Dependency issue with Scalding and Hadoop with sbt-assembly

Question

I'm trying to build a far with sbt of a simple hadoop job I'm trying to run in an attempt to run it on Amazon EMR. However when I run sbt assembly I get the following error:

[error] (*:assembly) deduplicate: different file contents found in the following:
[error] /Users/trenthauck/.ivy2/cache/org.mortbay.jetty/jsp-2.1/jars/jsp-2.1-6.1.14.jar:org/apache/jasper/compiler/Node$ChildInfo.class
[error] /Users/trenthauck/.ivy2/cache/tomcat/jasper-compiler/jars/jasper-compiler-5.5.12.jar:org/apache/jasper/compiler/Node$ChildInfo.class
[error] Total time: 10 s, completed Sep 14, 2013 4:49:24 PM

I attempted to follow the suggestion here https://groups.google.com/forum/#!topic/simple-build-tool/tzkq5TioIqM however it didn't work.

My build.sbt looks like:

import AssemblyKeys._

mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
  {
    case PathList("org", "apache", "jasper", xs @ _*) => MergeStrategy.last
    case x => old(x)
  }
}

assemblySettings

name := "Scaling Play"

version := "SNAPSHOT-0.1"

scalaVersion := "2.10.1"

libraryDependencies ++= Seq(
  "com.twitter" % "scalding-core_2.10" % "0.8.8",
  "com.twitter" % "scalding-args_2.10" % "0.8.8",
  "com.twitter" % "scalding-date_2.10" % "0.8.8",
  "org.apache.hadoop" % "hadoop-core" % "1.0.0"
  )

Answer 1

The order of the directives is important. You update the assembly settings, to overwrite it again a line later. First defining assemblySettings and then updating it will solve it.

The updated build.sbt:

import AssemblyKeys._

assemblySettings

mergeStrategy in assembly <<= (mergeStrategy in assembly) { (old) =>
  {
    case PathList("org", "apache", "jasper", xs @ _*) => MergeStrategy.last
    case x => old(x)
  }
}

…

After that you will discover that there are a lot more conflicting classes and other files. In this case you will require the following merges:

case PathList("org", "apache", xs @ _*) => MergeStrategy.last
case PathList("javax", "servlet", xs @ _*) => MergeStrategy.last
case PathList("com", "esotericsoftware", xs @ _*) => MergeStrategy.last
case PathList("project.clj") => MergeStrategy.last
case PathList("overview.html") => MergeStrategy.last
case x => old(x)

Note that using merge strategies for class files may give problems, caused by incompatible versions of that specific class. If that is the case then your problem is larger, because then the dependencies are incompatible with each other. You have then to resort to removing the dependency and find/make a compatible version.