Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/9143#discussion_r44085972
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/util/BatchedWriteAheadLog.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.streaming.util
+
+import java.nio.ByteBuffer
+import java.util.concurrent.{LinkedBlockingQueue, TimeoutException}
+import java.util.{Iterator => JIterator}
+
+import scala.collection.JavaConverters._
+import scala.collection.mutable.ArrayBuffer
+import scala.concurrent.{Await, ExecutionContext, Promise}
+import scala.concurrent.duration._
+import scala.util.control.NonFatal
+
+import org.apache.spark.Logging
+import org.apache.spark.util.{Utils, ThreadUtils}
+
+/**
+ * A wrapper for a WriteAheadLog that batches records before writing data.
All other methods will
+ * be passed on to the wrapped class.
+ *
+ * Parent exposed for testing.
+ */
+private[streaming] class BatchedWriteAheadLog(private[util] val parent:
WriteAheadLog)
+ extends WriteAheadLog with Logging {
+
+ import BatchedWriteAheadLog._
+
+ // exposed for tests
+ protected val walWriteQueue = new LinkedBlockingQueue[RecordBuffer]()
+
+ private val WAL_WRITE_STATUS_TIMEOUT = 5000 // 5 seconds
+
+ // Whether the writer thread is active
+ @volatile private var active: Boolean = true
+ protected val buffer = new ArrayBuffer[RecordBuffer]()
+
+ private val batchedWriterThread = startBatchedWriterThread()
+
+ /**
+ * Write a byte buffer to the log file. This method adds the byteBuffer
to a queue and blocks
+ * until the record is properly written by the parent.
+ */
+ override def write(byteBuffer: ByteBuffer, time: Long):
WriteAheadLogRecordHandle = {
+ val promise = Promise[WriteAheadLogRecordHandle]()
+ walWriteQueue.offer(RecordBuffer(byteBuffer, time, promise))
+ try {
+ Await.result(promise.future.recover { case _ => null
}(ThreadUtils.sameThread),
+ WAL_WRITE_STATUS_TIMEOUT.milliseconds)
+ } catch {
+ case e: TimeoutException =>
+ logWarning(s"Write to Write Ahead Log promise timed out after " +
+ s"$WAL_WRITE_STATUS_TIMEOUT millis for record.")
+ null
+ }
+ }
+
+ /**
+ * Read a segment from an existing Write Ahead Log. The data may be
aggregated, and the user
+ * should de-aggregate using [[BatchedWriteAheadLog.deaggregate]]
+ *
+ * This method is handled by the parent WriteAheadLog.
+ */
+ override def read(segment: WriteAheadLogRecordHandle): ByteBuffer = {
+ parent.read(segment)
+ }
+
+ /**
+ * Read all the existing logs from the log directory.
+ *
+ * This method is handled by the parent WriteAheadLog.
+ */
+ override def readAll(): JIterator[ByteBuffer] = {
+ parent.readAll().asScala.flatMap(deaggregate).asJava
--- End diff --
Are you thinking that the Scala iterator wrapper is putting all the objects
from the Java iterator into some list and then exposing as a scala iterator
from the list? No!! That would just defeat the purpose of the conversion, and
no one would be using this in Scala (scala is pretty mature man :).
`JavaIterator.asScala` just allocates one and only one extra object - a
wrapper object that wraps a Java iterator into a scala iterator, and forwards
the next, and hasNext of the scala iterator to the corresponding functions of
the Java iterator. That's it. If you want to see the code, search of
`JIteratorWrapper` in
[here](http://xuwei-k.github.io/scala-library-sxr/scala-library-2.10.0/scala/collection/convert/Wrappers.scala.html#undefined).
And the vice versa for `ScalaIterator.asJava`. So there is no list in the
middle, there is not collecting in a intermediate list, there is no buffering,
it is lazy all the way through.
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