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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-5660) Drill 1.10 queries fail due to Parquet Metadata "corruption" from DRILL-3867
Date Fri, 14 Jul 2017 14:38:00 GMT

    [ https://issues.apache.org/jira/browse/DRILL-5660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16087394#comment-16087394
] 

ASF GitHub Bot commented on DRILL-5660:
---------------------------------------

Github user paul-rogers commented on the issue:

    https://github.com/apache/drill/pull/877
  
    I wonder if you are very close to solving another long-running problem in Drill: Parquet
metadata file corruption (or, at least, the corruption causing queries to fail.)
    
    We have an issue that two Drillbits can write to metadata file concurrently and corrupt
it. We also have a case where Drillbit A writes the file while Drillbit B reads it. In both
cases, the reading Drillbit can't deserialize the file and "bad things happen."
    
    We need code that can try to deserialize a file, notice that the JSON in the file does
not fit our schema, and handle the deserialization exception. If we do that, it is a small
step to ignore all forms of read errors: file-not-found, file truncation, etc.
    
    In this case, we give an explicit version error if we can read the file and it has the
wrong version. If we can't deserialize the file, we log an error about "file is of a newer
version or corrupt."
    
    After this, the corruption can still occur, but the user view is that queries run slow
instead of just failing.


> Drill 1.10 queries fail due to Parquet Metadata "corruption" from DRILL-3867
> ----------------------------------------------------------------------------
>
>                 Key: DRILL-5660
>                 URL: https://issues.apache.org/jira/browse/DRILL-5660
>             Project: Apache Drill
>          Issue Type: Bug
>    Affects Versions: 1.11.0
>            Reporter: Paul Rogers
>            Assignee: Vitalii Diravka
>             Fix For: 1.11.0
>
>
> Drill recently accepted a PR for the following bug:
> DRILL-3867: Store relative paths in metadata file
> This PR turned out to have a flaw which affects version compatibility.
> The DRILL-3867 PR changed the format of Parquet metadata files to store relative paths.
All Drill servers after that PR create files with relative paths. But, the version number
of the file is unchanged, so that older Drillbits don't know that they can't understand the
file.
> Instead, if an older Drill tries to read the file, queries fail left and right. Drill
will resolve the paths, but does so relative to the user's HDFS home directory, which is wrong.
> What should have happened is that we should have bumped the parquet metadata file version
number so that older Drillbits can’t read the file. This ticket requests that we do that.
> Now, one could argue that the lack of version number change is fine. Once a user upgrades
Drill, they won't use an old Drillbit. But, things are not that simple:
> * A developer tests a branch based on a pre-DRILL-3867 build on a cluster in which metadata
files have been created by a post-DRILL-3867 build. (This has already occurred multiple times
in our shop.)
> * A user tries to upgrade to Drill 1.11, finds an issue, and needs to roll back to Drill
1.10. Doing so will cause queries to fail due to seemingly-corrupt metadata files.
> * A user tries to do a rolling upgrade: running 1.11 on some servers, 1.10 on others.
Once a 1.11 server is installed, the metadata is updated ("corrupted" from the perspective
of 1.10) and queries fail.
> Standard practice in this scenario is to:
> * Bump the file version number when the file format changes, and
> * Software refuses to read files with a version newer than the software was designed
for.
> Of course, it is highly desirable that newer servers read old files, but that is not
the issue here.



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