Github user tdas commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21200#discussion_r185351224
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousDataSourceRDD.scala
 ---
    @@ -0,0 +1,160 @@
    +/*
    + * 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.concurrent.TimeUnit
    +import javax.annotation.concurrent.GuardedBy
    +
    +import scala.collection.mutable
    +
    +import org.apache.spark._
    +import org.apache.spark.internal.Logging
    +import org.apache.spark.rdd.RDD
    +import org.apache.spark.sql.{Row, SQLContext}
    +import org.apache.spark.sql.catalyst.expressions.UnsafeRow
    +import 
org.apache.spark.sql.execution.datasources.v2.{DataSourceRDDPartition, 
RowToUnsafeDataReader}
    +import org.apache.spark.sql.sources.v2.reader._
    +import 
org.apache.spark.sql.sources.v2.reader.streaming.{ContinuousDataReader, 
PartitionOffset}
    +import org.apache.spark.util.ThreadUtils
    +
    +/**
    + * The bottom-most RDD of a continuous processing read task. Wraps a 
[[ContinuousQueuedDataReader]]
    + * to read from the remote source, and polls that queue for incoming rows.
    + *
    + * Note that continuous processing calls compute() multiple times, and the 
same
    + * [[ContinuousQueuedDataReader]] instance will/must be shared between 
each call for the same split.
    + */
    +class ContinuousDataSourceRDD(
    +    sc: SparkContext,
    +    sqlContext: SQLContext,
    +    @transient private val readerFactories: 
Seq[DataReaderFactory[UnsafeRow]])
    +  extends RDD[UnsafeRow](sc, Nil) {
    --- End diff --
    
    I think its a bad idea to have sqlContext in the constructor because this 
can accidentally serialize the SQLContext, and we dont want that. In fact, if 
you only need few confs from the SQLContext, then maybe, we should only have 
those in the constructor.


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