Andrew Kerr created TOREE-349:
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Summary: ClassCastException when reading Avro from another thread
(Toree master / Spark 2.0.0)
Key: TOREE-349
URL: https://issues.apache.org/jira/browse/TOREE-349
Project: TOREE
Issue Type: Bug
Reporter: Andrew Kerr
When using Toree (master branch commit e8ecd0623c65ad104045b1797fb27f69b8dfc23f)
with `--packages=com.databricks:spark-avro_2.11:3.0.1` in `SPARK_OPTS`
and attempting to load an avro file into a dataframe *in a separate thread*
then an exception is thrown
`java.lang.ClassCastException:
com.databricks.spark.avro.DefaultSource$SerializableConfiguration cannot be
cast to com.databricks.spark.avro.DefaultSource$SerializableConfiguration`
here
https://github.com/databricks/spark-avro/blob/v3.0.1/src/main/scala/com/databricks/spark/avro/DefaultSource.scala#L156
Will attach a Jupyter notebook that illustrates the problem and includes full
stack trace, with a script showing environment.
The class that throws the exception `DefaultSource` broadcasts Hadoop config
and returns an anonymous function that accesses that config. The exception
occurs when that function is executed and it attempts to access the config.
This looks like a class loader mismatch problem to me ("Class Identity Crisis").
With a bit of hacking of `spark-avro` I've seen the class loader for
`DefaultSource` when the config is broadcast to be
`scala.reflect.internal.util.ScalaClassLoader$URLClassLoader@31ac5411`
and when the config is read to be
`org.apache.spark.util.MutableURLClassLoader@3d3fcdb0`
If a fat jar including `spark-avro` is built and included with `--jars=...`
then the same problem occurs.
Interestingly the Spark's included support for CSV uses the same pattern as
Avro, broadcasting a config, but works as expected as shown in the notebook.
https://github.com/apache/spark/blob/v2.0.0/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVFileFormat.scala#L108
Avro also works as expected when an application fat jar is built and passed to
`spark-submit` without involving Toree.
Therefore this problem appears to require:
* Toree
* Broadcast variable
* Library added using `--packages` or `--jars`
* Library accessed from a thread different other than Toree interpreter's
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