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From "Sushanth Sowmyan (JIRA)" <>
Subject [jira] [Created] (HIVE-7368) datanucleus sometimes returns an empty result instead of an error or data
Date Tue, 08 Jul 2014 22:14:05 GMT
Sushanth Sowmyan created HIVE-7368:

             Summary: datanucleus sometimes returns an empty result instead of an error or
                 Key: HIVE-7368
             Project: Hive
          Issue Type: Bug
          Components: Metastore
    Affects Versions: 0.12.0
            Reporter: Sushanth Sowmyan

I investigated a scenario wherein a user needed to use a large number of concurrent hive clients
doing simple DDL tasks, while not using a standalone metastore server. Say, for eg., each
of them doing "drop table if exists tmp_blah_${i};"

This would consistently fail stating that it could not create a db, which is a funny error
to have when trying to drop a db "if exists". On digging in, it turned out that the error
was a mistaken report, coming instead from an attempt by the embedded metastore attempting
to create a "default" db when it did not exist. The funny thing being that the default db
did exist, and the getDatabase call would return empty, rather than returning an error or
returning a result. We could disable hive.metastore.checkForDefaultDb and the number of these
reports would drastically fall, but that only moved the problem, and we'd get phantom reports
from time to time of various other databases that existed that were being reported as non-existent.

On digging further, parallelism seemed to be an important factor in whether or not hive was
able to perform getDatabases without error. With about 20 simultaneous processes, there seemed
to be no errors whatsoever. At about 40 simultaneous processes, at least 1 would consistently
fail. At about 200, about 15-20 would consistently fail, in addition to taking a long time
to run.

I wrote a sample JDBC ping (actually a get_database mimic) utility to see whether the issue
was with connecting from that machine to the database server, and this had no errors whatsoever
up to 400 simultaneous processes. The mysql server in question was configured to serve up
to 650 connections, and it seemed to be serving responses quickly and did not seem overloaded.
We also disabled connection pooling in case that was exacerbating a connection availability
issue with that many concurrent processes, each with an embedded metastore. That, especially
in conjunction with disabling schema checking, and specifying a "datanucleus.connectionPool.testSQL=SELECT
1" did a fair amount for performance in this scenarios, but the errors (or rather, the null-result-successes
when there shouldn't have been one) continued.

On checking through hive again, if we modified hive to have datanucleus simply return a connection,
with which we did a direct sql get database, there would not be any error, but if we tried
to use jdo on datanucleus to construct a db object, we would get an empty result.

One of the biggest issues with this investigation, for me, was the difficulty of reproducibility.
When trying to reproduce in a lab, we were unable to create a similar enough environment that
caused the issue. Even in the client's environment, moving from RHEL5 to RHEL6 made the issue
go away.

Thus, we still have work to do on determining the underlying issue, I'm logging this issue
to collect information on similar issues we discover so we can work towards nailing down the
issue and then fixing it(in DN if need be)

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