title: Spark accumulableCollection does not work with mutable.Map
body: <p>I am using Spark to do employee record accumulation and for that I use Spark's accumulator. I am using Map[empId, emp] as accumulableCollection so that I can search employee by their ids. I have tried everything but it does not work. Can someone point if there is any logical issues with the way I am using accumulableCollection or Map is not supported. Following is my code</p>
  
  <pre><code>package demo
  
  import org.apache.spark.{SparkContext, SparkConf, Logging}
  
  import org.apache.spark.SparkContext._
  import scala.collection.mutable
  
  
  object MapAccuApp extends App with Logging {
    case class Employee(id:String, name:String, dept:String)
  
    val conf = new SparkConf().setAppName("Employees") setMaster ("local[4]")
    val sc = new SparkContext(conf)
  
    implicit def empMapToSet(empIdToEmp: mutable.Map[String, Employee]): mutable.MutableList[Employee] = {
      empIdToEmp.foldLeft(mutable.MutableList[Employee]()) { (l, e) =&gt; l += e._2}
    }
  
    val empAccu = sc.accumulableCollection[mutable.Map[String, Employee], Employee](mutable.Map[String,Employee]())
  
    val employees = List(
      Employee("10001", "Tom", "Eng"),
      Employee("10002", "Roger", "Sales"),
      Employee("10003", "Rafael", "Sales"),
      Employee("10004", "David", "Sales"),
      Employee("10005", "Moore", "Sales"),
      Employee("10006", "Dawn", "Sales"),
      Employee("10007", "Stud", "Marketing"),
      Employee("10008", "Brown", "QA")
    )
  
    System.out.println("employee count " + employees.size)
  
  
    sc.parallelize(employees).foreach(e =&gt; {
      empAccu += e
    })
  
    System.out.println("empAccumulator size " + empAccu.value.size)
  }
  </code></pre>