Scala + SBT - How to configure reference.conf for a shaded Akka library

Question

TL;DR

I am trying to shade a version of the akka library and bundle it with my application (to be able to run a spray-can server on the CDH 5.7 version of Spark 1.6). The shading process messes up akka's default configuration, and after manually providing a separate version of akka's reference.conf for the shaded akka, it still looks like the 2 versions get mixed up somehow.

Is shading akka versions known to cause problems? What am I doing wrong?

Background

I have a Scala/Spark application currently running on Spark 1.6.1 standalone. The application runs a spray-can http server using spray 1.3.3, which requires akka 2.3.9 (Spark 1.6.1 standalone includes a compatible akka 2.3.11).

I am trying to migrate the application to a new Cloudera-based Spark cluster running the CDH 5.7 version of Spark 1.6. The problem is that Spark 1.6 in CDH 5.7 is bundled with akka 2.2.3 which is not sufficient for spray 1.3.3 to function properly.

Attempted solution

Following the suggestion in this post, I decided to shade akka 2.3.9 and bundle it along with my application. Although this time I stumbled upon a new problem - akka has it's default configuration defined in a reference.conf file, which should be located on the application's classpath. Due to a known issue in sbt-assembly's shading feature, it seems that the shaded akka library would require a separate configuration.

So, I ended up shading akka with the following shade rule:

ShadeRule.rename("akka.**" -> "akka_2_3_9_shade.@1")
    .inLibrary("com.typesafe.akka" % "akka-actor_2.10" % "2.3.9")
    .inAll

and including an additional reference.conf file in my project, which is identical to akka's original reference.conf, but with all occurances of "akka" replaced with "akka_2_3_9_shade".

Now, though, it seems that the Spark-provided akka gets mixed up somehow with the shaded akka, as I'm getting the following error:

Exception in thread "main" java.lang.IllegalArgumentException: Cannot instantiate MailboxType [akka.dispatch.UnboundedMailbox], defined in [akka.actor.default-mailbox], make sure it has a public constructor with [akka.actor.ActorSystem.Settings, com.typesafe.config.Config] parameters
    at akka_2_3_9_shade.dispatch.Mailboxes$$anonfun$1.applyOrElse(Mailboxes.scala:197)
    at akka_2_3_9_shade.dispatch.Mailboxes$$anonfun$1.applyOrElse(Mailboxes.scala:195)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
    at scala.util.Failure$$anonfun$recover$1.apply(Try.scala:185)
    at scala.util.Try$.apply(Try.scala:161)
    at scala.util.Failure.recover(Try.scala:185)
    at akka_2_3_9_shade.dispatch.Mailboxes.lookupConfiguration(Mailboxes.scala:195)
    at akka_2_3_9_shade.dispatch.Mailboxes.lookup(Mailboxes.scala:78)
    at akka_2_3_9_shade.actor.LocalActorRefProvider.akka$actor$LocalActorRefProvider$$defaultMailbox$lzycompute(ActorRefProvider.scala:561)
    at akka_2_3_9_shade.actor.LocalActorRefProvider.akka$actor$LocalActorRefProvider$$defaultMailbox(ActorRefProvider.scala:561)
    at akka_2_3_9_shade.actor.LocalActorRefProvider$$anon$1.<init>(ActorRefProvider.scala:568)
    at akka_2_3_9_shade.actor.LocalActorRefProvider.rootGuardian$lzycompute(ActorRefProvider.scala:564)
    at akka_2_3_9_shade.actor.LocalActorRefProvider.rootGuardian(ActorRefProvider.scala:563)
    at akka_2_3_9_shade.actor.LocalActorRefProvider.init(ActorRefProvider.scala:618)
    at akka_2_3_9_shade.actor.ActorSystemImpl.liftedTree2$1(ActorSystem.scala:619)
    at akka_2_3_9_shade.actor.ActorSystemImpl._start$lzycompute(ActorSystem.scala:616)
    at akka_2_3_9_shade.actor.ActorSystemImpl._start(ActorSystem.scala:616)
    at akka_2_3_9_shade.actor.ActorSystemImpl.start(ActorSystem.scala:633)
    at akka_2_3_9_shade.actor.ActorSystem$.apply(ActorSystem.scala:142)
    at akka_2_3_9_shade.actor.ActorSystem$.apply(ActorSystem.scala:109)
    at akka_2_3_9_shade.actor.ActorSystem$.apply(ActorSystem.scala:100)
    at MyApp.api.Boot$delayedInit$body.apply(Boot.scala:45)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.App$$anonfun$main$1.apply(App.scala:71)
    at scala.collection.immutable.List.foreach(List.scala:318)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
    at scala.App$class.main(App.scala:71)
    at MyApp.api.Boot$.main(Boot.scala:28)
    at MyApp.api.Boot.main(Boot.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:601)
    at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:731)
    at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:181)
    at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:206)
    at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:121)
    at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)
Caused by: java.lang.ClassCastException: interface akka_2_3_9_shade.dispatch.MailboxType is not assignable from class akka.dispatch.UnboundedMailbox
    at akka_2_3_9_shade.actor.ReflectiveDynamicAccess$$anonfun$getClassFor$1.apply(DynamicAccess.scala:69)
    at akka_2_3_9_shade.actor.ReflectiveDynamicAccess$$anonfun$getClassFor$1.apply(DynamicAccess.scala:66)
    at scala.util.Try$.apply(Try.scala:161)
    at akka_2_3_9_shade.actor.ReflectiveDynamicAccess.getClassFor(DynamicAccess.scala:66)
    at akka_2_3_9_shade.actor.ReflectiveDynamicAccess.CreateInstanceFor(DynamicAccess.scala:84)
    ... 34 more

The relevant code from my application's Boot.scala file is the following:

[45]    implicit val system = ActorSystem()
...
[48]    val service = system.actorOf(Props[MyAppApiActor], "MyApp.Api")
...
[52]    val port = config.getInt("MyApp.server.port")
[53]    IO(Http) ? Http.Bind(service, interface = "0.0.0.0", port = port)

Answer 1

OK, so eventually I managed to solve this.

Turns out akka loads (some of the) configuration settings from the config file using keys that are defined as string literals. You can find a lot of these in akka/actor/ActorSystem.scala, for example.

And it seems that sbt-assembly does not change references to the shaded library/package name in string literals.

Also, some configuration keys are being changed by sbt-assembly's shading. I haven't really taken the time to find where and how exactly they are defined in akka's source, but the following exception, which is being thrown during the ActorSystem init code, proves that this is indeed the case:

ConfigException$Missing: No configuration setting found for key 'akka_2_3_9_shade'

So, the solution it to include a custom config file (call it for example akka_spray_shade.conf), and copy the following configuration sections in it:

• The contents of akka's original reference.conf, but having the akka prefix in the configuration values changed to akka_2_3_9_shade. (this is required for the hard-coded string literal config keys)
• The contents of akka's original reference.conf, but having the akka prefix in the configuration values changed to akka_2_3_9_shade and having the root configuration key changed from akka to akka_2_3_9_shade. (this is required for the config keys which do get modified by sbt-assembly)
• The contents of spray's original reference.conf, but having the akka prefix in the configuration values changed to akka_2_3_9_shade. (this is required to make sure that spray always refers to the shaded akka)

Now, this custom config file must be provided explicitly during the initialization of the ActorSystem in application's Boot.scala code:

val akkaShadeConfig = ConfigFactory.load("akka_spray_shade")
implicit val system = ActorSystem("custom-actor-system-name", akkaShadeConfig)

Answer 2

A small addition to the accepted answer.

It is not necessary to put this configuration in a custom-named file like akka_spray_shade.conf. The configuration can be placed into application.conf which is being loaded by default during ActorSystem creation when no custom configuration is explicitly specified: ActorSystem("custom-actor-system-name") effectively means ActorSystem("custom-actor-system-name", ConfigFactory.load("application")).

Answer 3

I struggled with this for a long time as well. It turns out that the default merge strategy in sbt-assembly excludes all the reference.conf files. Adding this to build.sbt solved it for me:

assemblyMergeStrategy in assembly := {
  case PathList("reference.conf") => MergeStrategy.concat
}