comments

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/MicroBatchExecution.scala

@@ -131,6 +131,9 @@ class MicroBatchExecution(
 
   protected var watermarkTracker: WatermarkTracker = _
 
+  // Store checkpointIDs for state store checkpoints to be committed or have been committed to
+  // the commit log.
+  // operatorID -> (partitionID -> uniqueID)
   private val currentCheckpointUniqueId = MutableMap[Long, Array[String]]()
 
   override lazy val logicalPlan: LogicalPlan = {
@@ -910,7 +913,7 @@ class MicroBatchExecution(
         execCtx.batchId == -1 || v == execCtx.batchId + 1,
         s"version $v doesn't match current Batch ID ${execCtx.batchId}")
     }
-    currentCheckpointUniqueId.put(opId, checkpointInfo.map{ c =>
+    currentCheckpointUniqueId.put(opId, checkpointInfo.map { c =>
       assert(c.checkpointId.isDefined)
       c.checkpointId.get
     })

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/RocksDB.scala

@@ -159,10 +159,20 @@ class RocksDB(
   @volatile private var changelogWriter: Option[StateStoreChangelogWriter] = None
   private val enableChangelogCheckpointing: Boolean = conf.enableChangelogCheckpointing
   @volatile private var loadedVersion = -1L   // -1 = nothing valid is loaded
+
+  // variables to manage checkpoint ID. Once a checkpoingting finishes, it nees to return
+  // the `lastCommittedCheckpointId` as the committed checkpointID, as well as
+  // `LastCommitBasedCheckpointId` as the checkpontID of the previous version that is based on.
+  // `loadedCheckpointId` is the checkpointID for the current live DB. After the batch finishes
+  // and checkpoint finishes, it will turn into `LastCommitBasedCheckpointId`.
+  // `sessionCheckpointId` store an ID to be used for future checkpoints. It is kept being used
+  // until we have to use a new one. We don't need to reuse any uniqueID, but reusing when possible
+  // can help debug problems.
   @volatile private var LastCommitBasedCheckpointId: Option[String] = None
   @volatile private var lastCommittedCheckpointId: Option[String] = None
   @volatile private var loadedCheckpointId: Option[String] = None
   @volatile private var sessionCheckpointId: Option[String] = None
+
   @volatile private var numKeysOnLoadedVersion = 0L
   @volatile private var numKeysOnWritingVersion = 0L
   @volatile private var fileManagerMetrics = RocksDBFileManagerMetrics.EMPTY_METRICS
@@ -338,6 +348,8 @@ class RocksDB(
         loadedVersion = -1  // invalidate loaded data
         LastCommitBasedCheckpointId = None
         lastCommittedCheckpointId = None
+        loadedCheckpointId = None
+        sessionCheckpointId = None
         throw t
     }
     if (enableChangelogCheckpointing && !readOnly) {

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/state/StateStore.scala

@@ -364,6 +364,11 @@ case class RangeKeyScanStateEncoderSpec(
  *   version of the data can be accessed. It is the responsible of the provider to populate
  *   this store with context information like the schema of keys and values, etc.
  *
+ *   If the checkpoint format version 2 is used, an additional argument `checkponitID` may be
+ *   provided as part of `getStore(version, checkpoinId)`. The provider needs to guarantee
+ *   that the loaded version is of this unique ID. It needs to load the version for this specific
+ *   ID from the checkpoint if needed.
+ *
  * - After the streaming query is stopped, the created provider instances are lazily disposed off.
  */
 trait StateStoreProvider {
@@ -405,13 +410,15 @@ trait StateStoreProvider {
   /** Called when the provider instance is unloaded from the executor */
   def close(): Unit
 
-  /** Return an instance of [[StateStore]] representing state data of the given version */
+  /** Return an instance of [[StateStore]] representing state data of the given version.
+   * If `checkpointUniqueId` is provided, the instance also needs to match the ID. */
   def getStore(
       version: Long,
       checkpointUniqueId: Option[String] = None): StateStore
 
   /**
-   * Return an instance of [[ReadStateStore]] representing state data of the given version.
+   * Return an instance of [[ReadStateStore]] representing state data of the given version
+   * and uniqueID if provided.
    * By default it will return the same instance as getStore(version) but wrapped to prevent
    * modification. Providers can override and return optimized version of [[ReadStateStore]]
    * based on the fact the instance will be only used for reading.