A StreamStatus element informs receiving operators whether or not they should continue to expect watermarks from the sending operator. There are 2 kinds of status, namely IDLE and ACTIVE. Watermark status elements are generated at the sources, and may be propagated through the operators of the topology using Output#emitWatermarkStatus(WatermarkStatus).
Sources and downstream operators should emit either of the status elements once it changes between "watermark-idle" and "watermark-active" states.
A source is considered "watermark-idle" if it will not emit records for an indefinite amount of time. This is the case, for example, for Flink's Kafka Consumer, where sources might initially have no assigned partitions to read from, or no records can be read from the assigned partitions. Once the source detects that it will resume emitting data, it is considered "watermark-active".
Downstream operators with multiple inputs (ex. head operators of a OneInputStreamTask or TwoInputStreamTask) should not wait for watermarks from an upstream operator that is "watermark-idle" when deciding whether or not to advance the operator's current watermark. When a downstream operator determines that all upstream operators are "watermark-idle" (i.e. when all input channels have received the watermark idle status element), then the operator is considered to also be "watermark-idle", as it will temporarily be unable to advance its own watermark. This is always the case for operators that only read from a single upstream operator. Once an operator is considered "watermark-idle", it should itself forward its idle status to inform downstream operators. The operator is considered to be back to "watermark-active" as soon as at least one of its upstream operators resume to be "watermark-active" (i.e. when at least one input channel receives the watermark active status element), and should also forward its active status to inform downstream operators.