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 org.apache.ambari.server.events.jpa.JPAEvent;
  import org.apache.ambari.server.orm.entities.ClusterEntity;
  
  import com.google.common.eventbus.EventBus;
  import com.google.inject.Singleton;
  
  /**
   * The {@link JPAEventPublisher} is used to publish events pertaining to
   * EclipseLink and the persistence layer. This class uses a synchronized bus and
   * will therefore run on the current thread. This is desirable since JPA events
   * are time-senstive and cannot be executed in an asynchronous manner most of
   * the time.
   * <p/>
   * This publisher was created specifically for AMBARI-17970. The headless server
   * thread which monitors the database for things to do (known as the action
   * scheduler) polls every second. However, it takes EclipseLink up to 1000ms to
   * update L1 persistence context cache references in other threads. Therefore,
   * it's possible that the action scheduler can pickup a {@link ClusterEntity}
   * with stale data, including stale configurations. Those stale configurations
   * can then be sent in commands down to agents.
   */
  @Singleton
  public class JPAEventPublisher {
    /**
     * A single threaded, synchronous event bus for processing JPA events.
     */
    private final EventBus m_eventBus = new EventBus("ambari-jpa-event-bus");
  
  
    /**
     * Publishes the specified event to all registered listeners that
     * {@link com.google.common.eventbus.Subscribe} to any of the
     * {@link JPAEventPublisher} instances.
     *
     * @param event
     *          the event
     */
    public void publish(JPAEvent event) {
      m_eventBus.post(event);
    }
  
    /**
     * Register a listener to receive events. The listener should use the
     * {@link com.google.common.eventbus.Subscribe} annotation.
     *
     * @param object
     *          the listener to receive events.
     */
    public void register(Object object) {
      m_eventBus.register(object);
    }
  }
  