Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/20828#discussion_r181181014
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/sources/ContinuousMemoryStream.scala
---
@@ -0,0 +1,211 @@
+/*
+ * 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.sources
+
+import java.{util => ju}
+import java.util.Optional
+import javax.annotation.concurrent.GuardedBy
+
+import scala.collection.JavaConverters._
+import scala.collection.mutable.ListBuffer
+
+import org.json4s.NoTypeHints
+import org.json4s.jackson.Serialization
+
+import org.apache.spark.SparkEnv
+import org.apache.spark.rpc.{RpcCallContext, RpcEndpointRef, RpcEnv,
ThreadSafeRpcEndpoint}
+import org.apache.spark.sql.{Encoder, Row, SQLContext}
+import org.apache.spark.sql.execution.streaming._
+import
org.apache.spark.sql.execution.streaming.sources.ContinuousMemoryStream.GetRecord
+import org.apache.spark.sql.sources.v2.{ContinuousReadSupport,
DataSourceOptions}
+import org.apache.spark.sql.sources.v2.reader.DataReaderFactory
+import
org.apache.spark.sql.sources.v2.reader.streaming.{ContinuousDataReader,
ContinuousReader, Offset, PartitionOffset}
+import org.apache.spark.sql.types.StructType
+import org.apache.spark.util.RpcUtils
+
+/**
+ * The overall strategy here is:
+ * * ContinuousMemoryStream maintains a list of records for each
partition. addData() will
+ * distribute records evenly-ish across partitions.
+ * * RecordEndpoint is set up as an endpoint for executor-side
+ * ContinuousMemoryStreamDataReader instances to poll. It returns the
record at the specified
+ * offset within the list, or null if that offset doesn't yet have a
record.
+ */
+class ContinuousMemoryStream[A : Encoder](id: Int, sqlContext: SQLContext)
+ extends MemoryStreamBase[A](sqlContext) with ContinuousReader with
ContinuousReadSupport {
+ private implicit val formats = Serialization.formats(NoTypeHints)
+ val NUM_PARTITIONS = 2
+
+ protected val logicalPlan =
+ StreamingRelationV2(this, "memory", Map(), attributes,
None)(sqlContext.sparkSession)
+
+ // ContinuousReader implementation
+
+ @GuardedBy("this")
+ private val records = Seq.fill(NUM_PARTITIONS)(new ListBuffer[A])
+
+ @GuardedBy("this")
+ private var startOffset: ContinuousMemoryStreamOffset = _
+
+ private val recordEndpoint = new RecordEndpoint()
+ @volatile private var endpointRef: RpcEndpointRef = _
+
+ def addData(data: TraversableOnce[A]): Offset = synchronized {
+ // Distribute data evenly among partition lists.
+ data.toSeq.zipWithIndex.map {
+ case (item, index) => records(index % NUM_PARTITIONS) += item
+ }
+
+ // The new target offset is the offset where all records in all
partitions have been processed.
+ ContinuousMemoryStreamOffset((0 until NUM_PARTITIONS).map(i => (i,
records(i).size)).toMap)
+ }
+
+ override def setStartOffset(start: Optional[Offset]): Unit =
synchronized {
+ // Inferred initial offset is position 0 in each partition.
+ startOffset = start.orElse {
+ ContinuousMemoryStreamOffset((0 until NUM_PARTITIONS).map(i => (i,
0)).toMap)
+ }.asInstanceOf[ContinuousMemoryStreamOffset]
+ }
+
+ override def getStartOffset: Offset = synchronized {
+ startOffset
+ }
+
+ override def deserializeOffset(json: String):
ContinuousMemoryStreamOffset = {
+ ContinuousMemoryStreamOffset(Serialization.read[Map[Int, Int]](json))
+ }
+
+ override def mergeOffsets(offsets: Array[PartitionOffset]):
ContinuousMemoryStreamOffset = {
+ ContinuousMemoryStreamOffset(
+ offsets.map {
+ case ContinuousMemoryStreamPartitionOffset(part, num) => (part,
num)
+ }.toMap
+ )
+ }
+
+ override def createDataReaderFactories():
ju.List[DataReaderFactory[Row]] = {
+ synchronized {
+ val endpointName = s"ContinuousMemoryStreamRecordReceiver-$id"
--- End diff --
Also, ContinuousMemoryStreamRecordReceiver ->
ContinuousMemoryStreamRecordEndpoint
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