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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-15625) S3A input stream to use etags to detect changed source files
Date Tue, 21 Aug 2018 17:05:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-15625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16587750#comment-16587750
] 

Steve Loughran commented on HADOOP-15625:
-----------------------------------------

Patch is better, but I think we still need to handle the situation of s3guard getFileStatus
may not have etag...if the tag is null then it mustn't be used in the parameters of the GET....but
the first GET which returns data will return that etag, which can then be attached to future
requests

> S3A input stream to use etags to detect changed source files
> ------------------------------------------------------------
>
>                 Key: HADOOP-15625
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15625
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 3.2.0
>            Reporter: Brahma Reddy Battula
>            Assignee: Brahma Reddy Battula
>            Priority: Major
>         Attachments: HADOOP-15625-001.patch, HADOOP-15625-002.patch
>
>
> S3A input stream doesn't handle changing source files any better than the other cloud
store connectors. Specifically: it doesn't noticed it has changed, caches the length from
startup, and whenever a seek triggers a new GET, you may get one of: old data, new data, and
even perhaps go from new data to old data due to eventual consistency.
> We can't do anything to stop this, but we could detect changes by
> # caching the etag of the first HEAD/GET (we don't get that HEAD on open with S3Guard,
BTW)
> # on future GET requests, verify the etag of the response
> # raise an IOE if the remote file changed during the read.
> It's a more dramatic failure, but it stops changes silently corrupting things.



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