Apache beam windowing: consider late data but emit only one pane

Question

I would like to emit a single pane when the watermark reaches x minutes past the end of the window. This let's me ensure I handle some late data, but still only emit one pane. I am currently working in java.

At the moment I can't find proper solutions to this problem. I could emit a single pane when the watermark reaches the end of the window, but then any late data is dropped. I could emit the pane at the end of the window and then again when I receive late data, however in this case I am not emitting a single pane.

I currently have code similar to this:

.triggering(
    // This is going to emit the pane, but I don't want emit the pane yet!                                  
    AfterWatermark.pastEndOfWindow()

    // This is going to emit panes each time I receive late data, however 
    // I would like to only emit one pane at the end of the allowedLateness
).withAllowedLateness(allowedLateness).accumulatingFiredPanes())

In case there is still confusion, I would like to only emit a single pane when the watermark passes the allowedLateness .

Answer 1

Thanks Guillem, in the end I used your answer to find this very useful link with lots of apache beam examples. From this I came up with the following solution:

 // We first specify to never emit any panes
 .triggering(Never.ever())

 // We then specify to fire always when closing the window. This will emit a
 // single final pane at the end of allowedLateness
 .withAllowedLateness(allowedLateness, Window.ClosingBehavior.FIRE_ALWAYS)
 .discardingFiredPanes())

Answer 2

What I would do is, first, to set Window.ClosingBehavior to FIRE_ALWAYS . This way, when the window is permanently closed it will send a final pane (even if there are no late records since the last pane) with PaneInfo.isLast set to true .

Then, I would proceed with the second option:

I could emit the pane at the end of the window and then again when I receive late data, however in this case I am not emitting a single pane.

But discarding downstream the panes that are not final with something like:

public void processElement(ProcessContext c) {
    if (c.pane().isLast) {
        c.output(c.element());
    }
}