Using Scala 2.12 with Spark 2.x

Question

At the Spark 2.1 docs it's mentioned that

Spark runs on Java 7+, Python 2.6+/3.4+ and R 3.1+. For the Scala API, Spark 2.1.0 uses Scala 2.11. You will need to use a compatible Scala version (2.11.x).

at the Scala 2.12 release news it's also mentioned that:

Although Scala 2.11 and 2.12 are mostly source compatible to facilitate cross-building, they are not binary compatible. This allows us to keep improving the Scala compiler and standard library.

But when I build an uber jar (using Scala 2.12) and run it on Spark 2.1. every thing work just fine.

and I know its not any official source but at the 47 degree blog they mentioned that Spark 2.1 does support Scala 2.12.

How can one explain those (conflicts?) pieces of information ?

Answer 1

Spark does not support Scala 2.12. You can follow SPARK-14220 (Build and test Spark against Scala 2.12) to get up to date status.

update: Spark 2.4 added an experimental Scala 2.12 support.

Answer 2

Scala 2.12 is officially supported (and required) as of Spark 3. Summary:

• Spark 2.0 - 2.3: Required Scala 2.11
• Spark 2.4: Supported Scala 2.11 and Scala 2.12, but not really cause almost all runtimes only supported Scala 2.11.
• Spark 3: Only Scala 2.12 is supported

Using a Spark runtime that's compiled with one Scala version and a JAR file that's compiled with another Scala version is dangerous and causes strange bugs. For example, as noted here, using a Scala 2.11 compiled JAR on a Spark 3 cluster will cause this error: java.lang.NoSuchMethodError: scala.Predef$.refArrayOps([Ljava/lang/Object;)Lscala/collection/mutable/ArrayOps.

Look at all the poor Spark users running into this very error.

Make sure to look into Scala cross compilation and understand the %% operator in SBT to limit your suffering. Maintaining Scala projects is hard and minimizing your dependencies is recommended.