content: /*
   * Copyright 1999-2018 Alibaba Group Holding Ltd.
   *
   * Licensed 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 com.alibaba.nacos.common.notify;
  
  import java.io.Serializable;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * An abstract class for event.
   *
   * @author <a href="mailto:liaochuntao@live.com">liaochuntao</a>
   * @author zongtanghu
   */
  @SuppressWarnings({"PMD.AbstractClassShouldStartWithAbstractNamingRule"})
  public abstract class Event implements Serializable {
      
      private static final long serialVersionUID = -3731383194964997493L;
      
      private static final AtomicLong SEQUENCE = new AtomicLong(0);
      
      private final long sequence = SEQUENCE.getAndIncrement();
      
      /**
       * Event sequence number, which can be used to handle the sequence of events.
       *
       * @return sequence num, It's best to make sure it's monotone.
       */
      public long sequence() {
          return sequence;
      }
      
      /**
       * Event scope.
       *
       * @return event scope, return null if for all scope
       */
      public String scope() {
          return null;
      }
      
      /**
       * Whether is plugin event. If so, the event can be dropped when no publish and subscriber without any hint. Default
       * false
       *
       * @return {@code true} if is plugin event, otherwise {@code false}
       */
      public boolean isPluginEvent() {
          return false;
      }
  }
  
  