Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/9143#discussion_r43810862
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/util/BatchedWriteAheadLog.scala
---
@@ -0,0 +1,204 @@
+/*
+ * 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.
+ */
+private[streaming] class BatchedWriteAheadLog(parent: WriteAheadLog)
+ extends WriteAheadLog with Logging {
+
+ import BatchedWriteAheadLog._
+
+ /** A thread pool for fulfilling log write promises */
+ private val batchWriterThreadPool = ExecutionContext.fromExecutorService(
+ ThreadUtils.newDaemonCachedThreadPool("wal-batch-writer-thead-pool"))
+
+ // exposed for tests
+ protected val walWriteQueue = new LinkedBlockingQueue[RecordBuffer]()
+
+ private val WAL_WRITE_STATUS_TIMEOUT = 5000 // 5 seconds
+
+ // Whether the writer thread is active
+ private var active: Boolean = true
+ protected val buffer = new ArrayBuffer[RecordBuffer]()
+
+ 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
}(batchWriterThreadPool),
+ 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
+ }
+
+ /**
+ * Delete the log files that are older than the threshold time.
+ *
+ * This method is handled by the parent WriteAheadLog.
+ */
+ override def clean(threshTime: Long, waitForCompletion: Boolean): Unit =
{
+ parent.clean(threshTime, waitForCompletion)
+ }
+
+
+ /**
+ * Stop the batched writer thread, fulfill promises with failures and
close parent writer.
+ */
+ override def close(): Unit = {
+ logInfo("BatchedWriteAheadLog shutting down.")
+ active = false
+ fulfillPromises()
+ batchWriterThreadPool.shutdownNow()
+ parent.close()
+ }
+
+ /**
+ * Respond to any promises that may have been left in the queue, to
unblock receivers during
+ * shutdown.
+ */
+ private def fulfillPromises(): Unit = {
+ while (!walWriteQueue.isEmpty) {
+ val RecordBuffer(_, _, promise) = walWriteQueue.poll()
+ promise.success(null)
+ }
+ }
+
+ /** Start the actual log writer on a separate thread. Visible
(protected) for testing. */
+ protected def startBatchedWriterThread(): Unit = {
+ val thread = new Thread(new Runnable {
+ override def run(): Unit = {
+ while (active) {
+ try {
+ flushRecords()
+ } catch {
+ case NonFatal(e) =>
+ logWarning("Encountered exception in Batched Writer
Thread.", e)
+ }
+ }
+ logInfo("Batched WAL Writer thread exiting.")
+ }
+ }, "Batched WAL Writer")
+ thread.setDaemon(true)
+ thread.start()
+ }
+
+ /** Write all the records in the buffer to the write ahead log. Visible
for testing. */
+ protected def flushRecords(): Unit = {
+ try {
+ buffer.append(walWriteQueue.take())
+ val numBatched = walWriteQueue.drainTo(buffer.asJava) + 1
+ logDebug(s"Received $numBatched records from queue")
+ } catch {
+ case _: InterruptedException =>
+ logWarning("Batch Write Ahead Log Writer queue interrupted.")
+ }
+ try {
+ var segment: WriteAheadLogRecordHandle = null
+ if (buffer.length > 0) {
+ logDebug(s"Batched ${buffer.length} records for Write Ahead Log
write")
+ // we take the latest record for the time to ensure that we don't
clean up files earlier
+ // than the expiration date of the records
+ val time = buffer.last.time
+ segment = parent.write(aggregate(buffer), time)
+ }
+ buffer.foreach(_.promise.success(segment))
+ } catch {
+ case NonFatal(e) =>
+ logWarning(s"Batch WAL Writer failed to write $buffer", e)
+ buffer.foreach(_.promise.success(null))
+ }
+ buffer.clear()
+ }
+}
+
+/** Static methods for aggregating and de-aggregating records. */
+private[streaming] object BatchedWriteAheadLog {
+ /**
+ * Wrapper class for representing the records that we will write to the
WriteAheadLog. Coupled
+ * with the timestamp for the write request of the record, and the
promise that will block the
+ * write request, while a separate thread is actually performing the
write.
+ */
+ private[util] case class RecordBuffer(
+ record: ByteBuffer,
+ time: Long,
+ promise: Promise[WriteAheadLogRecordHandle])
+
+ /** Aggregate multiple serialized ReceivedBlockTrackerLogEvents in a
single ByteBuffer. */
+ private[streaming] def aggregate(records: Seq[RecordBuffer]): ByteBuffer
= {
+
ByteBuffer.wrap(Utils.serialize[Array[Array[Byte]]](records.map(_.record.array()).toArray))
+ }
+
+ /**
+ * De-aggregate serialized ReceivedBlockTrackerLogEvents in a single
ByteBuffer.
+ * A stream may not have used batching initially, but started using it
after a restart. This
+ * method therefore needs to be backwards compatible.
+ */
+ private[streaming] def deaggregate(buffer: ByteBuffer):
Array[ByteBuffer] = {
--- End diff --
The whole class is already `private[streaming]`
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