Github user koeninger commented on a diff in the pull request:
https://github.com/apache/spark/pull/15702#discussion_r86066376
--- Diff: sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala ---
@@ -536,6 +535,41 @@ class Dataset[T] private[sql](
}
/**
+ * :: Experimental ::
+ * Defines an event time watermark for this [[Dataset]]. A watermark
tracks a point in time
+ * before which we assume no more late data is going to arrive.
+ *
+ * Spark will use this watermark for several purposes:
+ * - To know when a given time window aggregation can be finalized and
thus can be emitted when
+ * using output modes that do not allow updates.
+ * - To minimize the amount of state that we need to keep for on-going
aggregations.
--- End diff --
For append, this sounds like the intention is emit only once watermark has
passed, and drop state.
But for other output modes, it's not clear from reading this what the
effect of the watermark on emission and dropping state is.
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