HeartSaVioR commented on code in PR #39931:
URL: https://github.com/apache/spark/pull/39931#discussion_r1122424081
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sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/statefulOperators.scala:
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@@ -96,6 +98,25 @@ trait StateStoreReader extends StatefulOperator {
/** An operator that writes to a StateStore. */
trait StateStoreWriter extends StatefulOperator with PythonSQLMetrics { self:
SparkPlan =>
+ /**
+ * Produce the output watermark for given input watermark (ms).
+ *
+ * In most cases, this is same as the criteria of state eviction, as most
stateful operators
+ * produce the output from two different kinds:
+ *
+ * 1. without buffering
+ * 2. with buffering (state)
+ *
+ * The state eviction happens when event time exceeds a "certain threshold
of timestamp", which
+ * denotes a lower bound of event time values for output (output watermark).
+ *
+ * The default implementation provides the input watermark as it is. Most
built-in operators
+ * will evict based on min input watermark and ensure it will be minimum of
the event time value
+ * for the output so far (including output from eviction). Operators which
behave differently
+ * (e.g. different criteria on eviction) must override this method.
+ */
+ def produceOutputWatermark(inputWatermarkMs: Long): Option[Long] =
Some(inputWatermarkMs)
Review Comment:
OK. I'll leave a context as a code comment. I agree this is missing and
leads some confusion.
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