Bug: Support Concurrent Writes With S3A Magic Committer
Status: Resolved | Priority: Major | Resolution: Fixed
Created: 10/Jul/23 13:34
Affects Version/s: 3.4.0

h2. warning: no guarantee of safe/consistent results due to non-atomic job commit

Important: There is no guarantee that concurrent jobs writing to the same table are safe. This is as true for the classic FileOutputCommitter as it is for the S3A Magic Committer.

Why not? Because neither of these committers (or any other which works on a file-by-file basis) has an atomic job commit operation. If two jobs commit at the same time, the results are *completely undefined*. And this may not be detected by the applications.

If you want safe, concurrent parallel writes, use a manifest-file format with applications able to handle commit conflicts.

h2. problem

There is a failure in the commit process when multiple jobs are writing to a s3 directory *concurrently* using {*}magic committers{*}.

This issue is closely related HADOOP-17318.

When multiple Spark jobs write to the same S3A directory, they upload files simultaneously using "__magic" as the base directory for staging. Inside this directory, there are multiple "/job-some-uuid" directories, each representing a concurrently running job.

To fix some preoblems related to concunrrency a property was introduced in the previous fix: "spark.hadoop.fs.s3a.committer.abort.pending.uploads". When set to false, it ensures that during the cleanup stage, finalizing jobs do not abort pending uploads from other jobs. So we see in logs this line: 
{code:java}
DEBUG [main] o.a.h.fs.s3a.commit.AbstractS3ACommitter (819): Not cleanup up pending uploads to s3a ...{code}
(from [AbstractS3ACommitter.java#L952|https://github.com/apache/hadoop/blob/trunk/hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/commit/AbstractS3ACommitter.java#L952])

However, in the next step, the {*}"__magic" directory is recursively deleted{*}:
{code:java}
INFO  [main] o.a.h.fs.s3a.commit.magic.MagicS3GuardCommitter (98): Deleting magic directory s3a://my-bucket/my-table/__magic: duration 0:00.560s {code}
(from [AbstractS3ACommitter.java#L1112 |https://github.com/apache/hadoop/blob/trunk/hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/commit/AbstractS3ACommitter.java#L1112]and [MagicS3GuardCommitter.java#L137)|https://github.com/apache/hadoop/blob/trunk/hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/commit/magic/MagicS3GuardCommitter.java#L137)]

This deletion operation *affects the second job* that is still running because it loses pending uploads (i.e., ".pendingset" and ".pending" files).

The consequences can range from an exception in the best case to a silent loss of data in the worst case. The latter occurs when Job_1 deletes files just before Job_2 executes "listPendingUploadsToCommit" to list ".pendingset" files in the job attempt directory previous to complete the uploads with POST requests.

To resolve this issue, it's important {*}to ensure that only the prefix associated with the job currently finalizing is cleaned{*}.

Here's a possible solution:
{code:java}
/**
 * Delete the magic directory.
 */
public void cleanupStagingDirs() {
  final Path out = getOutputPath();
 //Path path = magicSubdir(getOutputPath());
  Path path = new Path(magicSubdir(out), formatJobDir(getUUID()));

  try(DurationInfo ignored = new DurationInfo(LOG, true,
      "Deleting magic directory %s", path)) {
    Invoker.ignoreIOExceptions(LOG, "cleanup magic directory", path.toString(),
        () -> deleteWithWarning(getDestFS(), path, true));
  }
} {code}
 

The side effect of this issue is that the "__magic" directory is never cleaned up. However, I believe this is a minor concern, even considering that other folders such as "_SUCCESS" also persist after jobs end.