has successfully iterated through all the elements (i.e. when a client has consumed all the rows). The completion method requests the given StateStore to commit changes followed by setting the store-specific metrics

doExecute is part of Spark SQL's SparkPlan abstraction.

Performance Metrics¶

FlatMapGroupsWithStateExec uses the performance metrics of StateStoreWriter.

StateStoreWriter¶

FlatMapGroupsWithStateExec is a stateful physical operator that can write to a state store (and MicroBatchExecution requests whether to run another batch or not based on the GroupStateTimeout).

FlatMapGroupsWithStateExec uses the GroupStateTimeout (and possibly the updated metadata) when asked whether to run another batch or not (when MicroBatchExecution is requested to construct the next streaming micro-batch when requested to run the activated streaming query).

Streaming Event-Time Watermark Support¶

FlatMapGroupsWithStateExec is a physical operator that supports streaming event-time watermark.

FlatMapGroupsWithStateExec is given the optional event time watermark when created.

The event-time watermark is initially undefined (None) when planned for execution (in FlatMapGroupsWithStateStrategy execution planning strategy).

The event-time watermark (with the StatefulOperatorStateInfo and the batchTimestampMs) is only defined to the current event-time watermark of the given OffsetSeqMetadata when IncrementalExecution query execution pipeline is requested to apply the state preparation rule (as part of the preparations rules).

IncrementalExecution is used as the lastExecution of the available streaming query execution engines. It is created in the queryPlanning phase (of the MicroBatchExecution and ContinuousExecution execution engines) based on the current OffsetSeqMetadata.

StateManager¶

stateManager: StateManager

While being created, FlatMapGroupsWithStateExec creates a StateManager (with the state encoder and the isTimeoutEnabled flag).

A StateManager is created per state format version that is given while creating a FlatMapGroupsWithStateExec (to choose between the available implementations).

The state format version is controlled by spark.sql.streaming.flatMapGroupsWithState.stateFormatVersion internal configuration property.

The StateManager is used exclusively when FlatMapGroupsWithStateExec physical operator is executed for the following:

• State schema (for the value schema of a StateStoreRDD)

• State data for a key in a StateStore while processing new data

• All state data (for all keys) in a StateStore while processing timed-out state data

• Removing the state for a key from a StateStore when all rows have been processed

• Persisting the state for a key in a StateStore when all rows have been processed

keyExpressions Method¶

keyExpressions: Seq[Attribute]

keyExpressions simply returns the grouping attributes.

keyExpressions is part of the WatermarkSupport abstraction.

Checking Out Whether Last Batch Execution Requires Another Non-Data Batch or Not¶

shouldRunAnotherBatch(
  newMetadata: OffsetSeqMetadata): Boolean

shouldRunAnotherBatch uses the GroupStateTimeout as follows:

• With EventTimeTimeout, shouldRunAnotherBatch is true only when the event-time watermark is defined and is older (below) the event-time watermark of the given OffsetSeqMetadata

• With NoTimeout (and other GroupStateTimeouts if there were any), shouldRunAnotherBatch is always false

• With ProcessingTimeTimeout, shouldRunAnotherBatch is always true

shouldRunAnotherBatch is part of the StateStoreWriter abstraction.

Internal Properties¶

isTimeoutEnabled Flag¶

Flag that says whether the GroupStateTimeout is not NoTimeout

Used when:

• FlatMapGroupsWithStateExec is created (and creates the internal StateManager)
• InputProcessor is requested to processTimedOutState

watermarkPresent Flag¶

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

Used when InputProcessor is requested to callFunctionAndUpdateState

Logging¶

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

Add the following line to conf/log4j.properties:

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

Refer to Logging.