Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/20828#discussion_r179617149
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousMemoryStream.scala
---
@@ -0,0 +1,212 @@
+/*
+ * 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.{util => ju}
+import java.util.Optional
+import java.util.concurrent.ArrayBlockingQueue
+import javax.annotation.concurrent.GuardedBy
+
+import scala.collection.JavaConverters._
+import scala.collection.mutable.ListBuffer
+import scala.reflect.ClassTag
+
+import org.json4s.NoTypeHints
+import org.json4s.jackson.Serialization
+
+import org.apache.spark.SparkEnv
+import org.apache.spark.internal.Logging
+import org.apache.spark.rpc.{RpcCallContext, RpcEndpointRef, RpcEnv,
ThreadSafeRpcEndpoint}
+import org.apache.spark.sql.{Dataset, Encoder, Row, SQLContext}
+import org.apache.spark.sql.catalyst.encoders.encoderFor
+import org.apache.spark.sql.catalyst.expressions.UnsafeRow
+import org.apache.spark.sql.execution.streaming._
+import org.apache.spark.sql.sources.v2.{ContinuousReadSupport,
DataSourceOptions}
+import org.apache.spark.sql.sources.v2.reader.{DataReader,
DataReaderFactory, SupportsScanUnsafeRow}
+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.
+ * * ContinuousMemoryStreamRecordBuffer is set up as an endpoint for
partition-level
+ * 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.
+ */
+
+private case class GetRecord(offset: ContinuousMemoryStreamPartitionOffset)
+
+private class ContinuousMemoryStreamRecordBuffer[A](
--- End diff --
This is not really a buffer as it only refers to buffers that are
externally managed. Its only serves as an endpoint to fetch stuff from the
buffer. I would prefer one way or the other - either all the buffer management
and endpoint management should be inside the class (adding and fetching,
synchronized by this class) OR this is an inner class of ContinuousMemoryStream
whose only purpose is to be endpoint relaying fetch requests.
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