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From "Ben Roling (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-16221) S3Guard: fail write that doesn't update metadata store
Date Wed, 01 May 2019 13:52:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-16221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16831010#comment-16831010
] 

Ben Roling commented on HADOOP-16221:
-------------------------------------

Thanks [~fabbri] for sharing your perspective.  I understand where you are coming from.

> 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: Sub-task
>          Components: fs/s3
>    Affects Versions: 3.2.0
>            Reporter: Ben Roling
>            Assignee: Ben Roling
>            Priority: Major
>             Fix For: 3.3.0
>
>
> 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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