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From "Robert Metzger (JIRA)" <j...@apache.org>
Subject [jira] [Created] (FLINK-3660) Measure latency of elements and expose it through web interface
Date Wed, 23 Mar 2016 10:18:25 GMT
Robert Metzger created FLINK-3660:

             Summary: 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

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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