spark-sql-streaming-StreamingQueryManager.adoc

StreamingQueryManager — Streaming Query Management

StreamingQueryManager is the management interface for streaming queries in a single SparkSession.

Table 1. StreamingQueryManager API

Method	Description
active	Gets all active structured queries active: Array[StreamingQuery]
addListener	Registers a StreamingQueryListener addListener(listener: StreamingQueryListener): Unit
awaitAnyTermination	Waits for any streaming query to be terminated awaitAnyTermination(): Unit awaitAnyTermination(timeoutMs: Long): Boolean
get	Gets a StreamingQuery by id get(id: String): StreamingQuery get(id: UUID): StreamingQuery
removeListener	De-registering a StreamingQueryListener removeListener(listener: StreamingQueryListener): Unit
resetTerminated	resetTerminated(): Unit

StreamingQueryManager is available using SparkSession and streams property.

val spark: SparkSession = ...
val queries = spark.streams

StreamingQueryManager is created when SessionState is created.

Figure 1. StreamingQueryManager

Tip	Refer to the Mastering Apache Spark 2 gitbook to learn about SessionState.

StreamingQueryManager is used (internally) to create a StreamingQuery (with its StreamExecution).

Figure 2. StreamingQueryManager Creates StreamingQuery (and StreamExecution)

StreamingQueryManager is notified about state changes of a structured query and passes them along (to query listeners).

StreamingQueryManager takes a single SparkSession when created.

Table 2. StreamingQueryManager’s Internal Registries and Counters (in alphabetical order)

Name	Description
activeQueries	Registry of StreamingQueries per UUID Used when StreamingQueryManager is requested for active streaming queries, get a streaming query by id, starts a streaming query and is notified that a streaming query has terminated.
activeQueriesLock
awaitTerminationLock
lastTerminatedQuery	StreamingQuery that has recently been terminated, i.e. stopped or due to an exception. null when no streaming query has terminated yet or resetTerminated. Used in awaitAnyTermination to know when a streaming query has terminated Set when StreamingQueryManager is notified that a streaming query has terminated
listenerBus	StreamingQueryListenerBus (for the current SparkSession) Used to: register or deregister a StreamingQueryListener Post a streaming event (and notify StreamingQueryListener listeners about streaming events)
stateStoreCoordinator	StateStoreCoordinatorRef with the StateStoreCoordinator RPC Endpoint Created when StreamingQueryManager is created Used when: StreamingQueryManager is notified that a streaming query has terminated Stateful operators are executed, i.e. FlatMapGroupsWithStateExec, StateStoreRestoreExec, StateStoreSaveExec, StreamingDeduplicateExec and StreamingSymmetricHashJoinExec Creating StateStoreRDD (with storeUpdateFunction aborting StateStore when a task fails)

Getting All Active Streaming Queries — active Method

active: Array[StreamingQuery]

active gets all active streaming queries.

Getting Active Continuous Query By Name — get Method

get(name: String): StreamingQuery

get method returns a StreamingQuery by name.

It may throw an IllegalArgumentException when no StreamingQuery exists for the name.

java.lang.IllegalArgumentException: There is no active query with name hello
  at org.apache.spark.sql.StreamingQueryManager$$anonfun$get$1.apply(StreamingQueryManager.scala:59)
  at org.apache.spark.sql.StreamingQueryManager$$anonfun$get$1.apply(StreamingQueryManager.scala:59)
  at scala.collection.MapLike$class.getOrElse(MapLike.scala:128)
  at scala.collection.AbstractMap.getOrElse(Map.scala:59)
  at org.apache.spark.sql.StreamingQueryManager.get(StreamingQueryManager.scala:58)
  ... 49 elided

Registering StreamingQueryListener — addListener Method

addListener(listener: StreamingQueryListener): Unit

addListener requests the StreamingQueryListenerBus to add the input listener.

De-Registering StreamingQueryListener — removeListener Method

removeListener(listener: StreamingQueryListener): Unit

removeListener requests StreamingQueryListenerBus to remove the input listener.

Waiting for Any Streaming Query Termination — awaitAnyTermination Method

awaitAnyTermination(): Unit
awaitAnyTermination(timeoutMs: Long): Boolean

awaitAnyTermination acquires a lock on awaitTerminationLock and waits until any streaming query has finished (i.e. lastTerminatedQuery is available) or timeoutMs has expired.

awaitAnyTermination re-throws the StreamingQueryException from lastTerminatedQuery if it reported one.

resetTerminated Method

resetTerminated(): Unit

resetTerminated forgets about the past-terminated query (so that awaitAnyTermination can be used again to wait for a new streaming query termination).

Internally, resetTerminated acquires a lock on awaitTerminationLock and simply resets lastTerminatedQuery (i.e. sets it to null).

Creating Serializable StreamingQuery (StreamingQueryWrapper with StreamExecution) — createQuery Internal Method

createQuery(
  userSpecifiedName: Option[String],
  userSpecifiedCheckpointLocation: Option[String],
  df: DataFrame,
  sink: Sink,
  outputMode: OutputMode,
  useTempCheckpointLocation: Boolean,
  recoverFromCheckpointLocation: Boolean,
  trigger: Trigger,
  triggerClock: Clock): StreamingQueryWrapper

createQuery creates a StreamingQueryWrapper (for a StreamExecution per the input user-defined properties).

Internally, createQuery first finds the name of the checkpoint directory of a query (aka checkpoint location) in the following order:

1. Exactly the input userSpecifiedCheckpointLocation if defined

2. spark.sql.streaming.checkpointLocation Spark property if defined for the parent directory with a subdirectory per the optional userSpecifiedName (or a randomly-generated UUID)

3. (only when useTempCheckpointLocation is enabled) A temporary directory (as specified by java.io.tmpdir JVM property) with a subdirectory with temporary prefix.

Note	userSpecifiedCheckpointLocation can be any path that is acceptable by Hadoop’s Path.

If the directory name for the checkpoint location could not be found, createQuery reports a AnalysisException.

checkpointLocation must be specified either through option("checkpointLocation", ...) or SparkSession.conf.set("spark.sql.streaming.checkpointLocation", ...)

createQuery reports a AnalysisException when the input recoverFromCheckpointLocation flag is turned off but there is offsets directory in the checkpoint location.

createQuery makes sure that the logical plan of the structured query is analyzed (i.e. no logical errors have been found).

Unless spark.sql.streaming.unsupportedOperationCheck Spark property is turned on, createQuery checks the logical plan of the streaming query for unsupported operations.

(only when spark.sql.adaptive.enabled Spark property is turned on) createQuery prints out a WARN message to the logs:

WARN spark.sql.adaptive.enabled is not supported in streaming DataFrames/Datasets and will be disabled.

In the end, createQuery creates a StreamingQueryWrapper with a new MicroBatchExecution.

Note

recoverFromCheckpointLocation flag corresponds to recoverFromCheckpointLocation flag that StreamingQueryManager uses to start a streaming query and which is enabled by default (and is in fact the only place where createQuery is used). memory sink has the flag enabled for Complete output mode only foreach sink has the flag always enabled console sink has the flag always disabled all other sinks have the flag always enabled

Note	userSpecifiedName corresponds to queryName option (that can be defined using DataStreamWriter's queryName method) while userSpecifiedCheckpointLocation is checkpointLocation option.

Note	createQuery is used exclusively when StreamingQueryManager is requested to start executing a streaming query.

Starting Streaming Query Execution — startQuery Internal Method

startQuery(
  userSpecifiedName: Option[String],
  userSpecifiedCheckpointLocation: Option[String],
  df: DataFrame,
  extraOptions: Map[String, String],
  sink: BaseStreamingSink,
  outputMode: OutputMode,
  useTempCheckpointLocation: Boolean = false,
  recoverFromCheckpointLocation: Boolean = true,
  trigger: Trigger = ProcessingTime(0),
  triggerClock: Clock = new SystemClock()): StreamingQuery

startQuery starts a streaming query.

Note	trigger defaults to 0 milliseconds (as ProcessingTime(0)).

Internally, startQuery first creates a StreamingQueryWrapper, registers it in activeQueries internal registry (by the id), requests it for the underlying StreamExecution and starts it.

In the end, startQuery returns the StreamingQueryWrapper (as part of the fluent API so you can chain operators) or throws the exception that was reported when attempting to start the query.

startQuery throws an IllegalArgumentException when there is another query registered under name. startQuery looks it up in the activeQueries internal registry.

Cannot start query with name [name] as a query with that name is already active

startQuery throws an IllegalStateException when a query is started again from checkpoint. startQuery looks it up in activeQueries internal registry.

Cannot start query with id [id] as another query with same id is already active. Perhaps you are attempting to restart a query from checkpoint that is already active.

Note	startQuery is used exclusively when DataStreamWriter is requested to start.

Posting StreamingQueryListener Event to StreamingQueryListenerBus — postListenerEvent Internal Method

postListenerEvent(event: StreamingQueryListener.Event): Unit

postListenerEvent simply posts the input event to StreamingQueryListenerBus.

Figure 3. StreamingQueryManager Propagates StreamingQueryListener Events

Note	postListenerEvent is used exclusively when StreamExecution posts a streaming event.

Marking Streaming Query as Terminated (and Deactivating Query in StateStoreCoordinator) — notifyQueryTermination Internal Method

notifyQueryTermination(terminatedQuery: StreamingQuery): Unit

notifyQueryTermination removes the terminatedQuery from activeQueries internal registry (by the query id).

notifyQueryTermination records the terminatedQuery in lastTerminatedQuery internal registry (when no earlier streaming query was recorded or the terminatedQuery terminated due to an exception).

notifyQueryTermination notifies others that are blocked on awaitTerminationLock.

In the end, notifyQueryTermination requests StateStoreCoordinator to deactivate all active runs of the streaming query.

Figure 4. StreamingQueryManager’s Marking Streaming Query as Terminated

Note	notifyQueryTermination is used exclusively when StreamExecution has finished (running streaming batches) (possibly due to an exception).