[SPARK-14729][SCHEDULER] Refactored YARN scheduler creation code to use newly added ExternalClusterManager

## What changes were proposed in this pull request?
With the addition of ExternalClusterManager(ECM) interface in PR apache#11723, any cluster manager can now be integrated with Spark. It was suggested in  ExternalClusterManager PR that one of the existing cluster managers should start using the new interface to ensure that the API is correct. Ideally, all the existing cluster managers should eventually use the ECM interface but as a first step yarn will now use the ECM interface. This PR refactors YARN code from SparkContext.createTaskScheduler function  into YarnClusterManager that implements ECM interface.

## How was this patch tested?
Since this is refactoring, no new tests has been added. Existing tests have been run. Basic manual testing with YARN was done too.

Author: Hemant Bhanawat <[email protected]>

Closes apache#12641 from hbhanawat/yarnClusterMgr.

Author: Hemant Bhanawat

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -2389,53 +2389,6 @@ object SparkContext extends Logging {
         }
         (backend, scheduler)
 
-      case "yarn" if deployMode == "cluster" =>
-        val scheduler = try {
-          val clazz = Utils.classForName("org.apache.spark.scheduler.cluster.YarnClusterScheduler")
-          val cons = clazz.getConstructor(classOf[SparkContext])
-          cons.newInstance(sc).asInstanceOf[TaskSchedulerImpl]
-        } catch {
-          // TODO: Enumerate the exact reasons why it can fail
-          // But irrespective of it, it means we cannot proceed !
-          case e: Exception =>
-            throw new SparkException("YARN mode not available ?", e)
-        }
-        val backend = try {
-          val clazz =
-            Utils.classForName("org.apache.spark.scheduler.cluster.YarnClusterSchedulerBackend")
-          val cons = clazz.getConstructor(classOf[TaskSchedulerImpl], classOf[SparkContext])
-          cons.newInstance(scheduler, sc).asInstanceOf[CoarseGrainedSchedulerBackend]
-        } catch {
-          case e: Exception =>
-            throw new SparkException("YARN mode not available ?", e)
-        }
-        scheduler.initialize(backend)
-        (backend, scheduler)
-
-      case "yarn" if deployMode == "client" =>
-        val scheduler = try {
-          val clazz = Utils.classForName("org.apache.spark.scheduler.cluster.YarnScheduler")
-          val cons = clazz.getConstructor(classOf[SparkContext])
-          cons.newInstance(sc).asInstanceOf[TaskSchedulerImpl]
-
-        } catch {
-          case e: Exception =>
-            throw new SparkException("YARN mode not available ?", e)
-        }
-
-        val backend = try {
-          val clazz =
-            Utils.classForName("org.apache.spark.scheduler.cluster.YarnClientSchedulerBackend")
-          val cons = clazz.getConstructor(classOf[TaskSchedulerImpl], classOf[SparkContext])
-          cons.newInstance(scheduler, sc).asInstanceOf[CoarseGrainedSchedulerBackend]
-        } catch {
-          case e: Exception =>
-            throw new SparkException("YARN mode not available ?", e)
-        }
-
-        scheduler.initialize(backend)
-        (backend, scheduler)
-
       case MESOS_REGEX(mesosUrl) =>
         MesosNativeLibrary.load()
         val scheduler = new TaskSchedulerImpl(sc)
@@ -2464,6 +2417,7 @@ object SparkContext extends Logging {
           cm.initialize(scheduler, backend)
           (backend, scheduler)
         } catch {
+          case se: SparkException => throw se
           case NonFatal(e) =>
             throw new SparkException("External scheduler cannot be instantiated", e)
         }

core/src/test/scala/org/apache/spark/SparkContextSchedulerCreationSuite.scala

@@ -129,26 +129,6 @@ class SparkContextSchedulerCreationSuite
     }
   }
 
-  def testYarn(master: String, deployMode: String, expectedClassName: String) {
-    try {
-      val sched = createTaskScheduler(master, deployMode)
-      assert(sched.getClass === Utils.classForName(expectedClassName))
-    } catch {
-      case e: SparkException =>
-        assert(e.getMessage.contains("YARN mode not available"))
-        logWarning("YARN not available, could not test actual YARN scheduler creation")
-      case e: Throwable => fail(e)
-    }
-  }
-
-  test("yarn-cluster") {
-    testYarn("yarn", "cluster", "org.apache.spark.scheduler.cluster.YarnClusterScheduler")
-  }
-
-  test("yarn-client") {
-    testYarn("yarn", "client", "org.apache.spark.scheduler.cluster.YarnScheduler")
-  }
-
   def testMesos(master: String, expectedClass: Class[_], coarse: Boolean) {
     val conf = new SparkConf().set("spark.mesos.coarse", coarse.toString)
     try {

yarn/src/main/resources/META-INF/services/org.apache.spark.scheduler.ExternalClusterManager

@@ -0,0 +1 @@
+org.apache.spark.scheduler.cluster.YarnClusterManager

yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnClusterManager.scala

@@ -0,0 +1,56 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.scheduler.cluster
+
+import org.apache.spark.{SparkContext, SparkException}
+import org.apache.spark.scheduler.{ExternalClusterManager, SchedulerBackend, TaskScheduler, TaskSchedulerImpl}
+
+/**
+ * Cluster Manager for creation of Yarn scheduler and backend
+ */
+private[spark] class YarnClusterManager extends ExternalClusterManager {
+
+  override def canCreate(masterURL: String): Boolean = {
+    masterURL == "yarn"
+  }
+
+  override def createTaskScheduler(sc: SparkContext, masterURL: String): TaskScheduler = {
+    sc.deployMode match {
+      case "cluster" => new YarnClusterScheduler(sc)
+      case "client" => new YarnScheduler(sc)
+      case _ => throw new SparkException(s"Unknown deploy mode '${sc.deployMode}' for Yarn")
+    }
+  }
+
+  override def createSchedulerBackend(sc: SparkContext,
+      masterURL: String,
+      scheduler: TaskScheduler): SchedulerBackend = {
+    sc.deployMode match {
+      case "cluster" =>
+        new YarnClusterSchedulerBackend(scheduler.asInstanceOf[TaskSchedulerImpl], sc)
+      case "client" =>
+        new YarnClientSchedulerBackend(scheduler.asInstanceOf[TaskSchedulerImpl], sc)
+      case  _ =>
+        throw new SparkException(s"Unknown deploy mode '${sc.deployMode}' for Yarn")
+    }
+  }
+
+  override def initialize(scheduler: TaskScheduler, backend: SchedulerBackend): Unit = {
+    scheduler.asInstanceOf[TaskSchedulerImpl].initialize(backend)
+  }
+}