Github user vanzin commented on a diff in the pull request:
https://github.com/apache/spark/pull/9214#discussion_r43666862
--- Diff:
core/src/main/scala/org/apache/spark/shuffle/ShuffleOutputCoordinator.scala ---
@@ -0,0 +1,105 @@
+/*
+ * 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.shuffle
+
+import java.io.{FileOutputStream, FileInputStream, File}
+
+import com.google.common.annotations.VisibleForTesting
+
+import org.apache.spark.storage.ShuffleMapStatusBlockId
+import org.apache.spark.{SparkEnv, Logging}
+import org.apache.spark.scheduler.MapStatus
+import org.apache.spark.serializer.SerializerInstance
+
+/**
+ * Ensures that on each executor, there are no conflicting writes to the
same shuffle files. It
+ * implements "first write wins", by atomically moving all shuffle files
into their final location,
+ * only if the files did not already exist. See SPARK-8029
+ */
+private[spark] object ShuffleOutputCoordinator extends Logging {
+
+ /**
+ * If any of the destination files do not exist, then move all of the
temporary files to their
+ * destinations, and return (true, the given MapStatus). If all
destination files exist, then
+ * delete all temporary files, and return (false, the MapStatus from
previously committed shuffle
+ * output).
+ *
+ * Note that this will write to all destination files. If the tmp file
is missing, then a
+ * zero-length destination file will be created. This is so the
ShuffleOutputCoordinator can work
+ * even when there is a non-determinstic data, where the output exists
in one attempt, but is
+ * empty in another attempt.
+ *
+ * @param tmpToDest Seq of (temporary, destination) file pairs
+ * @param mapStatus the [[MapStatus]] for the output already written to
the the temporary files
+ * @return pair of: (1) true iff the set of temporary files was moved to
the destination and (2)
+ * the MapStatus of the committed attempt.
+ *
+ */
+ def commitOutputs(
+ shuffleId: Int,
+ partitionId: Int,
+ tmpToDest: Seq[(File, File)],
+ mapStatus: MapStatus,
+ sparkEnv: SparkEnv): (Boolean, MapStatus) = synchronized {
+ val mapStatusFile = sparkEnv.blockManager.diskBlockManager.getFile(
+ ShuffleMapStatusBlockId(shuffleId, partitionId))
+ val ser = sparkEnv.serializer.newInstance()
+ commitOutputs(shuffleId, partitionId, tmpToDest, mapStatus,
mapStatusFile, ser)
+ }
+
+ @VisibleForTesting
+ def commitOutputs(
+ shuffleId: Int,
+ partitionId: Int,
+ tmpToDest: Seq[(File, File)],
+ mapStatus: MapStatus,
+ mapStatusFile: File,
+ serializer: SerializerInstance): (Boolean, MapStatus) = synchronized
{
+ val destAlreadyExists = tmpToDest.forall{_._2.exists()} &&
mapStatusFile.exists()
+ if (!destAlreadyExists) {
+ tmpToDest.foreach { case (tmp, dest) =>
+ // If *some* of the destination files exist, but not all of them,
then its not clear
+ // what to do. There could be a task already reading from this
dest file when we delete
--- End diff --
So, this justification feels a little weird to me. Let's say you have t1
and t2.
* if t1 finishes and writes "n" files, and t2 finishes and writes "n + 1"
files, then you'll get the output of t2
* if instead t2 also creates "n" files, you'll get the output of t1 instead.
Or am I misunderstanding something?
Also, the "if a task is already reading from the file" case will probably
make this whole block of code fail on Windows; I believe `File.delete()` will
fail if the file is open, unlike on POSIX fses.
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