[jira] [Commented] (FLINK-3660) Measure latency of elements and expose it through web interface

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-3660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15473681#comment-15473681 ] 

ASF GitHub Bot commented on FLINK-3660:
---------------------------------------

Github user rmetzger commented on the issue:

    https://github.com/apache/flink/pull/2386
  
    @aljoscha I updated the pull request, which now builds green (just rebasing fixed the issues).
    I also keep metrics now per operator, not only for the sinks.


> Measure latency of elements and expose it through web interface
> ---------------------------------------------------------------
>
>                 Key: FLINK-3660
>                 URL: https://issues.apache.org/jira/browse/FLINK-3660
>             Project: Flink
>          Issue Type: Sub-task
>          Components: Streaming
>            Reporter: Robert Metzger
>            Assignee: Robert Metzger
>             Fix For: pre-apache
>
>
> It would be nice to expose the end-to-end latency of a streaming job in the webinterface.
> To achieve this, my initial thought was to attach an ingestion-time timestamp at the sources to each record.
> However, this introduces overhead for a monitoring feature users might not even use (8 bytes for each element + System.currentTimeMilis() on each element).
> Therefore, I suggest to implement this feature by periodically sending special events, similar to watermarks through the topology. 
> Those {{LatencyMarks}} are emitted at a configurable interval at the sources and forwarded by the tasks. The sinks will compare the timestamp of the latency marks with their current system time to determine the latency.
> The latency marks will not add to the latency of a job, but the marks will be delayed similarly than regular records, so their latency will approximate the record latency.
> Above suggestion expects the clocks on all taskmanagers to be in sync. Otherwise, the measured latencies would also include the offsets between the taskmanager's clocks.
> In a second step, we can try to mitigate the issue by using the JobManager as a central timing service. The TaskManagers will periodically query the JM for the current time in order to determine the offset with their clock.
> This offset would still include the network latency between TM and JM but it would still lead to reasonably good estimations.



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