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From "Geoffrey Jacoby (Jira)" <>
Subject [jira] [Created] (PHOENIX-5604) Index rebuilds should not skip WAL
Date Wed, 04 Dec 2019 23:42:00 GMT
Geoffrey Jacoby created PHOENIX-5604:

             Summary: Index rebuilds should not skip WAL
                 Key: PHOENIX-5604
             Project: Phoenix
          Issue Type: Bug
            Reporter: Geoffrey Jacoby
            Assignee: Geoffrey Jacoby

Currently both Index read repairs and IndexTool build/rebuilds in the new design continue
to skip the WAL, following the same pattern the old Indexer used. However, there are key differences
between the old and new logic that make this no longer the correct choice.

First, recall that all HBase replication is based on tailing the WAL, and that any transaction
that skips the WAL doesn't get replicated. 

In the old logic, the data table write (and WAL append) would be accompanied by an IndexedKeyValue
which would contain enough information to reconstitute the index edit in the event of a failure
before the index edit could be committed. So skipping the WAL during recovery was _potentially_ OK,
because writing to the WAL would be redundant locally. (But that still seems to me wrong in
a case with replication, since I don't believe IndexedKeyValues are replicated, since they
use the "magic" METAFAMILY cf.)  

In the new logic, on a normal write, we write to the index first (which will go into a WAL),
then the data table (into a potentially different RS's WAL), and lastly the verified flag
flip into the Index, into the original index write's WAL. If something goes wrong with stage
2 or 3, read repair will fix it, but if the repair action – whether a put or delete –
doesn't go into the WAL, a DR buddy of the index will be out of sync. 

This is even more important on an async initial build of an index, where if I understand right,
there is no WAL append for the index write at all in the current UngroupedAggregateRegionObserver
rebuild logic. The same would be the case of a rebuild of a new-style index in the event of
non-Phoenix related corruption (such as HDFS or raw HBase level). 

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