content: /*
   * 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.ambari.server.events.publishers;
  
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.ThreadPoolExecutor;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import org.apache.ambari.server.configuration.Configuration;
  import org.apache.ambari.server.controller.utilities.ScalingThreadPoolExecutor;
  import org.apache.ambari.server.events.AlertEvent;
  
  import com.google.common.eventbus.AsyncEventBus;
  import com.google.common.eventbus.EventBus;
  import com.google.common.eventbus.Subscribe;
  import com.google.inject.Inject;
  import com.google.inject.Singleton;
  
  /**
   * The {@link AlertEventPublisher} is used to wrap a customized instance of an
   * {@link AsyncEventBus} that is only used for alerts. In general, Ambari should
   * have its own application-wide event bus for application events (session
   * information, state changes, etc), but since alerts can contain many events
   * being published concurrently, it makes sense to encapsulate a specific alert
   * bus in this publisher.
   */
  @Singleton
  public final class AlertEventPublisher {
  
    /**
     * A multi-threaded event bus that can handle dispatching {@link AlertEvent}s.
     */
    private final EventBus m_eventBus;
  
    /**
     * Constructor.
     */
    @Inject
    public AlertEventPublisher(Configuration config) {
      // create an executor which will scale with the number of queued work items
      // when handling incoming alerts
      int corePoolSize = config.getAlertEventPublisherCorePoolSize();
      int maxPoolSize = config.getAlertEventPublisherMaxPoolSize();
      int workerQueueSize = config.getAlertEventPublisherWorkerQueueSize();
  
      ThreadPoolExecutor executor = new ScalingThreadPoolExecutor(corePoolSize, maxPoolSize, 0L,
          TimeUnit.SECONDS, workerQueueSize);
  
      executor.allowCoreThreadTimeOut(false);
      executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy());
      executor.setThreadFactory(new AlertEventBusThreadFactory());
  
      m_eventBus = new AsyncEventBus(executor);
    }
  
    /**
     * Publishes the specified event to all registered listeners that
     * {@link Subscribe} to any of the {@link AlertEvent} instances.
     *
     * @param event
     */
    public void publish(AlertEvent event) {
      m_eventBus.post(event);
    }
  
    /**
     * Register a listener to receive events. The listener should use the
     * {@link Subscribe} annotation.
     *
     * @param object
     *          the listener to receive events.
     */
    public void register(Object object) {
      m_eventBus.register(object);
    }
  
    /**
     * A custom {@link ThreadFactory} for the threads that will handle published
     * {@link AlertEvent}. Threads created will have slightly reduced priority
     * since {@link AlertEvent} instances are not critical to the system.
     */
    private static final class AlertEventBusThreadFactory implements
        ThreadFactory {
  
      private static final AtomicInteger s_threadIdPool = new AtomicInteger(1);
  
      /**
       * {@inheritDoc}
       */
      @Override
      public Thread newThread(Runnable r) {
        Thread thread = new Thread(r, "alert-event-bus-"
            + s_threadIdPool.getAndIncrement());
  
        thread.setDaemon(false);
        thread.setPriority(Thread.NORM_PRIORITY - 1);
  
        return thread;
      }
    }
  }
  