Github user zsxwing commented on a diff in the pull request:
https://github.com/apache/spark/pull/14553#discussion_r83953473
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Source.scala
---
@@ -30,16 +30,30 @@ trait Source {
/** Returns the schema of the data from this source */
def schema: StructType
- /** Returns the maximum available offset for this source. */
+ /**
+ * Returns the maximum available offset for this source.
+ * Returns `None` if this source has never received any data.
+ */
def getOffset: Option[Offset]
/**
- * Returns the data that is between the offsets (`start`, `end`]. When
`start` is `None` then
- * the batch should begin with the first available record. This method
must always return the
- * same data for a particular `start` and `end` pair.
+ * Returns the data that is between the offsets (`start`, `end`]. When
`start` is `None`,
+ * then the batch should begin with the first record. This method must
always return the
+ * same data for a particular `start` and `end` pair; even after the
Source has been restarted
+ * on a different node.
+ *
+ * Higher layers will always call this method with a value of `start`
greater than or equal
+ * to the last value passed to `commit` and a value of `end` less than
or equal to the
+ * last value returned by `getMaxOffset`
--- End diff --
nit: please fix the comment about `getMaxOffset` as well
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