Ben Roling created HADOOP-16221:
-----------------------------------
Summary: S3Guard: fail write that doesn't update metadata store
Key: HADOOP-16221
URL: https://issues.apache.org/jira/browse/HADOOP-16221
Project: Hadoop Common
Issue Type: Improvement
Components: fs/s3
Affects Versions: 3.2.0
Reporter: Ben Roling
Right now, a failure to write to the S3Guard metadata store (e.g. DynamoDB) is
[merely
logged|https://github.com/apache/hadoop/blob/rel/release-3.1.2/hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java#L2708-L2712].
It does not fail the S3AFileSystem write operation itself. As such, the writer
has no idea that anything went wrong. The implication of this is that S3Guard
doesn't always provide the consistency it advertises.
For example [this
article|https://blog.cloudera.com/blog/2017/08/introducing-s3guard-s3-consistency-for-apache-hadoop/]
states:
{quote}If a Hadoop S3A client creates or moves a file, and then a client lists
its directory, that file is now guaranteed to be included in the listing.
{quote}
Unfortunately, this is sort of untrue and could result in exactly the sort of
problem S3Guard is supposed to avoid:
{quote}Missing data that is silently dropped. Multi-step Hadoop jobs that
depend on output of previous jobs may silently omit some data. This omission
happens when a job chooses which files to consume based on a directory listing,
which may not include recently-written items.
{quote}
Imagine the typical multi-job Hadoop processing pipeline. Job 1 runs and
succeeds, but one (or more) S3Guard metadata write failed under the covers. Job
2 picks up the output directory from Job 1 and runs its processing, potentially
seeing an inconsistent listing, silently missing some of the Job 1 output files.
S3Guard should at least provide a configuration option to fail if the metadata
write fails. It seems even ideally this should be the default?
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