Re: Exceptions when launching counts on a Flink DataSet concurrently

[Juan Rodríguez Hortalá <ju...@gmail.com>]

Thanks for your answer Fabian.

In my opinion this is not just a possible new feature for an optimization,
but a bigger problem because the client program crashes with an exception
when concurrent counts or collects are triggered on the same data set, and
this also happens non deterministically depending on how threads are
executed. So that should be documented somewhere.

Just my two cents

Thanks,

Juan

On Mon, Apr 29, 2019 at 02:02 Fabian Hueske <fh...@apache.org> wrote:

> Hi Juan,
>
> count() and collect() trigger the execution of a job.
> Since Flink does not cache intermediate results (yet), all operations from
> the sink (count()/collect()) to the sources are executed.
> So in a sense a DataSet is immutable (given that the input of the sources
> do not change) but completely recomputed for every execution.
>
> There are currently some efforts [1] on the way to improve Flink behavior
> for interactive sessions.
>
> Best, Fabian
>
> [1]
> https://cwiki.apache.org/confluence/display/FLINK/FLIP-36%3A+Support+Interactive+Programming+in+Flink
> [2]
> https://lists.apache.org/thread.html/5f4961f1dfe23204631fd6f2b3227724ce9831f462737f51742a52c1@%3Cdev.flink.apache.org%3E
>
> Am Fr., 26. Apr. 2019 um 17:03 Uhr schrieb Juan Rodríguez Hortalá <
> juan.rodriguez.hortala@gmail.com>:
>
>> Hi Timo,
>>
>> Thanks for your answer. I was surprised to have problems calling those
>> methods concurrently, because I though data sets were immutable. Now I
>> understand calling count or collect mutates the data set, not its contents
>> but some kind of execution plan included in the data set.
>>
>> I suggest adding a remark about this lack of thread safety to the
>> documentation. Maybe it’s already there but I haven’t seen it. I also
>> understand repeated calls to collect and count the safe data set are ok as
>> long as they are done sequentially, and not concurrently.
>>
>> Thanks,
>>
>> Juan
>>
>> On Fri, Apr 26, 2019 at 02:00 Timo Walther <tw...@apache.org> wrote:
>>
>>> Hi Juan,
>>>
>>> as far as I know we do not provide any concurrency guarantees for
>>> count() or collect(). Those methods need to be used with caution anyways as
>>> the result size must not exceed a certain threshold. I will loop in Fabian
>>> who might know more about the internals of the execution?
>>>
>>> Regards,
>>> Timo
>>>
>>>
>>> Am 26.04.19 um 03:13 schrieb Juan Rodríguez Hortalá:
>>>
>>> Any thoughts on this?
>>>
>>> On Sun, Apr 7, 2019, 6:56 PM Juan Rodríguez Hortalá <
>>> juan.rodriguez.hortala@gmail.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> I have a very simple program using the local execution environment,
>>>> that throws NPE and other exceptions related to concurrent access when
>>>> launching a count for a DataSet from different threads. The program is
>>>> https://gist.github.com/juanrh/685a89039e866c1067a6efbfc22c753e which
>>>> is basically this:
>>>>
>>>> def doubleCollectConcurrent = {
>>>>   val env = ExecutionEnvironment.createLocalEnvironment(3)
>>>>   val xs = env.fromCollection(1 to 100).map{_+1}
>>>>   implicit val ec = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(10))
>>>>
>>>>   val pendingActions = Seq.fill(10)(
>>>>     Future { println(s"xs.count = ${xs.count}") }
>>>>   )
>>>>   val pendingActionsFinished = Future.fold(pendingActions)(Unit){ (u1, u2) =>
>>>>     println("pending action finished")
>>>>     Unit  }
>>>>   Await.result(pendingActionsFinished, 10 seconds)
>>>>
>>>>   ok}
>>>>
>>>>
>>>> It looks like the issue is on OperatorTranslation.java at
>>>> https://github.com/apache/flink/blob/master/flink-java/src/main/java/org/apache/flink/api/java/operators/OperatorTranslation.java#L51,
>>>> when a sink is added to the sinks list while that list is being traversed.
>>>> I have the impression that this is by design, so I'd like to confirm that
>>>> this is the expected behaviour, and whether this is happening only for the
>>>> local execution environment, or if this affects all execution environments
>>>> implementations. Other related questions I have are:
>>>>
>>>>    - Is this documented somewhere? I'm quite new to Flink, so I might
>>>>    have missed this. Is there any known workaround for concurrently launching
>>>>    counts and other sink computations on the same DataSet?
>>>>    - Is it safe performing a sequence of calls to DataSet sink methods
>>>>    like count or collect, on the same DataSet, as long as they are performed
>>>>    from the same thread? From my experience it looks like it is, but I'd like
>>>>    to get a confirmation if possible.
>>>>
>>>> This might be related to
>>>> https://stackoverflow.com/questions/51035465/concurrentmodificationexception-in-flink
>>>> but I'm not sure.
>>>>
>>>> Thanks a lot for your help.
>>>>
>>>> Greetings,
>>>>
>>>> Juan
>>>>
>>>
>>>