Flink keyed windows watermark

Question

I'm using flink with event time keyed windows. It seems like some of the windows are not being emitted.
Is the watermark being advanced for each key individually?
For example, if my key is (id,type), and a specific pair of id and type are not being ingested to the source, will their specific window watermark will not advance? If this is the case, how can i make sure that all my keyd windows will get evicted after some time? (we have many keys so sending a periodic dummy message for each key is not an option).

I'll appreciate any help

Answer 1

Flink has separate watermarks for each task (i.e., each parallel instance) -- otherwise there would have to some sort of horribly expensive global coordination -- but not for each key. In the case of a keyed window, each instance of the window operator will be handling the events for some disjoint subset of the keyspace, and all of the windows for those keys will be using the same watermark.

Keep in mind that empty windows do not produce results. So if there is some key for which there are no events during a window, that window will not produce results for that key.

Or it could be that you have an idle source holding back the watermarks. If one of your source tasks becomes idle, then its watermark won't advance. You could inspect the current watermark in the web UI, and check to see if it is advancing in every task.