Tzu-Li (Gordon) Tai created FLINK-4576:
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Summary: 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
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 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 the 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)}}
New internal task interface {{LowWatermarkCooperatingTask}} in flink-runtime
New public interface {{LowWatermarkListener}} in flink-streaming-java
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