Jamie Grier created FLINK-10886:
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Summary: Event time synchronization across sources
Key: FLINK-10886
URL: https://issues.apache.org/jira/browse/FLINK-10886
Project: Flink
Issue Type: Improvement
Components: Streaming Connectors
Reporter: Jamie Grier
Assignee: Jamie Grier
When reading from a source with many parallel partitions, especially when
reading lots of historical data (or recovering from downtime and there is a
backlog to read), it's quite common for there to develop an event-time skew
across those partitions.
When doing event-time windowing -- or in fact any event-time driven processing
-- the event time skew across partitions results directly in increased
buffering in Flink and of course the corresponding state/checkpoint size growth.
As the event-time skew and state size grows larger this can have a major effect
on application performance and in some cases result in a "death spiral" where
the application performance get's worse and worse as the state size grows and
grows.
So, one solution to this problem, outside of core changes in Flink itself,
seems to be to try to coordinate sources across partitions so that they make
progress through event time at roughly the same rate. In fact if there is
large skew the idea would be to slow or even stop reading from some partitions
with newer data while first reading the partitions with older data. Anyway, to
do this we need to share state somehow amongst sub-tasks.
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