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https://issues.apache.org/jira/browse/FLINK-4576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15535824#comment-15535824
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Tzu-Li (Gordon) Tai edited comment on FLINK-4576 at 9/30/16 2:57 PM:
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Hi [~aljoscha],
I'm struggling a bit on implementation choice of recording and accessing the
last emitted watermark of each source operator.
When a {{SourceStreamTask}} receives a request to return its last emitted
watermark to JobManager, it accesses the last emitted watermark of the head
operator (a {{StreamSource}} operator) by calling
{{headOperator.getLastEmittedWatermark()}}.
I'm currently jumping around two possible ways to achieve this:
1. Record the last emitted watermark within {{SourceFunction.SourceContext}} s,
and expose it through a getter method.
{{headOperator.getLastEmittedWatermark()}} simply uses this getter method on
the source context.
2. Extend the {{o.a.f.streaming.api.operators.Output}} interface to have a new
{{getLastEmittedWatermark}} method, and let
{{headOperator.getLastEmittedWatermark()}} call that instead.
I'm not sure which would be a better approach?
Another problem I'd like to confirm is that the low watermark service should
only be relevant to watermarks emitted from {{StreamSource}} operators, and not
the watermarks emitted at the end of the whole operator chain of a
{{SourceStreamTask}}, correct?
I'm thinking about this because I'm a bit unsure with how user-supplied
watermark assigners should work with the JM low watermark service, especially
when watermarks are not generated at source subtasks.
I've completed the communication between JM / TM / tasks, so I'm down to this
last bit and can open the PR afterwards. I'm not sure if my questions make
sense as I'm still relatively new to this part of the code, so any feedback
will be helpful :) !
was (Author: tzulitai):
Hi [~aljoscha],
I'm struggling a bit on implementation choice of recording and accessing the
last emitted watermark of each source operator.
When a {{SourceStreamTask}} receives a request to return its last emitted
watermark to JobManager, it accesses the last emitted watermark of the head
operator (a {{StreamSource}} operator) by calling
{{headOperator.getLastEmittedWatermark()}}.
I'm currently jumping around two possible ways to achieve this:
1. Record the last emitted watermark within {{SourceFunction.SourceContext}} s,
and expose it through a getter method.
{{headOperator.getLastEmittedWatermark()}} simply uses this getter method on
the source context.
2. Extend the {{o.a.f.streaming.api.operators.Output}} interface to have a new
{{getLastEmittedWatermark}} method, and let
{{headOperator.getLastEmittedWatermark()}} call that instead.
I'm not sure which would be a better approach?
Another problem I'd like to confirm is that the low watermark service should
only be relevant to watermarks emitted from {{StreamSource}} operators, and not
the watermarks emitted at the end of the whole operator chain of a
{{SourceStreamTask}}, correct?
I'm thinking about this because I'm a bit unsure with how user-supplied
watermark assigners should work with the JM low watermark service.
I've completed the communication between JM / TM / tasks, so I'm down to this
last bit and can open the PR afterwards. I'm not sure if my questions make
sense as I'm still relatively new to this part of the code, so any feedback
will be helpful :) !
> Low Watermark Service in JobManager for Streaming Sources
> ---------------------------------------------------------
>
> Key: FLINK-4576
> URL: https://issues.apache.org/jira/browse/FLINK-4576
> Project: Flink
> Issue Type: New Feature
> Components: JobManager, Streaming, TaskManager
> Reporter: Tzu-Li (Gordon) Tai
> Assignee: Tzu-Li (Gordon) Tai
> Priority: Blocker
> Fix For: 1.2.0
>
>
> As per discussion in FLINK-4341 by [~aljoscha] and [~StephanEwen], we need a
> low watermark service in the JobManager to support transparent resharding /
> partition discovery for our Kafka and Kinesis consumers (and any future
> streaming connectors in general for which the external system may elastically
> scale up and down independently of the parallelism of sources in Flink). The
> main idea is to let source subtasks that don't emit their own watermarks
> (because they currently don't have data partitions to consume) emit the low
> watermark across all subtasks, instead of simply emitting a Long.MAX_VALUE
> watermark and forbidding them to be assigned partitions in the future.
> The proposed implementation, from a high-level: a {{LowWatermarkCoordinator}}
> will be added to execution graphs, periodically triggering only the source
> vertices with a {{RetrieveLowWatermark}} message. The tasks reply to the
> JobManager through the actor gateway (or a new interface after FLINK-4456
> gets merged) with a {{ReplyLowWatermark}} message. When the coordinator
> collects all low watermarks for a particular source vertex and determines the
> aggregated low watermark for this round (accounting only values that are
> larger than the aggregated low watermark of the last round), it sends a
> {{NotifyNewLowWatermark}} message to the source vertex's tasks.
> The messages will only be relevant to tasks that implement an internal
> {{LowWatermarkCooperatingTask}} interface. For now, only {{SourceStreamTask}}
> should implement {{LowWatermarkCooperatingTask}}.
> Source functions should implement a public {{LowWatermarkListener}} interface
> if they wish to get notified of the aggregated low watermarks across
> subtasks. Connectors like the Kinesis consumer can choose to emit this
> watermark if the subtask currently does not have any shards, so that
> downstream operators may still properly advance time windows (implementation
> for this is tracked as a separate issue).
> Overall, the service will include -
> New messages between JobManager <-> TaskManager:
> {{RetrieveLowWatermark(jobId, jobVertexId, taskId, timestamp)}}
> {{ReplyLowWatermark(jobId, jobVertexId, taskId, currentLowWatermark)}}
> {{NotifyNewLowWatermark(jobId, jobVertexId, taskId, newLowWatermark,
> timestamp)}}
> New internal task interface {{LowWatermarkCooperatingTask}} in flink-runtime
> New public interface {{LowWatermarkListener}} in flink-streaming-java
> Might also need to extend {{SourceFunction.SourceContext}} to support
> retrieving the current low watermark of sources.
> Any feedback for this is appreciated!
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