I'm actually not seeing output from TumblingEventTimeWindows even if I set the watermark strategy to Kafka source. Is there a complete example about this, I'm probably missing something basic?

This is usually because the watermark isn’t advancing. Maybe there is no data in some source tasks which makes the watermark isn’t advancing. You can try one of the following solutions: • Set the parallelism of the job to 1 to make sure there is data in the source task. • Configure idle source (https://nightlies.apache.org/flink/flink-docs-master/docs/dev/datastream/event-time/generating_watermarks/#dealing-with-idle-sources):

WatermarkStrategy.for_monotonous_timestamps().with_idleness(Duration.of_seconds(10)).with_timestamp_assigner(SensorTimestampAssigner()

Idleness was the thing, thanks so much!