[GitHub] [spark] HeartSaVioR commented on a change in pull request #31986: [SPARK-34888][SS] Introduce UpdatingSessionIterator adjusting session window on elements

[GitBox <gi...@apache.org>]

HeartSaVioR commented on a change in pull request #31986:
URL: https://github.com/apache/spark/pull/31986#discussion_r611255893



##########
File path: sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/UpdatingSessionsIterator.scala
##########
@@ -0,0 +1,223 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.execution.aggregate
+
+import scala.collection.mutable
+
+import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.expressions._
+import org.apache.spark.sql.catalyst.expressions.codegen.GenerateUnsafeProjection
+import org.apache.spark.sql.execution.ExternalAppendOnlyUnsafeRowArray
+
+/**
+ * This class calculates and updates the session window for each element in the given iterator,
+ * which makes elements in the same session window having same session spec. Downstream can apply
+ * aggregation to finally merge these elements bound to the same session window.
+ *
+ * This class works on the precondition that given iterator is sorted by "group keys + start time
+ * of session window", and this iterator still retains the characteristic of the sort.
+ *
+ * This class copies the elements to safely update on each element, as well as buffers elements
+ * which are bound to the same session window. Due to such overheads, [[MergingSessionsIterator]]
+ * should be used whenever possible.
+ */
+class UpdatingSessionsIterator(
+    iter: Iterator[InternalRow],
+    groupingExpressions: Seq[NamedExpression],
+    sessionExpression: NamedExpression,
+    inputSchema: Seq[Attribute],
+    inMemoryThreshold: Int,
+    spillThreshold: Int) extends Iterator[InternalRow] {
+
+  private val groupingWithoutSession: Seq[NamedExpression] =
+    groupingExpressions.diff(Seq(sessionExpression))
+  private val groupingWithoutSessionAttributes: Seq[Attribute] =
+    groupingWithoutSession.map(_.toAttribute)
+  private[this] val groupingWithoutSessionProjection: UnsafeProjection =
+    UnsafeProjection.create(groupingWithoutSession, inputSchema)
+
+  private val valuesExpressions: Seq[Attribute] = inputSchema.diff(groupingWithoutSession)
+
+  private[this] val sessionProjection: UnsafeProjection =
+    UnsafeProjection.create(Seq(sessionExpression), inputSchema)
+
+  private var currentKeys: InternalRow = _
+  private var currentSession: UnsafeRow = _
+
+  private var currentRows: ExternalAppendOnlyUnsafeRowArray = new ExternalAppendOnlyUnsafeRowArray(
+    inMemoryThreshold, spillThreshold)
+
+  private var returnRows: ExternalAppendOnlyUnsafeRowArray = _
+  private var returnRowsIter: Iterator[InternalRow] = _
+  private var errorOnIterator: Boolean = false
+
+  private val processedKeys: mutable.HashSet[InternalRow] = new mutable.HashSet[InternalRow]()
+
+  override def hasNext: Boolean = {
+    assertIteratorNotCorrupted()
+
+    if (returnRowsIter != null && returnRowsIter.hasNext) {
+      return true
+    }
+
+    if (returnRowsIter != null) {
+      returnRowsIter = null
+      returnRows.clear()
+    }
+
+    iter.hasNext
+  }
+
+  override def next(): InternalRow = {
+    assertIteratorNotCorrupted()
+
+    if (returnRowsIter != null && returnRowsIter.hasNext) {
+      return returnRowsIter.next()
+    }
+
+    var exitCondition = false
+    while (iter.hasNext && !exitCondition) {
+      // we are going to modify the row, so we should make sure multiple objects are not
+      // referencing same memory, which could be possible when optimizing iterator
+      // without this, multiple rows in same key will be returned with same content
+      val row = iter.next().copy()
+
+      val keys = groupingWithoutSessionProjection(row)
+      val session = sessionProjection(row)
+      val sessionStruct = session.getStruct(0, 2)
+      val sessionStart = getSessionStart(sessionStruct)
+      val sessionEnd = getSessionEnd(sessionStruct)
+
+      if (currentKeys == null) {
+        startNewSession(row, keys, sessionStruct)
+      } else if (keys != currentKeys) {
+        closeCurrentSession(keyChanged = true)
+        processedKeys.add(currentKeys)
+        startNewSession(row, keys, sessionStruct)
+        exitCondition = true
+      } else {
+        if (sessionStart < getSessionStart(currentSession)) {
+          handleBrokenPreconditionForSort()
+        } else if (sessionStart <= getSessionEnd(currentSession)) {
+          // expanding session length if needed
+          expandEndOfCurrentSession(sessionEnd)
+          currentRows.add(row.asInstanceOf[UnsafeRow])
+        } else {
+          closeCurrentSession(keyChanged = false)
+          startNewSession(row, keys, sessionStruct)
+          exitCondition = true
+        }
+      }
+    }
+
+    if (!iter.hasNext) {
+      // no further row: closing session
+      closeCurrentSession(keyChanged = false)
+    }
+
+    // here returnRowsIter should be able to provide at least one row
+    require(returnRowsIter != null && returnRowsIter.hasNext)
+
+    returnRowsIter.next()
+  }
+
+  private def startNewSession(
+      currentRow: InternalRow,
+      groupingKey: UnsafeRow,
+      sessionStruct: UnsafeRow): Unit = {
+    if (processedKeys.contains(groupingKey)) {
+      handleBrokenPreconditionForSort()
+    }
+
+    currentKeys = groupingKey.copy()
+    currentSession = sessionStruct.copy()
+
+    currentRows.clear()
+    currentRows.add(currentRow.asInstanceOf[UnsafeRow])
+  }
+
+  private def getSessionStart(sessionStruct: UnsafeRow): Long = {
+    sessionStruct.getLong(0)
+  }
+
+  private def getSessionEnd(sessionStruct: UnsafeRow): Long = {
+    sessionStruct.getLong(1)
+  }
+
+  def updateSessionEnd(sessionStruct: UnsafeRow, sessionEnd: Long): Unit = {
+    sessionStruct.setLong(1, sessionEnd)
+  }
+
+  private def expandEndOfCurrentSession(sessionEnd: Long): Unit = {
+    if (sessionEnd > getSessionEnd(currentSession)) {
+      updateSessionEnd(currentSession, sessionEnd)
+    }
+  }
+
+  private def handleBrokenPreconditionForSort(): Unit = {
+    errorOnIterator = true
+    throw new IllegalStateException("The iterator must be sorted by key and session start!")
+  }
+
+  private val join = new JoinedRow
+  private val join2 = new JoinedRow
+
+  private val groupingKeyProj = GenerateUnsafeProjection.generate(groupingExpressions,
+    groupingWithoutSessionAttributes :+ sessionExpression.toAttribute)
+  private val valueProj = GenerateUnsafeProjection.generate(valuesExpressions, inputSchema)
+  private val restoreProj = GenerateUnsafeProjection.generate(inputSchema,
+    groupingExpressions.map(_.toAttribute) ++ valuesExpressions.map(_.toAttribute))
+
+  private def generateGroupingKey(): UnsafeRow = {
+    val newRow = new SpecificInternalRow(Seq(sessionExpression.toAttribute).toStructType)
+    newRow.update(0, currentSession)
+    val joined = join(currentKeys, newRow)
+
+    groupingKeyProj(joined)
+  }
+
+  private def closeCurrentSession(keyChanged: Boolean): Unit = {
+    returnRows = currentRows
+    currentRows = new ExternalAppendOnlyUnsafeRowArray(

Review comment:
       Please check that we back up current array to `returnRows` for providing iterator. When we close the current session, we have a row which needs to be bound to the next session. The new row and following rows bound to the next session will be buffered to the next array.
   
   The origin reason of not re-using an ExternalAppendOnlyUnsafeRowArray instance is, the iterator implementation of ExternalAppendOnlyUnsafeRowArray invalidates itself (meaning throwing exception) if the array is modified afterwards.
   
   So it's not simply done as creating array and iterator, and appending data into array and letting consumer to retrieve data from iterator at the same time. We need to call stop on buffering a specific array and provide iterator, and create a new array to buffer.
   
   Probably initializing `currentRows` is better to be placed in `startNewSession`.




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