Hi, I have a job like 'Source -> assignmentTimestamp -> flatmap -> Window -> Sink' and I get back pressure from 'Source' to the 'FlatMap' operators form the 'BackPressure' tab in the Web UI. When trying to find which operator is the source of back pressure, I use metrics provided by the Web UI, specifically, 'inPoolUsage' and 'outPoolUsage'. Firstly, As far as I know, when both of the metrics are 0, the operator should not be defined as 'back pressured', but when I check the 'AssignmentTimestamp' operator, where 8 subtasks running, I find 1 or 2 of them have 0 value about the back pressure index, and the others have the index higher than 0.80, and all of them are marked in 'HIGH' status. However, the two metrics, 'in/outPoolUsage', are always be 0. So maybe the operator is not back pressured actually? Or is there any problem with my Flink WebUI? Second question is, from my experience, I think the source of the back pressure should be the Window operator because the outPoolUsage of the 'FlatMap' are 1, and the 'Window' is the first downstream operator from the 'Flatmap', but the inPoolUsage and the outPoolUsage are also 0. So the cause of the back pressure should be the network bottleneck between window and flatmap? Am I right? Thanks for your reading, and I'm looking forward for your ideas.
Haocheng
