shanyu zhao created SPARK-31029:
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Summary: Occasional class not found error in user's Future code
using global ExecutionContext
Key: SPARK-31029
URL: https://issues.apache.org/jira/browse/SPARK-31029
Project: Spark
Issue Type: Bug
Components: YARN
Affects Versions: 2.4.5
Reporter: shanyu zhao
*Problem:*
When running tpc-ds test (https://github.com/databricks/spark-sql-perf),
occasionally we see error related to class not found:
2020-02-04 20:00:26,673 ERROR yarn.ApplicationMaster: User class threw
exception: scala.ScalaReflectionException: class
com.databricks.spark.sql.perf.ExperimentRun in JavaMirror with
sun.misc.Launcher$AppClassLoader@28ba21f3 of type class
sun.misc.Launcher$AppClassLoader with classpath [...]
and parent being sun.misc.Launcher$ExtClassLoader@3ff5d147 of type class
sun.misc.Launcher$ExtClassLoader with classpath [...]
and parent being primordial classloader with boot classpath [...] not found.
*Root cause:*
Spark driver starts ApplicationMaster in the main thread, which starts a user
thread and set MutableURLClassLoader to that thread's ContextClassLoader.
userClassThread = startUserApplication()
The main thread then setup YarnSchedulerBackend RPC endpoints, which handles
these calls using scala Future with the default global ExecutionContext:
- doRequestTotalExecutors
- doKillExecutors
If main thread starts a future to handle doKillExecutors() before user thread
does then the default thread pool thread's ContextClassLoader would be the
default (AppClassLoader).
If user thread starts a future first then the thread pool thread will have
MutableURLClassLoader.
So if user's code uses a future which references a user provided class (only
MutableURLClassLoader can load), and before the future if there are executor
lost, you will see errors related to class not found.
*Proposed Solution:*
Set the same class loader (userClassLoader) to both the main thread and user
thread in ApplicationMaster.scala
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