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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: Custom TimestampExtractor and FlinkKafkaConsumer082
Date Mon, 30 Nov 2015 16:29:08 GMT
Hi,
as an addition. I don’t have a solution yet, for the general problem of what happens when
a parallel instance of a source never receives elements. This watermark business is very tricky...

Cheers,
Aljoscha
> On 30 Nov 2015, at 17:20, Aljoscha Krettek <aljoscha@apache.org> wrote:
> 
> Hi Konstantin,
> I finally nailed down the problem. :-)
> 
> The basis of the problem is the fact that there is a mismatch in the parallelism of the
Flink Kafka Consumer and the number of partitions in the Kafka Stream. I would assume that
in your case the Kafka Stream has 1 partition. This means, that only one of the parallel instances
of the Flink Kafka Consumer ever receives element, which in turn means that only one of the
parallel instances of the timestamp extractor ever receives elements. This means that no watermarks
get emitted for the other parallel instances which in turn means that the watermark does not
advance downstream because the watermark at an operator is the minimum over all upstream watermarks.
This explains why ExampleTimestampExtractor1 only works in the case with parallelism=1. 
> 
> The reason why ExampleTimestampExtractor2 works in all parallelism settings is not very
obvious. The secret is in this method:
> 
> @Override
> public long getCurrentWatermark() {
>   return lastTimestamp - maxDelay;
> }
> 
> In the parallel instances that never receive any element lastTimestamp is set to Long.MIN_VALUE.
So “lastTimestamp - maxDelay” is (Long.MAX_VALUE - maxDelay (+1)). Now, because the watermark
at an operator is always the minimum over all watermarks from upstream operators the watermark
at the window operator always tracks the watermark of the parallel instance that receives
elements. 
> 
> I hope this helps, but please let me know if I should provide more explanation. This
is a very tricky topic.
> 
> Cheers,
> Aljoscha
> 
>> On 29 Nov 2015, at 21:18, Konstantin Knauf <konstantin.knauf@tngtech.com> wrote:
>> 
>> Hi Aljoscha,
>> 
>> I have put together a gist [1] with two classes, a short processing
>> pipeline, which shows the behavior and a data generator to write records
>> into Kafka. I hope I remembered everything we discussed correctly.
>> 
>> So basically in the example it works with "TimestampExtractor1" only for
>> parallelism 1, with "TimestampExtractor2" it works regardless of the
>> parallelism. Run from the IDE.
>> 
>> Let me know if you need anything else.
>> 
>> Cheers,
>> 
>> Konstantin
>> 
>> [1] https://gist.github.com/knaufk/d57b5c3c7db576f3350d
>> 
>> On 25.11.2015 21:15, Konstantin Knauf wrote:
>>> Hi Aljoscha,
>>> 
>>> sure, will do. I have neither found a solution. I won't have time to put
>>> a minimal example together before the weekend though.
>>> 
>>> Cheers,
>>> 
>>> Konstantin
>>> 
>>> On 25.11.2015 19:10, Aljoscha Krettek wrote:
>>>> Hi Konstantin,
>>>> I still didn’t come up with an explanation for the behavior. Could you
maybe send me example code (and example data if it is necessary to reproduce the problem.)?
This would really help me pinpoint the problem.
>>>> 
>>>> Cheers,
>>>> Aljoscha
>>>>> On 17 Nov 2015, at 21:42, Konstantin Knauf <konstantin.knauf@tngtech.com>
wrote:
>>>>> 
>>>>> Hi Aljoscha,
>>>>> 
>>>>> Are you sure? I am running the job from my IDE at the moment.
>>>>> 
>>>>> If I set
>>>>> 
>>>>> StreamExecutionEnvironment.setParallelism(1);
>>>>> 
>>>>> I works with the old TimestampExtractor (returning Long.MIN_VALUE from
>>>>> getCurrentWatermark() and emitting a watermark at every record)
>>>>> 
>>>>> If I set
>>>>> 
>>>>> StreamExecutionEnvironment.setParallelism(5);
>>>>> 
>>>>> it does not work.
>>>>> 
>>>>> So, if I understood you correctly, it is the opposite of what you were
>>>>> expecting?!
>>>>> 
>>>>> Cheers,
>>>>> 
>>>>> Konstantin
>>>>> 
>>>>> 
>>>>> On 17.11.2015 11:32, Aljoscha Krettek wrote:
>>>>>> Hi,
>>>>>> actually, the bug is more subtle. Normally, it is not a problem that
the TimestampExtractor sometimes emits a watermark that is lower than the one before. (This
is the result of the bug with Long.MIN_VALUE I mentioned before). The stream operators wait
for watermarks from all upstream operators and only advance the watermark monotonically in
lockstep with them. This way, the watermark cannot decrease at an operator.
>>>>>> 
>>>>>> In your case, you have a topology with parallelism 1, I assume. In
that case the operators are chained. (There is no separate operators but basically only one
operator and element transmission happens in function calls). In this setting the watermarks
are directly forwarded to operators without going through the logic I mentioned above.
>>>>>> 
>>>>>> Cheers,
>>>>>> Aljoscha
>>>>>>> On 16 Nov 2015, at 18:13, Konstantin Knauf <konstantin.knauf@tngtech.com>
wrote:
>>>>>>> 
>>>>>>> Hi Aljoscha,
>>>>>>> 
>>>>>>> I changed the Timestamp Extraktor to save the lastSeenTimestamp
and only
>>>>>>> emit with getCurrentWatermark [1] as you suggested. So basically
I do
>>>>>>> the opposite than before (only watermarks per events vs only
watermarks
>>>>>>> per autowatermark). And now it works :). The question remains,
why it
>>>>>>> did not work before. As far as I see, it is an issue with the
first
>>>>>>> TimestmapExtractor itself?!
>>>>>>> 
>>>>>>> Does getCurrentWatermark(..) somehow "overpower" the extracted
watermarks?
>>>>>>> 
>>>>>>> Cheers,
>>>>>>> 
>>>>>>> Konstantin
>>>>>>> 
>>>>>>> [1]
>>>>>>> 
>>>>>>> final private long maxDelay;
>>>>>>> private long lastTimestamp = Long.MIN_VALUE;
>>>>>>> 
>>>>>>> public PojoTimestampExtractor(long maxDelay) {
>>>>>>>     this.maxDelay = maxDelay;
>>>>>>> }
>>>>>>> 
>>>>>>> @Override
>>>>>>> public long extractTimestamp(Pojo pojo, long l) {
>>>>>>>     lastTimestamp = pojo.getTime();
>>>>>>>     return pojo.getTime();
>>>>>>> }
>>>>>>> 
>>>>>>> @Override
>>>>>>> public long extractWatermark(Pojo pojo, long l) {
>>>>>>>     return Long.MIN_VALUE;
>>>>>>> }
>>>>>>> 
>>>>>>> @Override
>>>>>>> public long getCurrentWatermark() {
>>>>>>>     return lastTimestamp - maxDelay;
>>>>>>> }
>>>>>>> 
>>>>>>> 
>>>>>>> On 16.11.2015 13:37, Aljoscha Krettek wrote:
>>>>>>>> Hi,
>>>>>>>> yes, at your data-rate emitting a watermark for every element
should not be a problem. It could become a problem with higher data-rates since the system
can get overwhelmed if every element also generates a watermark. In that case I would suggest
storing the lastest element-timestamp in an internal field and only emitting in getCurrentWatermark(),
since then, then the watermark interval can be tunes using the auto-watermark interval setting.
>>>>>>>> 
>>>>>>>> But that should not be the cause of the problem that you
currently have. Would you maybe be willing to send me some (mock) example data and the code
so that I can reproduce the problem and have a look at it? to aljoscha at apache.org.
>>>>>>>> 
>>>>>>>> Cheers,
>>>>>>>> Aljoscha
>>>>>>>>> On 16 Nov 2015, at 13:05, Konstantin Knauf <konstantin.knauf@tngtech.com>
wrote:
>>>>>>>>> 
>>>>>>>>> Hi Aljoscha,
>>>>>>>>> 
>>>>>>>>> ok, now I at least understand, why it works with fromElements(...).
For
>>>>>>>>> the rest I am not so sure.
>>>>>>>>> 
>>>>>>>>>> What this means in your case is that the watermark
can only advance if
>>>>>>>>> a new element arrives, because only then is the watermark
updated.
>>>>>>>>> 
>>>>>>>>> But new elements arrive all the time, about 50/s, or
do you mean
>>>>>>>>> something else?
>>>>>>>>> 
>>>>>>>>> getCurrentWatermark returning Long.MIN_VALUE still seems
to be an ok
>>>>>>>>> choice, if i understand the semantics correctly. It just
affects
>>>>>>>>> watermarking in the absence of events, right?
>>>>>>>>> 
>>>>>>>>> Cheers,
>>>>>>>>> 
>>>>>>>>> Konstantin
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> On 16.11.2015 12:31, Aljoscha Krettek wrote:
>>>>>>>>>> Hi,
>>>>>>>>>> it could be what Gyula mentioned. Let me first go
a bit into how the TimestampExtractor works internally.
>>>>>>>>>> 
>>>>>>>>>> First, the timestamp extractor internally keeps the
value of the last emitted watermark. Then, the semantics of the TimestampExtractor are as
follows :
>>>>>>>>>> - the result of extractTimestamp is taken and it
replaces the internal timestamp of the element
>>>>>>>>>> - if the result of extractWatermark is larger than
the last watermark the new value is emitted as a watermark and the value is stored
>>>>>>>>>> - getCurrentWatermark is called on the specified
auto-watermark interval, if the returned value is larger than the last watermark it is emitted
and stored as last watermark
>>>>>>>>>> 
>>>>>>>>>> What this means in your case is that the watermark
can only advance if a new element arrives, because only then is the watermark updated.
>>>>>>>>>> 
>>>>>>>>>> The reason why you see results if you use fromElements
is that the window-operator also emits all the windows that it currently has buffered if the
program closes. This happens in the case of fromElements because only a finite number of elements
is emitted, after which the source closes, thereby finishing the whole program.
>>>>>>>>>> 
>>>>>>>>>> Cheers,
>>>>>>>>>> Aljoscha
>>>>>>>>>>> On 16 Nov 2015, at 10:42, Gyula Fóra <gyula.fora@gmail.com>
wrote:
>>>>>>>>>>> 
>>>>>>>>>>> Could this part of the extractor be the problem
Aljoscha?
>>>>>>>>>>> 
>>>>>>>>>>> @Override
>>>>>>>>>>> public long getCurrentWatermark() {
>>>>>>>>>>>    return Long.MIN_VALUE;
>>>>>>>>>>> }
>>>>>>>>>>> 
>>>>>>>>>>> Gyula
>>>>>>>>>>> 
>>>>>>>>>>> Konstantin Knauf <konstantin.knauf@tngtech.com>
ezt írta (időpont: 2015. nov. 16., H, 10:39):
>>>>>>>>>>> Hi Aljoscha,
>>>>>>>>>>> 
>>>>>>>>>>> thanks for your answer. Yes I am using the same
TimestampExtractor-Class.
>>>>>>>>>>> 
>>>>>>>>>>> The timestamps look good to me. Here an example.
>>>>>>>>>>> 
>>>>>>>>>>> {"time": 1447666537260, ...} And parsed: 2015-11-16T10:35:37.260+01:00
>>>>>>>>>>> 
>>>>>>>>>>> The order now is
>>>>>>>>>>> 
>>>>>>>>>>> stream
>>>>>>>>>>> .map(dummyMapper)
>>>>>>>>>>> .assignTimestamps(...)
>>>>>>>>>>> .timeWindow(...)
>>>>>>>>>>> 
>>>>>>>>>>> Is there a way to print out the assigned timestamps
after
>>>>>>>>>>> stream.assignTimestamps(...)?
>>>>>>>>>>> 
>>>>>>>>>>> Cheers,
>>>>>>>>>>> 
>>>>>>>>>>> Konstantin
>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> On 16.11.2015 10:31, Aljoscha Krettek wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>> are you also using the timestamp extractor
when you are using env.fromCollection().
>>>>>>>>>>>> 
>>>>>>>>>>>> Could you maybe insert a dummy mapper after
the Kafka source that just prints the element and forwards it? To see if the elements come
with a good timestamp from Kafka.
>>>>>>>>>>>> 
>>>>>>>>>>>> Cheers,
>>>>>>>>>>>> Aljoscha
>>>>>>>>>>>>> On 15 Nov 2015, at 22:55, Konstantin
Knauf <konstantin.knauf@tngtech.com> wrote:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>>> 
>>>>>>>>>>>>> I have the following issue with Flink
(0.10) and Kafka.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> I am using a very simple TimestampExtractor
like [1], which just
>>>>>>>>>>>>> extracts a millis timestamp from a POJO.
In my streaming job, I read in
>>>>>>>>>>>>> these POJOs from Kafka using the FlinkKafkaConsumer082
like this:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> stream = env.addSource(new FlinkKafkaConsumer082<
>>>>>>>>>>>>> (parameterTool.getRequired("topic"),
>>>>>>>>>>>>>           new AvroPojoDeserializationSchema(),
>>>>>>>>>>>>> parameterTool.getProperties()))
>>>>>>>>>>>>> 
>>>>>>>>>>>>> I have timestampEnabled() and the TimeCharacteristics
are EventTime,
>>>>>>>>>>>>> AutoWatermarkIntervall is 500.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> The problem is, when I do something like:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> stream.assignTimestamps(new PojoTimestampExtractor(6000))
>>>>>>>>>>>>> .timeWindowAll(Time.of(1, TimeUnit.SECONDS)
>>>>>>>>>>>>> .sum(..)
>>>>>>>>>>>>> .print()
>>>>>>>>>>>>> 
>>>>>>>>>>>>> env.execute();
>>>>>>>>>>>>> 
>>>>>>>>>>>>> the windows never get triggered.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> If I use ProcessingTime it works.
>>>>>>>>>>>>> If I use env.fromCollection(...) instead
of the KafkaSource it works
>>>>>>>>>>>>> with EventTime, too.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> Any ideas what I could be doing wrong
are highly appreciated.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> Cheers,
>>>>>>>>>>>>> 
>>>>>>>>>>>>> Konstantin
>>>>>>>>>>>>> 
>>>>>>>>>>>>> [1]:
>>>>>>>>>>>>> 
>>>>>>>>>>>>> public class PojoTimestampExtractor implements
TimestampExtractor<Pojo> {
>>>>>>>>>>>>> 
>>>>>>>>>>>>> final private long maxDelay;
>>>>>>>>>>>>> 
>>>>>>>>>>>>> public  PojoTimestampExtractor(long maxDelay)
{
>>>>>>>>>>>>>   this.maxDelay = maxDelay;
>>>>>>>>>>>>> }
>>>>>>>>>>>>> 
>>>>>>>>>>>>> @Override
>>>>>>>>>>>>> public long extractTimestamp(Pojo fightEvent,
long l) {
>>>>>>>>>>>>>   return pojo.getTime();
>>>>>>>>>>>>> }
>>>>>>>>>>>>> 
>>>>>>>>>>>>> @Override
>>>>>>>>>>>>> public long extractWatermark(Pojo pojo,
long l) {
>>>>>>>>>>>>>   return pojo.getTime() - maxDelay;
>>>>>>>>>>>>> }
>>>>>>>>>>>>> 
>>>>>>>>>>>>> @Override
>>>>>>>>>>>>> public long getCurrentWatermark() {
>>>>>>>>>>>>>   return Long.MIN_VALUE;
>>>>>>>>>>>>> }
>>>>>>>>>>>> 
>>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> --
>>>>>>>>>>> Konstantin Knauf * konstantin.knauf@tngtech.com
* +49-174-3413182
>>>>>>>>>>> TNG Technology Consulting GmbH, Betastr. 13a,
85774 Unterföhring
>>>>>>>>>>> Geschäftsführer: Henrik Klagges, Christoph
Stock, Dr. Robert Dahlke
>>>>>>>>>>> Sitz: Unterföhring * Amtsgericht München *
HRB 135082
>>>>>>>>>> 
>>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> -- 
>>>>>>>>> Konstantin Knauf * konstantin.knauf@tngtech.com * +49-174-3413182
>>>>>>>>> TNG Technology Consulting GmbH, Betastr. 13a, 85774 Unterföhring
>>>>>>>>> Geschäftsführer: Henrik Klagges, Christoph Stock, Dr.
Robert Dahlke
>>>>>>>>> Sitz: Unterföhring * Amtsgericht München * HRB 135082
>>>>>>>> 
>>>>>>>> 
>>>>>>> 
>>>>>>> -- 
>>>>>>> Konstantin Knauf * konstantin.knauf@tngtech.com * +49-174-3413182
>>>>>>> TNG Technology Consulting GmbH, Betastr. 13a, 85774 Unterföhring
>>>>>>> Geschäftsführer: Henrik Klagges, Christoph Stock, Dr. Robert
Dahlke
>>>>>>> Sitz: Unterföhring * Amtsgericht München * HRB 135082
>>>>>> 
>>>>>> 
>>>>> 
>>>>> -- 
>>>>> Konstantin Knauf * konstantin.knauf@tngtech.com * +49-174-3413182
>>>>> TNG Technology Consulting GmbH, Betastr. 13a, 85774 Unterföhring
>>>>> Geschäftsführer: Henrik Klagges, Christoph Stock, Dr. Robert Dahlke
>>>>> Sitz: Unterföhring * Amtsgericht München * HRB 135082
>>>> 
>>>> 
>>> 
>> 
>> -- 
>> Konstantin Knauf * konstantin.knauf@tngtech.com * +49-174-3413182
>> TNG Technology Consulting GmbH, Betastr. 13a, 85774 Unterföhring
>> Geschäftsführer: Henrik Klagges, Christoph Stock, Dr. Robert Dahlke
>> Sitz: Unterföhring * Amtsgericht München * HRB 135082
> 


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