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https://issues.apache.org/jira/browse/DRILL-5660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16121408#comment-16121408
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ASF GitHub Bot commented on DRILL-5660:
---------------------------------------

Github user arina-ielchiieva commented on a diff in the pull request:

    https://github.com/apache/drill/pull/877#discussion_r132413275
  
    --- Diff: 
exec/java-exec/src/main/java/org/apache/drill/exec/store/parquet/ParquetGroupScan.java
 ---
    @@ -268,11 +276,15 @@ private FileSelection expandIfNecessary(FileSelection 
selection) throws IOExcept
         // use the cacheFileRoot if provided (e.g after partition pruning)
         Path metaFilePath = new Path(cacheFileRoot != null ? cacheFileRoot : 
selectionRoot, Metadata.METADATA_FILENAME);
         if (!fs.exists(metaFilePath)) { // no metadata cache
    +      MetadataContext metaContext = selection.getMetaContext();
    --- End diff --
    
    It seems you set metadata context, when there can be the cases when it is 
still null? Also can you please explain why there are so many checks in the 
code if meta context is null, can we make sure it always exists?


> 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
>              Labels: doc-impacting
>             Fix For: 1.12.0
>
>
> 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.
> *Main technical points of working of parquet metadata caching for now.*
> Only process of reading the parquet metadata is changed (the process of 
> writing isn't changed):
> +1. Metadata files are valid:+
> Metadata objects are created by deserialization of parquet metadata files in 
> the process of creating ParquetGroupScan physical operator. 
> All supported versions are stored in the "MetadataVersion.Constants" class 
> and in the Jackson annotations for Metadata.ParquetTableMetadataBase class.
> +2. Metadata files version isn't supported (created by newer Drill version). 
> Drill table has at least one metadata file of unsupported version:+
> JsonMappingException is obtained and swallowed without creating metadata 
> object. Error message is logged. The state is stored in MetadataContext, 
> therefore further there will be no attempt to deserialize metadata file again 
> in context of performing current query. The physical plan will be created 
> without using parquet metadata caching. Warning message is logged for every 
> further check "is metadata corrupted".
> +3. Drill table has at least one corrupted metadata file, which can't be 
> deserialized:+
> JsonParseException is obtained. Then the same behaviour as for the 
> unsupported version files.
> +4. The metadata file was removed by other process:+
> FileNotFound is obtained. Then the same behaviour as for the unsupported 
> version files.
> The new versions of metadata should be added in such manner:
> 1. Increasing of the metadata major version if metadata structure is changed.
> 2. Increasing of the metadata minor version if only metadata content is 
> changed, but metadata structure is the same.
> For the first case a new metadata structure (class) should be created 
> (possible an improvement of deserializing metadata files of any version into 
> one strucure by using special converting)
> For the second case only annotation for the last metadata structure can be 
> updated.
> *Summary*
> 1. Drill will read and use metadata files if files are valid, all present and 
> supported. Under supported we mean that files were created before and under 
> current Drill version.
> 2. Drill will ignore reading metadata files if at least one file is missing, 
> empty, corrupted or unsupported. Under unsupported we mean files created 
> after current Drill version. When metadata files are not used, warning 
> message will be written to the logs.



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