[jira] [Commented] (SPARK-25245) Explain regarding limiting modification on "spark.sql.shuffle.partitions" for structured streaming

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-25245?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16727501#comment-16727501 ] 

ASF GitHub Bot commented on SPARK-25245:
----------------------------------------

srowen closed pull request #22238: [SPARK-25245][DOCS][SS] Explain regarding limiting modification on "spark.sql.shuffle.partitions" for structured streaming
URL: https://github.com/apache/spark/pull/22238
 
 
   

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diff --git a/docs/structured-streaming-programming-guide.md b/docs/structured-streaming-programming-guide.md
index 73de1892977ac..8c3622c857240 100644
--- a/docs/structured-streaming-programming-guide.md
+++ b/docs/structured-streaming-programming-guide.md
@@ -2812,6 +2812,16 @@ See [Input Sources](#input-sources) and [Output Sinks](#output-sinks) sections f
 
 # Additional Information
 
+**Notes**
+
+- Several configurations are not modifiable after the query has run. To change them, discard the checkpoint and start a new query. These configurations include:
+  - `spark.sql.shuffle.partitions`
+    - This is due to the physical partitioning of state: state is partitioned via applying hash function to key, hence the number of partitions for state should be unchanged.
+    - If you want to run fewer tasks for stateful operations, `coalesce` would help with avoiding unnecessary repartitioning.
+      - After `coalesce`, the number of (reduced) tasks will be kept unless another shuffle happens.
+  - `spark.sql.streaming.stateStore.providerClass`: To read the previous state of the query properly, the class of state store provider should be unchanged.
+  - `spark.sql.streaming.multipleWatermarkPolicy`: Modification of this would lead inconsistent watermark value when query contains multiple watermarks, hence the policy should be unchanged.
+
 **Further Reading**
 
 - See and run the
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
index ef3ce98fd7add..ca100da9f019c 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
@@ -266,7 +266,9 @@ object SQLConf {
     .createWithDefault(Long.MaxValue)
 
   val SHUFFLE_PARTITIONS = buildConf("spark.sql.shuffle.partitions")
-    .doc("The default number of partitions to use when shuffling data for joins or aggregations.")
+    .doc("The default number of partitions to use when shuffling data for joins or aggregations. " +
+      "Note: For structured streaming, this configuration cannot be changed between query " +
+      "restarts from the same checkpoint location.")
     .intConf
     .createWithDefault(200)
 
@@ -868,7 +870,9 @@ object SQLConf {
       .internal()
       .doc(
         "The class used to manage state data in stateful streaming queries. This class must " +
-          "be a subclass of StateStoreProvider, and must have a zero-arg constructor.")
+          "be a subclass of StateStoreProvider, and must have a zero-arg constructor. " +
+          "Note: For structured streaming, this configuration cannot be changed between query " +
+          "restarts from the same checkpoint location.")
       .stringConf
       .createWithDefault(
         "org.apache.spark.sql.execution.streaming.state.HDFSBackedStateStoreProvider")


 

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> Explain regarding limiting modification on "spark.sql.shuffle.partitions" for structured streaming
> --------------------------------------------------------------------------------------------------
>
>                 Key: SPARK-25245
>                 URL: https://issues.apache.org/jira/browse/SPARK-25245
>             Project: Spark
>          Issue Type: Documentation
>          Components: Structured Streaming
>    Affects Versions: 2.4.0
>            Reporter: Jungtaek Lim
>            Assignee: Jungtaek Lim
>            Priority: Minor
>             Fix For: 3.0.0
>
>
> Couple of users wondered why "spark.sql.shuffle.partitions" keeps unchanged when they changed the config value after running the query. Some of them even submitted the patch as this behavior as a bug. But it is based on how state is partitioned and the behavior is intentional.
> Looks like it's worth to explain it to guide doc so that no more users will be wondered.



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