StateStore Contract — Kay-Value Store for Streaming State Data

StateStore is the abstraction of a versioned and possibly fault-tolerant key-value stores for streaming state data (e.g. for persisting running aggregates in Streaming Aggregation).

StateStore supports incremental checkpointing in which only the key-value "Row" pairs that changed can be committed or aborted (without touching other key-value pairs).

StateStore is identified with the aggregating operator id and the partition id (among other properties for identification).

Note
HDFSBackedStateStore is the default and only known implementation of the StateStore Contract in Spark Structured Streaming.
Table 1. StateStore Contract
Method Description

abort

abort(): Unit

Aborts (discards) changes to the state store

Used when:

commit

commit(): Long

Commits the changes to the state store (and returns the current version)

Used when:

get

get(key: UnsafeRow): UnsafeRow

Gets the value of the (non-null) key

Used when:

getRange

getRange(
  start: Option[UnsafeRow],
  end: Option[UnsafeRow]): Iterator[UnsafeRowPair]

Gets the key-value pairs for the specified range (with optional approximate start and end extents)

Used when:

Note
All the uses above assume the start and end as None that basically is iterator.

hasCommitted

hasCommitted: Boolean

Returns whether all changes have been committed (true) or not (false)

Used when:

id

id: StateStoreId

The ID of the state store

Used when:

iterator

iterator(): Iterator[UnsafeRowPair]

Returns an iterator with all the kay-value pairs in the state store

Used when:

metrics

metrics: StateStoreMetrics

StateStoreMetrics of the state store

Used when:

put

put(key: UnsafeRow, value: UnsafeRow): Unit

Stores (puts) the value for the (non-null) key

Used when:

remove

remove(key: UnsafeRow): Unit

Removes the (non-null) key from the state store

Used when:

version

version: Long

Version of the state store

Used exclusively when HDFSBackedStateStore state store is requested for a new version (that simply the current version incremented)

Note

Read the motivation and design in State Store for Streaming Aggregations.

Tip

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

Add the following line to conf/log4j.properties:

log4j.logger.org.apache.spark.sql.execution.streaming.state.StateStore$=ALL

Refer to Logging.

Creating (and Caching) RPC Endpoint Reference to StateStoreCoordinator for Executors — coordinatorRef Internal Object Method

coordinatorRef: Option[StateStoreCoordinatorRef]

coordinatorRef requests the SparkEnv helper object for the current SparkEnv.

If the SparkEnv is available and the _coordRef is not assigned yet, coordinatorRef prints out the following DEBUG message to the logs followed by requesting the StateStoreCoordinatorRef for the StateStoreCoordinator endpoint.

Getting StateStoreCoordinatorRef

If the SparkEnv is available, coordinatorRef prints out the following INFO message to the logs:

Retrieved reference to StateStoreCoordinator: [_coordRef]
Note
coordinatorRef is used when StateStore helper object is requested to reportActiveStoreInstance (when StateStore object helper is requested to find the StateStore by StateStoreProviderId) and verifyIfStoreInstanceActive (when StateStore object helper is requested to doMaintenance).

Unloading State Store Provider — unload Method

unload(storeProviderId: StateStoreProviderId): Unit

unload…​FIXME

Note
unload is used when StateStore helper object is requested to stop and doMaintenance.

stop Object Method

stop(): Unit

stop…​FIXME

Note
stop seems only be used in tests.

Announcing New StateStoreProvider — reportActiveStoreInstance Internal Object Method

reportActiveStoreInstance(storeProviderId: StateStoreProviderId): Unit

reportActiveStoreInstance takes the current host and executorId (from the BlockManager on the Spark executor) and requests the StateStoreCoordinatorRef to reportActiveInstance.

Note
reportActiveStoreInstance uses SparkEnv to access the BlockManager.

In the end, reportActiveStoreInstance prints out the following INFO message to the logs:

Reported that the loaded instance [storeProviderId] is active
Note
reportActiveStoreInstance is used exclusively when StateStore helper object is requested to find the StateStore by StateStoreProviderId.

MaintenanceTask Daemon Thread

MaintenanceTask is a daemon thread that triggers maintenance work of every registered StateStoreProvider.

When an error occurs, MaintenanceTask clears loadedProviders registry.

MaintenanceTask is scheduled on state-store-maintenance-task thread pool that runs periodically every spark.sql.streaming.stateStore.maintenanceInterval configuration property (default: 60s).

Looking Up StateStore by ID and Version — get Factory Method

get(
  storeProviderId: StateStoreProviderId,
  keySchema: StructType,
  valueSchema: StructType,
  indexOrdinal: Option[Int],
  version: Long,
  storeConf: StateStoreConf,
  hadoopConf: Configuration): StateStore

get finds StateStore for the given StateStoreProviderId.

Internally, get looks up the StateStoreProvider (for storeProviderId) in loadedProviders registry. If unavailable, get creates and initializes one.

get will also start the periodic maintenance task (unless already started) and announce the new StateStoreProvider.

In the end, get gets the StateStore (for the version).

Note

get is used when:

Starting Periodic Maintenance Task (Unless Already Started) — startMaintenanceIfNeeded Internal Object Method

startMaintenanceIfNeeded(): Unit

startMaintenanceIfNeeded schedules MaintenanceTask to start after and every spark.sql.streaming.stateStore.maintenanceInterval (defaults to 60s).

Note
startMaintenanceIfNeeded does nothing when the maintenance task has already been started and is still running.
Note
startMaintenanceIfNeeded is used exclusively when StateStore is requested to find the StateStore by StateStoreProviderId.

Doing Maintenance of Registered State Store Providers — doMaintenance Internal Object Method

doMaintenance(): Unit

Internally, doMaintenance prints the following DEBUG message to the logs:

Doing maintenance

doMaintenance then requests every StateStoreProvider (registered in loadedProviders) to do its own internal maintenance (only when a StateStoreProvider is still active).

When a StateStoreProvider is inactive, doMaintenance removes it from the provider registry and prints the following INFO message to the logs:

Unloaded [provider]
Note
doMaintenance is used exclusively in MaintenanceTask daemon thread.

verifyIfStoreInstanceActive Internal Object Method

verifyIfStoreInstanceActive(storeProviderId: StateStoreProviderId): Boolean

verifyIfStoreInstanceActive…​FIXME

Note
verifyIfStoreInstanceActive is used exclusively when StateStore helper object is requested to doMaintenance (from a running MaintenanceTask daemon thread).

Internal Properties

Name Description

loadedProviders

Loaded providers, i.e. StateStoreProviders per StateStoreProviderId

Used in…​FIXME

_coordRef

StateStoreCoordinator RPC endpoint (a RpcEndpointRef to StateStoreCoordinator)

Used in…​FIXME

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