/*
 * 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.solr.common.cloud;

import java.util.Set;

/**
 * Callback registered with {@link ZkStateReader#registerCollectionStateWatcher(String, CollectionStateWatcher)}
 * and called whenever there is a change in the collection state <em>or</em> in the list of liveNodes.
 * 
 * @see DocCollectionWatcher
 */
public interface CollectionStateWatcher {

  /**
   * Called when either the collection we are registered against has a change of state <em>or</em> there is a change to the live nodes of our collection.
   *
   * <p>
   * Note that, due to the way Zookeeper watchers are implemented, a single call may be
   * the result of several state changes. Also, multiple calls to this method can be made
   * with the same state, ie. without any new updates.
   * </p>
   *
   * @param liveNodes       the set of live nodes
   * @param collectionState the new collection state (may be null if the collection has been
   *                        deleted)
   * @return true if the watcher should be removed
   */
  boolean onStateChanged(Set<String> liveNodes, DocCollection collectionState);

}