GitHub user tdas opened a pull request:


    ## What changes were proposed in this pull request?
    CacheKafkaConsumer in the project `kafka-0-10-sql` is designed to maintain 
a pool of KafkaConsumers that can be reused. However, it was built with the 
assumption there will be only one task using trying to read the same Kafka 
TopicPartition at the same time. Hence, the cache was keyed by the 
TopicPartition a consumer is supposed to read. And any cases where this 
assumption may not be true, we have SparkPlan flag to disable the use of a 
cache. So it was up to the planner to correctly identify when it was not safe 
to use the cache and set the flag accordingly. 
    Fundamentally, this is the wrong way to approach the problem. It is HARD 
for a high-level planner to reason about the low-level execution model, whether 
there will be multiple tasks in the same query trying to read the same 
partition. Case in point, 2.3.0 introduced stream-stream joins, and you can 
build a streaming self-join query on Kafka. It's pretty non-trivial to figure 
out how this leads to two tasks reading the same partition twice, possibly 
concurrently. And due to the non-triviality, it is hard to figure this out in 
the planner and set the flag to avoid the cache / consumer pool. And this can 
inadvertently lead to ConcurrentModificationException ,or worse, silent reading 
of incorrect data.
    Here is a better way to design this. The planner shouldnt have to 
understand these low-level optimizations. Rather the consumer pool should be 
smart enough avoid concurrent use of a cached consumer. Currently, it tries to 
do so but incorrectly (the flag inuse is not checked when returning a cached 
consumer, see 
 If there is another request for the same partition as a currently in-use 
consumer, the pool should automatically return a fresh consumer that should be 
closed when the task is done. Then the planner does not have to have a flag to 
avoid reuses.
    This PR is a step towards that goal. It does the following. 
    - There are effectively two kinds of consumer that may be generated 
      - Cached consumer - this should be returned to the pool at task end
      - Non-cached consumer - this should be closed at task end
    - A trait called KafkaConsumer is introduced to hide this difference from 
the users of the consumer so that the client code does not have to reason about 
whether to stop and release. They simply called `val consumer = 
KafkaConsumer.acquire` and then `consumer.release()`.
    - If there is request for a consumer that is in-use, then a new consumer is 
    - If there is a concurrent attempt of the same task, then a new consumer is 
generated, and the existing cached consumer is marked for close upon release. 
    This PR does not remove the planner flag to avoid reuse to make this patch 
safe enough for merging in branch-2.3. This can be done later in master-only.
    ## How was this patch tested?
    (Please explain how this patch was tested. E.g. unit tests, integration 
tests, manual tests)
    (If this patch involves UI changes, please attach a screenshot; otherwise, 
remove this)
    Please review before opening a 
pull request.

You can merge this pull request into a Git repository by running:

    $ git pull SPARK-23623

Alternatively you can review and apply these changes as the patch at:

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #20767
commit 97510c6952a865caf41e6b6f19c3af7e714c3ad6
Author: Tathagata Das <tathagata.das1565@...>
Date:   2018-03-08T02:23:45Z




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