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https://issues.apache.org/jira/browse/SPARK-16963?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15431779#comment-15431779
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Frederick Reiss commented on SPARK-16963:
-----------------------------------------
The proposed changes in the attached PR are now ready for review. [~marmbrus]
can you please have a look at your convenience? [~prashant_] can you also
please have a look with a particular focus on whether the changes fit with the
MQTT connector?
> Change Source API so that sources do not need to keep unbounded state
> ---------------------------------------------------------------------
>
> Key: SPARK-16963
> URL: https://issues.apache.org/jira/browse/SPARK-16963
> Project: Spark
> Issue Type: Improvement
> Components: Streaming
> Affects Versions: 2.0.0
> Reporter: Frederick Reiss
>
> The version of the Source API in Spark 2.0.0 defines a single getBatch()
> method for fetching records from the source, with the following Scaladoc
> comments defining the semantics:
> {noformat}
> /**
> * 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.
> */
> def getBatch(start: Option[Offset], end: Offset): DataFrame
> {noformat}
> These semantics mean that a Source must retain all past history for the
> stream that it backs. Further, a Source is also required to retain this data
> across restarts of the process where the Source is instantiated, even when
> the Source is restarted on a different machine.
> These restrictions make it difficult to implement the Source API, as any
> implementation requires potentially unbounded amounts of distributed storage.
> See the mailing list thread at
> [http://apache-spark-developers-list.1001551.n3.nabble.com/Source-API-requires-unbounded-distributed-storage-td18551.html]
> for more information.
> This JIRA will cover augmenting the Source API with an additional callback
> that will allow Structured Streaming scheduler to notify the source when it
> is safe to discard buffered data.
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