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From Liang-Chi Hsieh <vii...@gmail.com>
Subject [DISCUSS] Disable streaming query with possible correctness issue by default
Date Sat, 07 Nov 2020 06:59:44 GMT
Hi devs,

In Spark structured streaming, chained stateful operators possibly produces
incorrect results under the global watermark. SPARK-33259
(https://issues.apache.org/jira/browse/SPARK-33259) has an example
demostrating what the correctness issue could be.

Currently we don't prevent users running such queries. Because the possible
correctness in chained stateful operators in streaming query is not
straightforward for users. From users perspective, it will possibly be
considered as a Spark bug like SPARK-33259. It is also possible the worse
case, users are not aware of the correctness issue and use wrong results.

IMO, it is better to disable such queries and let users choose to run the
query if they understand there is such risk, instead of implicitly running
the query and let users to find out correctness issue by themselves.

I would like to propose to disable the streaming query with possible
correctness issue in chained stateful operators. The behavior can be
controlled by a SQL config, so if users understand the risk and still want
to run the query, they can disable the check.

In the PR (https://github.com/apache/spark/pull/30210), the concern I got
for now is, this changes current behavior and by default it will break some
existing streaming queries. But I think it is pretty easy to disable the
check with the new config. In the PR currently there is no objection but
suggestion to hear more voices. Please let me know if you have some
thoughts.

Thanks.
Liang-Chi Hsieh



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