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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 Mon, 04 Mar 2019 23:21:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-15625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16783884#comment-16783884
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Steve Loughran commented on HADOOP-15625:
-----------------------------------------

* s3a doesn't work with snowball at all, I do know of some interest in distcp to it working,
with the result then being fedexed to the final Se3 store. The support needed will be minimal

A key thing: we have to plan for some endpoints not having etags. And if someone went from
unversioned to versioned, the may be unversioned files in the store

* S3Select. Good point. For the s3guard integration, maybe the strategy will be: do a HEAD
until either the etag matches or a stability threshold has been reached, but not worry about
changes during a read. Be interesting to experiment to see what happens...you could probably
infer something from that behaviour. (e.g some cursor on the read for the paged lists, every
time you ask for a new page it scans some more.

Not had a chance to look @ your patch yet, will do it tomorrow. 


> 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--15625-006.patch, HADOOP-15625-001.patch, HADOOP-15625-002.patch,
HADOOP-15625-003.patch, HADOOP-15625-004.patch, HADOOP-15625-005.patch, HADOOP-15625-006.patch,
HADOOP-15625-007.patch, HADOOP-15625-008.patch, HADOOP-15625-009.patch, HADOOP-15625-010.patch,
HADOOP-15625-011.patch, HADOOP-15625-012.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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