Aljoscha Krettek commented on FLINK-4576:

first off, sorry for the late response, I was on vacation until this Tuesday.

I think you analysed the problems correctly:
 1. Should both sources and intermediate operators be able to interact with 
this new service?
 2. How to get the watermark from operators and how to give it to operators?

I think you already have the answer to 1.: yes. Because we allow users to 
assign watermarks in the middle of a pipeline and even provide "sources" 
ourselves that do this (Kinesis, ContinuousFileSource).

This leads naturally to an answer for 2.: I think we need to introduce an 
interface like this:
interface WatermarkOperator {
   Watermark currentWatermark();
   void notifyOfCurrentGlobalWatermark(Watermark watermark);

sources would implement this interface and any operators that we have 
in-between would also implement it. This then means, of course, that basically 
all stream tasks have to be able to handle watermarks and always ask all the 
operators in the chain and update them if they implement the interface.

[~StephanEwen] do you maybe also have some thoughts on this?

> 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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