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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-15999) S3Guard: Better support for out-of-band operations
Date Fri, 08 Mar 2019 15:34:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-15999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16787977#comment-16787977
] 

Steve Loughran commented on HADOOP-15999:
-----------------------------------------

This is ~ready to go in; I've only got one change to the code (see the bottom).

I do think we need to be sure that we've got all opportunties for inconsistencies to arise
covered, and I'm now considering performance too.

h3. Deletions

I think the whole of S3Guard is potentially brittle to 
* OOB deletions: you skip it here, so no worse, but because the S3AInputStream retries on
FNFE, so as to "debounce" cached 404s, it's potentially going to retry forever
* OOB creation of a file which has a deletion tombstone marker. 

You are already documenting this, so the next step to think about is code: 

*Proposed*: write a test to simulate that deletion problem, to see what happens. I'm actually
curious now. We ought to have the S3AInputStream retry briefly on that initial GET failing,
but only on that initial one. (after setting "fs.s3a.retry.limit" to something low & the
interval down to 10ms or so to fail fast)

sequences

{code}
1. create; delete; open; read -> fail after retry
2. create; open, read, delete, read -> fail fast on second read
{code}

The StoreStatistics of the filesystem's IGNORED_ERRORS stat will be increased on the ignored
error, so on sequence 1 will have increased, whereas on sequence 2 it will not have. If either
 of these tests don't quite fail as expected, we can disable the tests and continue, at least
now with some tests to simulate a condition we don't have a fix for



*Proposed* add a JIRA on this for us all to worry about. For both we just need to have some
model of how long it takes for debouncing to stabilise. Then in this new check, if an FNFE
is raised *and* the check is happening > (modtime+ debounce-delay) then its a real FNFE.


h3. Timestamp ordering 


I'm going to add a new complication here. When you initiate a PUT, AFAIK (and [~Thomas Demoor]
should be able to confirm), the modified time is that of the time the PUT began, not when
the PUT completed. Which means I can have a workflow of 

{code}
write1 = fs.create(path, true)
write2 = fs.create(path, true)
write2.close()
status  = fs.getFileStatus(path)
write1.write(128MB of data)
write1.close()
status2  = fs.getFileStatus(path)

assertTrue(status2.getLastModified() < status1.getLastModified())
{code}

There's no way we are going to be able to defend against that except by tracking versions
in the DDB tables, and the S3a Status including that when known. What we'll have to do then
is make sure that this issue is documented today, and for the extension to do tag tracking
in S3Guard, it keeps an eye on versions

*Proposed*: mention this problem in the docs. Once version tracking goes in to s3guard, we'll
need to move the ("is newer than") operator out of this modtime check into somewhere else
(proposed: do it in the S3AFileStatus, which will look @ version info if set, falling back
to etags). (actually, if version checking is on in the GET, we'd never see the updated file,
would we?)


h3. Performance impact

this is going to reinstate the HEAD on every read, so making non-auth S3Guard a bit slower.
We could think about addressing that by moving the checks into the input stream itself. That
is: the first GET which returns data will also act as the metadata check. That'd mean the
read context will need updating with some "metastoreProcessHeader" callback to invoke on the
first GET.

*Proposed*: Add a JIRA for this to become an optimization

The good news is that because it's reading a file, its only one HTTP HEAD request: no need
to do any of the other two directory probes except in the case that the file isn't there.

h2. code review


h3. ITestS3GuardOutOfBandOperations

Check your import ordering: new files are where we should start off with getting things "correct"
according to our style rules.

> S3Guard: Better support for out-of-band operations
> --------------------------------------------------
>
>                 Key: HADOOP-15999
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15999
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 3.1.0
>            Reporter: Sean Mackrory
>            Assignee: Gabor Bota
>            Priority: Major
>         Attachments: HADOOP-15999-007.patch, HADOOP-15999.001.patch, HADOOP-15999.002.patch,
HADOOP-15999.003.patch, HADOOP-15999.004.patch, HADOOP-15999.005.patch, HADOOP-15999.006.patch,
HADOOP-15999.008.patch, out-of-band-operations.patch
>
>
> S3Guard was initially done on the premise that a new MetadataStore would be the source
of truth, and that it wouldn't provide guarantees if updates were done without using S3Guard.
> I've been seeing increased demand for better support for scenarios where operations are
done on the data that can't reasonably be done with S3Guard involved. For example:
> * A file is deleted using S3Guard, and replaced by some other tool. S3Guard can't tell
the difference between the new file and delete / list inconsistency and continues to treat
the file as deleted.
> * An S3Guard-ed file is overwritten by a longer file by some other tool. When reading
the file, only the length of the original file is read.
> We could possibly have smarter behavior here by querying both S3 and the MetadataStore
(even in cases where we may currently only query the MetadataStore in getFileStatus) and use
whichever one has the higher modified time.
> This kills the performance boost we currently get in some workloads with the short-circuited
getFileStatus, but we could keep it with authoritative mode which should give a larger performance
boost. At least we'd get more correctness without authoritative mode and a clear declaration
of when we can make the assumptions required to short-circuit the process. If we can't consider
S3Guard the source of truth, we need to defer to S3 more.
> We'd need to be extra sure of any locality / time zone issues if we start relying on
mod_time more directly, but currently we're tracking the modification time as returned by
S3 anyway.



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