Github user vanzin commented on a diff in the pull request:
https://github.com/apache/spark/pull/9182#discussion_r44824670
--- Diff:
yarn/src/main/scala/org/apache/spark/scheduler/cluster/SchedulerExtensionService.scala
---
@@ -0,0 +1,160 @@
+/*
+ * 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 java.util.concurrent.atomic.AtomicBoolean
+
+import org.apache.hadoop.yarn.api.records.{ApplicationAttemptId,
ApplicationId}
+
+import org.apache.spark.{Logging, SparkContext}
+import org.apache.spark.util.Utils
+
+/**
+ * An extension service that can be loaded into a Spark YARN scheduler.
+ * A Service that can be started and stopped.
+ *
+ * 1. For implementations to be loadable by `SchedulerExtensionServices`,
+ * they must provide an empty constructor.
+ * 2. The `stop()` operation MUST be idempotent, and succeed even if
`start()` was
+ * never invoked.
+ */
+trait SchedulerExtensionService {
+
+ /**
+ * Start the extension service. This should be a no-op if
+ * called more than once.
+ * @param binding binding to the spark application and YARN
+ */
+ def start(binding: SchedulerExtensionServiceBinding): Unit
+
+ /**
+ * Stop the service
+ * The `stop()` operation MUST be idempotent, and succeed even if
`start()` was
+ * never invoked.
+ */
+ def stop(): Unit
+}
+
+/**
+ * Binding information for a [[SchedulerExtensionService]].
+ *
+ * The attempt ID will be set if the service is started within a YARN
application master;
+ * there is then a different attempt ID for every time that AM is
restarted.
+ * When the service binding is instantiated on a client, there's no
attempt ID, as it lacks
+ * this information.
+ * @param sparkContext current spark context
+ * @param applicationId YARN application ID
+ * @param attemptId YARN attemptID -if known.
+ */
+case class SchedulerExtensionServiceBinding(
+ sparkContext: SparkContext,
+ applicationId: ApplicationId,
+ attemptId: Option[ApplicationAttemptId] = None)
+
+/**
+ * Container for [[SchedulerExtensionService]] instances.
+ *
+ * Loads Extension Services from the configuration property
+ * `"spark.yarn.services"`, instantiates and starts them.
+ * When stopped, it stops all child entries.
+ *
+ * The order in which child extension services are started and stopped
+ * is undefined.
+ *
+ */
+private[spark] class SchedulerExtensionServices extends
SchedulerExtensionService
+ with Logging {
+ private var serviceOption: Option[String] = None
+ private var services: List[SchedulerExtensionService] = Nil
+ private val started = new AtomicBoolean(false)
+ private var binding: SchedulerExtensionServiceBinding = _
+
+ /**
+ * Binding operation will load the named services and call bind on them
too; the
+ * entire set of services are then ready for `init()` and `start()`
calls.
+ *
+ * @param binding binding to the spark application and YARN
+ */
+ def start(binding: SchedulerExtensionServiceBinding): Unit = {
+ if (started.getAndSet(true)) {
+ logWarning("Ignoring re-entrant start operation")
+ return
+ }
+ require(binding.sparkContext != null, "Null context parameter")
+ require(binding.applicationId != null, "Null appId parameter")
+ this.binding = binding
+ val sparkContext = binding.sparkContext
+ val appId = binding.applicationId
+ val attemptId = binding.attemptId
+ logInfo(s"Starting Yarn extension services with app
${binding.applicationId}" +
--- End diff --
nit: if you use `$appId` then the whole statement fits in the same line.
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