[jira] [Commented] (PHOENIX-5528) Race condition in index verification causes multiple index rows to be returned for single data table row

["Kadir OZDEMIR (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/PHOENIX-5528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16952350#comment-16952350 ] 

Kadir OZDEMIR commented on PHOENIX-5528:
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[~vincentpoon], I think the scenario you have described can happen because whenever GlobalIndexChecker detects an unverified row, it rebuilds the row first, then it closes the current scanner, and finally opens a new region scanner on the index table region to make sure that the row that has just been rebuilt is visible to the scanner. When it does this, it reuses the original scan object. Phoenix sets the time range on the scan to [0, HBase max timestamp], which means that retrieve the latest and greatest. The solution to this problem is to update the scan time range to replace the max timestamp value with the current wall clock time of the server when the scanner is opened by GlobalIndexChecker in the very first time, so that even when the new region scanner is opened, the newly updated rows outside the time range of the scan will not be visible to the scan.  

> Race condition in index verification causes multiple index rows to be returned for single data table row
> --------------------------------------------------------------------------------------------------------
>
>                 Key: PHOENIX-5528
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-5528
>             Project: Phoenix
>          Issue Type: Bug
>            Reporter: Vincent Poon
>            Priority: Major
>
> Warning: This is an artificially generated scenario that likely has a very low probability of happening in practice.  But a race condition nevertheless.  Unfortunately I don't have a test case, but was able to produce this by debugging a local regionserver and adding breakpoints at the right places to produce the ordering here.
> The core problem is that when we do an update to the data table, we produce two unverified index rows at first.  When we scan both of these index rows and attempt to verify via rebuilding the data table row, we cannot guarantee that both verifications happen before the data table update, or both happen after the data table update.
> I use multiple index regions here to demonstrate, but I believe it could happen within a single region as well.
> Steps:
> 1) Create a test table with "pk" and "indexed_val" columns, and a global index on "indexed_val".
> 2) upsert into test values ('test_pk', 'test_val');
> 3) Split the index table on 'test_pk':
>    hbase shell: split 'test_index', 'test_pk'.
>    This creates two regions, call them regionA and regionB (which holds the existing index row)
> 3) start an update: upsert into test values ('test_pk', 'new_val');
>    The first thing the indexing code does is create two unverified index rows: one is a new version of the existing index row, and the other is for the new indexed value.
>    We pause the thread after this is done, before the row locks and data table write happens.
> 4) select indexed_val from test;
>    This scans both the index regions in parallel.  Each scan picks up a unverified row in its region.  We pause in GlobalIndexChecker.
>    Let the regionB scan proceed.  It will attempt to rebuild the data table row.  The data table still has 'test_val' as the indexed value.  The rebuild succeeds.
>    scan on regionA still paused.
> 5) The original update proceeds to update the data table indexed value to 'new_val'.
> 6) The scan on regionA proceeds, and attempted to rebuild the data table row.  The rebuild succeeds with 'new_val' as the indexed value.
> 7) Both 'test_val' and 'new_val' are returned to the client, because both rebuilds succeeded.



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