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https://issues.apache.org/jira/browse/HADOOP-15625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16767309#comment-16767309
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Ben Roling commented on HADOOP-15625:
-------------------------------------
{quote}With the way the code is structured now, it would take another remote
call to even know whether the new file is longer or shorter as non-matching
eTag results in a null return value from getObject() call.
{quote}
I just realized it really wouldn't be necessary to introduce another remote
call to detect this. I don't really need to use the
withMatchingETagConstraint() on GetObjectRequest. I could omit that and just
check the eTag on the response to see whether it has changed. That said
though, I don't know that it makes sense to me to have any different behavior
in terms of the exception that would be thrown whether the new file is shorter
or longer so I still am not sure why you might still want tests of both.
> 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,
> HADOOP-15625-003.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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