Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/21200#discussion_r185668041
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousQueuedDataReader.scala
---
@@ -0,0 +1,199 @@
+/*
+ * 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.streaming.continuous
+
+import java.io.Closeable
+import java.util.concurrent.{ArrayBlockingQueue, BlockingQueue, TimeUnit}
+import java.util.concurrent.atomic.AtomicBoolean
+
+import org.apache.spark.{Partition, SparkEnv, SparkException, TaskContext}
+import org.apache.spark.internal.Logging
+import org.apache.spark.sql.catalyst.expressions.UnsafeRow
+import org.apache.spark.sql.sources.v2.reader.{DataReader,
DataReaderFactory}
+import org.apache.spark.sql.sources.v2.reader.streaming.PartitionOffset
+import org.apache.spark.util.ThreadUtils
+
+/**
+ * The record types in a continuous processing buffer.
+ */
+sealed trait ContinuousRecord
+case object EpochMarker extends ContinuousRecord
+case class ContinuousRow(row: UnsafeRow, offset: PartitionOffset) extends
ContinuousRecord
+
+/**
+ * A wrapper for a continuous processing data reader, including a reading
queue and epoch markers.
+ *
+ * This will be instantiated once per partition - successive calls to
compute() in the
+ * [[ContinuousDataSourceRDD]] will reuse the same reader. This is
required to get continuity of
+ * offsets across epochs.
+ *
+ * The RDD is responsible for advancing two fields here, since they need
to be updated in line
+ * with the data flow:
+ * * currentOffset - contains the offset of the most recent row which a
compute() iterator has sent
+ * upwards. The RDD is responsible for advancing this.
+ * * currentEpoch - the epoch which is currently occurring. The RDD is
responsible for incrementing
+ * this before ending the compute() iterator.
+ */
+class ContinuousQueuedDataReader(
+ factory: DataReaderFactory[UnsafeRow],
+ context: TaskContext,
+ dataQueueSize: Int,
+ epochPollIntervalMs: Long) extends Closeable {
+ private val reader = factory.createDataReader()
+
+ // Important sequencing - we must get our starting point before the
provider threads start running
+ var currentOffset: PartitionOffset =
ContinuousDataSourceRDD.getBaseReader(reader).getOffset
+ var currentEpoch: Long =
context.getLocalProperty(ContinuousExecution.START_EPOCH_KEY).toLong
+
+ private val queue = new
ArrayBlockingQueue[ContinuousRecord](dataQueueSize)
+
+ private val epochPollFailed = new AtomicBoolean(false)
+ private val dataReaderFailed = new AtomicBoolean(false)
+
+ private val coordinatorId =
context.getLocalProperty(ContinuousExecution.EPOCH_COORDINATOR_ID_KEY)
+
+ private val epochMarkerExecutor =
ThreadUtils.newDaemonSingleThreadScheduledExecutor(
+ s"epoch-poll--$coordinatorId--${context.partitionId()}")
+ private val epochMarkerGenerator = new EpochMarkerGenerator(queue,
context, epochPollFailed)
+ epochMarkerExecutor.scheduleWithFixedDelay(
+ epochMarkerGenerator, 0, epochPollIntervalMs, TimeUnit.MILLISECONDS)
+
+ private val dataReaderThread = new DataReaderThread(reader, queue,
context, dataReaderFailed)
+ dataReaderThread.setDaemon(true)
+ dataReaderThread.start()
+
+ context.addTaskCompletionListener(_ => {
+ this.close()
+ })
+
+ def next(): ContinuousRecord = {
+ val POLL_TIMEOUT_MS = 1000
+ var currentEntry: ContinuousRecord = null
+
+ while (currentEntry == null) {
+ if (context.isInterrupted() || context.isCompleted()) {
+ // Force the epoch to end here. The writer will notice the context
is interrupted
+ // or completed and not start a new one. This makes it possible to
achieve clean
+ // shutdown of the streaming query.
+ // TODO: The obvious generalization of this logic to multiple
stages won't work. It's
+ // invalid to send an epoch marker from the bottom of a task if
all its child tasks
+ // haven't sent one.
+ currentEntry = EpochMarker
+ } else {
+ if (dataReaderFailed.get()) {
--- End diff --
Mentioned this above, we can simply check `dataReaderThread.failureReason`
here instead of having another flag.
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