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MAPREDUCE-7282. Move away from V2 commit algorithm #2349

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steveloughran wants to merge 2 commits into from
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MAPREDUCE-7282. Move away from V2 commit algorithm #2349

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MAPREDUCE-7282. Move away from V2 commit algorithm
steveloughran Sep 29, 2020
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MAPREDUCE-7282 feedback on message and comments
steveloughran Oct 1, 2020
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12 changes: 11 additions & 1 deletion ...client-core/src/main/java/org/apache/hadoop/mapreduce/lib/output/FileOutputCommitter.java
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Expand Up @@ -68,7 +68,7 @@ public class FileOutputCommitter extends PathOutputCommitter {
"mapreduce.fileoutputcommitter.marksuccessfuljobs";
public static final String FILEOUTPUTCOMMITTER_ALGORITHM_VERSION =
"mapreduce.fileoutputcommitter.algorithm.version";
public static final int FILEOUTPUTCOMMITTER_ALGORITHM_VERSION_DEFAULT = 2;
public static final int FILEOUTPUTCOMMITTER_ALGORITHM_VERSION_DEFAULT = 1;
// Skip cleanup _temporary folders under job's output directory
public static final String FILEOUTPUTCOMMITTER_CLEANUP_SKIPPED =
"mapreduce.fileoutputcommitter.cleanup.skipped";
Expand Down Expand Up @@ -348,6 +348,16 @@ public Path getWorkPath() throws IOException {
* @param context the job's context
*/
public void setupJob(JobContext context) throws IOException {
// Downgrade v2 to v1 with a warning.
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if (algorithmVersion == 2) {
Logger log = LoggerFactory.getLogger(
"org.apache.hadoop.mapreduce.lib.output."
+ "FileOutputCommitter.Algorithm");

log.warn("The v2 commit algorithm is deprecated;"
+ " please switch to the v1 algorithm");

@jbrennan333 jbrennan333 Sep 29, 2020

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I don't think we should use the word deprecated. That implies that this algorithm will be removed in a future release

@steveloughran steveloughran Oct 1, 2020

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what do you suggest?

@steveloughran steveloughran Oct 1, 2020

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switching to your text

}

if (hasOutputPath()) {
Path jobAttemptPath = getJobAttemptPath(context);
FileSystem fs = jobAttemptPath.getFileSystem(
Expand Down
33 changes: 29 additions & 4 deletions ...adoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml
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Expand Up @@ -1562,10 +1562,35 @@

<property>
<name>mapreduce.fileoutputcommitter.algorithm.version</name>
<value>2</value>
<description>The file output committer algorithm version
valid algorithm version number: 1 or 2
default to 2, which is the original algorithm
<value>1</value>
<description>The file output committer algorithm version.

There are two algorithm versions in Hadoop, "1" and "2".

The version 2 algorithm is deprecated and no longer the default
as task commits were not atomic.

@jbrennan333 jbrennan333 Sep 29, 2020

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Similarly, remove "deprecated".

If a first task attempt fails part-way
through its task commit, the output directory could end up
with data from that failed commit, alongside the data
from any subsequent attempts.

See https://issues.apache.org/jira/browse/MAPREDUCE-7282

Although no-longer the default, this algorithm is safe to use if
all task attempts for a single task meet the following requirements
-they generate exactly the same set of files
-the contents of each file are exactly the same in each task attempt

That is:
1. If a second attempt commits work, there will be no leftover files from
a first attempt which failed during its task commit.
2. If a network partition causes the first task attempt to overwrite
some/all of the output of a second attempt, the result will be
exactly the same as if it had not done so.

To avoid the warning message on job setup, set the log level of the log
org.apache.hadoop.mapreduce.lib.output.FileOutputCommitter.Algorithm
to ERROR.

@jbrennan333 jbrennan333 Sep 29, 2020

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I think this section should be moved to the end of the Algorithm 2 section below. You can add (see below for details) to the end of the line that says why algorithm v2 in no longer the default.

@steveloughran steveloughran Oct 1, 2020

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ok


In algorithm version 1,

Expand Down