Re: Apache Flink CEP how to detect if event did not occur within x seconds?

[fudian.fd]

Hi Florin,

Are you using processing time or event time? The JIRA FLINK-7384 allows to emit 
timed-out patterns without having to wait for the next element ONLY in 
processing time. For event time, it still needs the watermark to trigger the 
emitting of matched or timed-out patterns. Besides, the watermark is global for 
the whole job, not per key. In your application, SNMP will not send anymore 
data for one device when it's disconnected. But if there are many devices to be 
monitored, the watermark will still progress as long as there are some devices 
connected. If it's the case that there are no elements at all, the mentioned 
assigner will look like the following:

class PeriodicExtractor implements AssignerWithPeriodicWatermarks<Long> {

   private volatile long maxTimestamp = Long.MIN_VALUE;

   private static final long MAX_IDLE = 5 * 60 * 1000; // 5 minutes
   private static final long OFFSET = 30 * 1000;    // 30 seconds
   private volatile long lastTimeReceivedElement = 0L;

   @Override
   public long extractTimestamp(Long element, long previousElementTimestamp) {
      maxTimestamp = Math.max(maxTimestamp, element);
      lastTimeReceivedElement = System.currentTimeMillis();
      return element;
   }

   @Nullable
   @Override
   public Watermark getCurrentWatermark() {
      if (System.currentTimeMillis() - lastTimeReceivedElement >= MAX_IDLE) {
         maxTimestamp = Math.max(maxTimestamp, System.currentTimeMillis() - 
OFFSET);
      }
      return new Watermark(maxTimestamp);
   }
}

Regards,
Dian

> 在 2018年12月13日，上午12:06，Andrey Zagrebin <and...@data-artisans.com> 写道：
> 
> Hi Florin,
> 
> I think Dawid might help you. I am pulling him into the discussion.
> 
> Best,
> Andrey
> 
>> On 12 Dec 2018, at 16:24, Spico Florin <spicoflo...@gmail.com 
>> <mailto:spicoflo...@gmail.com>> wrote:
>> 
>> Hello!
>> I'm using the same questions as in this stackoverflow post 
>> https://stackoverflow.com/questions/50356981/apache-flink-cep-how-to-detect-if-event-did-not-occur-within-x-seconds?rq=1
>>  
>> <https://stackoverflow.com/questions/50356981/apache-flink-cep-how-to-detect-if-event-did-not-occur-within-x-seconds?rq=1>,
>>  due to fact that I need the same functionality.
>> 
>> My use case is to detect when an device is disconnected from a server (for 
>> example).
>> From SNMP I'm sending the data to Kafka then  to Flink CEP and check whether 
>> I have the established connection within X seconds.
>> If the window of X seconds timedout, then I can consider that the device is 
>> disconnected.
>> The issue is that, when the device is disconnected, SNMP does't send anymore 
>> data about that port that was used. Thus no event is sent to CEP. Only when 
>> the device is connected again then, the disconnected event is triggered.
>> 
>> The same functionality was requested in this post too: 
>> https://stackoverflow.com/questions/51399973/flink-time-window-based-on-event-time-output-nothing
>>  
>> <https://stackoverflow.com/questions/51399973/flink-time-window-based-on-event-time-output-nothing>.
>> 
>> I checked the status of the jira ticket regarding this feature:
>> 
>> https://issues.apache.org/jira/browse/FLINK-7384 
>> <https://issues.apache.org/jira/browse/FLINK-7384>
>> 
>> but unfortunately, is still open.
>> 
>> In the official documentation, 
>> https://ci.apache.org/projects/flink/flink-docs-stable/dev/event_time.html 
>> <https://ci.apache.org/projects/flink/flink-docs-stable/dev/event_time.html>,
>>  in the paragraph related with Idling sources, somehow the situation 
>> described resembles what we need:
>> "Currently, with pure event time watermarks generators, watermarks can not 
>> progress if there are no elements to be processed. That means in case of gap 
>> in the 
>> incoming data, event time will not progress and for example the window 
>> operator will not be triggered and thus existing windows will not be able to 
>> produce any output data.
>> To circumvent this one can use periodic watermark assigners that don’t only 
>> assign based on element timestamps. An example solution could be 
>> an assigner that switches to using current processing time as the time basis 
>> after not observing new events for a while."
>> 
>> Unfortunately, no example of how the mentioned assigner looks like. A full 
>> example will help us to check whether the mentioned solution works or not.
>> 
>> Therefore, can you please share your thoughts about this feature? 
>> Is feasible? If yes,please provide the Flink version that solve it and the 
>> example.
>> Is still a limitation and there is no plan to solve it in near future?
>> 
>> I look forward for your answers.
>> 
>> Best regards,
>> Florin
>> 
>> 
> 

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