[jira] [Comment Edited] (HADOOP-13884) s3a create(overwrite=true) to only look for dir/ and list entries, not file

["Steve Loughran (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-13884?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16837281#comment-16837281 ] 

Steve Loughran edited comment on HADOOP-13884 at 5/10/19 1:27 PM:
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Revisiting: 

#. sometimes that negative entry can last minutes rather than a few seconds.
# we also do existence checks on rename, so the same risk can occur there: you copy to a destination and the new file isn't immediately visible.

Proposed: a new operation checkIsNotDirectory(key): boolean. which returns true if there is either nothing at the far end or a file, that is "no directory entry"

we can have a sequence of

* S3Guard: check the store to make sure it's not a directory entry.
* S3: HEAD key + "/"; LIST key.

Use in createFile and when renaming a file on rename/2 if the source is a file. 

It's a bit odder on rename/2 as if the dest is a directory it is inserted underneath. on 

Rename 3 copy(file, dest) will fail if one of
* isDirectory(dest)
* isFile(dest) and !overwrite
This makes it possible to use the {{checkIsNotDirectory(key):}} probe when source is file and overwrite is true.




was (Author: stevel@apache.org):
Revisiting: 

#. sometimes that negative entry can last minutes rather than a few seconds.
# we also do existence checks on rename, so the same risk can occur there: you copy to a destination and the new file isn't immediately visible.

Proposed: a new operation checkIsNotDirectory(key): boolean. which returns true if there is either nothing at the far end or a file, that is "no directory entry"

we can have a sequence of

* S3Guard: check the store to make sure it's not a directory entry.
* S3: HEAD key + "/"; LIST key.


* 

> s3a create(overwrite=true) to only look for dir/ and list entries, not file
> ---------------------------------------------------------------------------
>
>                 Key: HADOOP-13884
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13884
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 2.9.0
>            Reporter: Steve Loughran
>            Priority: Minor
>
> before doing a create(), s3a does a getFileStatus() to make sure there isn't a directory there, and, if overwrite=false, that there isn't a file.
> Because S3 caches negative HEAD/GET requests, if there isn't a file, then even after the PUT, a later GET/HEAD may return 404; we are generating create consistency where none need exist. 
> when overwrite=true we don't care whether the file exists or not, only that the path isn't a directory. So we can just do the HEAD path +"/' and the LIST calls, skipping the {{HEAD path}}. This will save an HTTP round trip of a few hundred millis, and ensure that there's no 404 cached in the S3 front end for later callers



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