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Hyukjin Kwon 9ccae0c9e7 [SPARK-26362][CORE] Remove 'spark.driver.allowMultipleContexts' to disallow multiple creation of SparkContexts 
## What changes were proposed in this pull request?

Multiple SparkContexts are discouraged and it has been warning for last 4 years, see SPARK-4180. It could cause arbitrary and mysterious error cases, see SPARK-2243.

Honestly, I didn't even know Spark still allows it, which looks never officially supported, see SPARK-2243.

I believe It should be good timing now to remove this configuration.

## How was this patch tested?

Each doc was manually checked and manually tested:

```
$ ./bin/spark-shell --conf=spark.driver.allowMultipleContexts=true
...
scala> new SparkContext()
org.apache.spark.SparkException: Only one SparkContext should be running in this JVM (see SPARK-2243).The currently running SparkContext was created at:
org.apache.spark.sql.SparkSession$Builder.getOrCreate(SparkSession.scala:939)
...
org.apache.spark.SparkContext$.$anonfun$assertNoOtherContextIsRunning$2(SparkContext.scala:2435)
  at scala.Option.foreach(Option.scala:274)
  at org.apache.spark.SparkContext$.assertNoOtherContextIsRunning(SparkContext.scala:2432)
  at org.apache.spark.SparkContext$.markPartiallyConstructed(SparkContext.scala:2509)
  at org.apache.spark.SparkContext.<init>(SparkContext.scala:80)
  at org.apache.spark.SparkContext.<init>(SparkContext.scala:112)
  ... 49 elided
```

Closes #23311 from HyukjinKwon/SPARK-26362.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>
2018-12-15 13:55:24 +08:00
..
catalyst [MINOR][SQL] Some errors in the notes. 2018-12-14 14:23:21 -06:00
core [SPARK-26362][CORE] Remove 'spark.driver.allowMultipleContexts' to disallow multiple creation of SparkContexts 2018-12-15 13:55:24 +08:00
hive [SPARK-26319][SQL][TEST] Add appendReadColumns Unit Test for HiveShimSuite 2018-12-10 22:26:28 +08:00
hive-thriftserver [SPARK-26090][CORE][SQL][ML] Resolve most miscellaneous deprecation and build warnings for Spark 3 2018-11-19 09:16:42 -06:00
create-docs.sh [MINOR][DOCS] Minor doc fixes related with doc build and uses script dir in SQL doc gen script 2017-08-26 13:56:24 +09:00
gen-sql-markdown.py [SPARK-21485][FOLLOWUP][SQL][DOCS] Describes examples and arguments separately, and note/since in SQL built-in function documentation 2017-08-05 10:10:56 -07:00
mkdocs.yml [SPARK-21485][SQL][DOCS] Spark SQL documentation generation for built-in functions 2017-07-26 09:38:51 -07:00
README.md [MINOR][DOC] Fix some typos and grammar issues 2018-04-06 13:37:08 +08:00

README.md

Spark SQL

This module provides support for executing relational queries expressed in either SQL or the DataFrame/Dataset API.

Spark SQL is broken up into four subprojects:

  • Catalyst (sql/catalyst) - An implementation-agnostic framework for manipulating trees of relational operators and expressions.
  • Execution (sql/core) - A query planner / execution engine for translating Catalyst's logical query plans into Spark RDDs. This component also includes a new public interface, SQLContext, that allows users to execute SQL or LINQ statements against existing RDDs and Parquet files.
  • Hive Support (sql/hive) - Includes an extension of SQLContext called HiveContext that allows users to write queries using a subset of HiveQL and access data from a Hive Metastore using Hive SerDes. There are also wrappers that allow users to run queries that include Hive UDFs, UDAFs, and UDTFs.
  • HiveServer and CLI support (sql/hive-thriftserver) - Includes support for the SQL CLI (bin/spark-sql) and a HiveServer2 (for JDBC/ODBC) compatible server.

Running sql/create-docs.sh generates SQL documentation for built-in functions under sql/site.