Github user marmbrus commented on a diff in the pull request:
https://github.com/apache/spark/pull/15702#discussion_r86203412
--- 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.
+ *
+ * The current event time is computed by looking at the
`MAX(eventTime)` seen in an epoch across
+ * all of the partitions in the query minus a user specified
`delayThreshold`. Due to the cost
+ * of coordinating this value across partitions, the actual watermark
used is only guaranteed
+ * to be at least `delayThreshold` behind the actual event time. In
some cases we may still
+ * process records that arrive more than `delayThreshold` late.
+ *
+ * @param eventTime the name of the column that contains the event time
of the row.
+ * @param delayThreshold the minimum delay to wait to data to arrive
late, relative to the latest
+ * record that has been processed in the form of
an interval
+ * (e.g. "1 minute" or "5 hours").
--- End diff --
That seems like more of an implementation detail, rather than a contract of
the API. The real contract is stated above as `the actual watermark used is
only guaranteed to be at least 'delayThreshold' behind the actual event time`.
There aren't really any bounds we can promise without knowing more about the
query (even ms).
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