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https://issues.apache.org/jira/browse/PHOENIX-5528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16956401#comment-16956401
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Geoffrey Jacoby commented on PHOENIX-5528:
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[~kozdemir] - I see the changes to GlobalIndexChecker to filter out the extra
index rows, but not the changes to IndexRegionObserver to throw the retry
exception described in the comment above.
Couple of questions about the algorithm:
# How do you know that the first index row you find for a given data table key
is the correct index row?
# In the case where everything's unverified (say after an upgrade to the new
index framework), and there's a very broad scan, do we risk an OOME from trying
to hold a region's worth of data table keys in the index coproc's memory? Would
storing a hash large enough to avoid collisions be safer?
> 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
> Assignee: Kadir OZDEMIR
> Priority: Major
> Attachments: PHOENIX-5528.master.001.patch
>
>
> 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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