code: /*
   * 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.skywalking.oap.server.core.cluster;
  
  import java.util.ArrayList;
  import java.util.List;
  import lombok.extern.slf4j.Slf4j;
  import org.apache.skywalking.oap.server.library.module.ModuleStartException;
  
  @Slf4j
  public abstract class ClusterCoordinator implements ClusterRegister, ClusterNodesQuery, ClusterWatcherRegister {
      private final List<ClusterWatcher> clusterWatchers = new ArrayList<>();
  
      /**
       * Initialize the required resources, such as healthy checker and listener.
       */
      public abstract void start() throws ModuleStartException;
  
      @Override
      public void registerWatcher(final ClusterWatcher watcher) {
          this.clusterWatchers.add(watcher);
      }
  
      protected void notifyWatchers(List<RemoteInstance> remoteInstances) {
          if (log.isDebugEnabled()) {
              log.debug("Notify watchers and update cluster instances:{}", remoteInstances.toString());
          }
          this.clusterWatchers.forEach(
              clusterWatcher -> clusterWatcher.onClusterNodesChanged(remoteInstances));
      }
  }
  