ASF GitHub Bot commented on FLINK-3660:

Github user rmetzger commented on the issue:

    I had an offline discussion with @aljoscha about this PR, and we decided to 
remove the `isSink` flag from the `AbstractStreamOperator`.
    I updated the PR.

> 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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