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https://issues.apache.org/jira/browse/FLINK-3660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15295688#comment-15295688
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Stephan Ewen commented on FLINK-3660:
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I would break this into two tasks:
- measure latency and exposes them via a metric
- visualize that metric in the Web Dashboard
> 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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