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Common functionality for HBase
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/*
 * 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.hadoop.hbase.conf;

import java.util.Collections;
import java.util.Set;
import java.util.WeakHashMap;
import org.apache.hadoop.conf.Configuration;
import org.apache.yetus.audience.InterfaceAudience;
import org.apache.yetus.audience.InterfaceStability;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Maintains the set of all the classes which would like to get notified when the Configuration is
 * reloaded from the disk in the Online Configuration Change mechanism, which lets you update
 * certain configuration properties on-the-fly, without having to restart the cluster.
 * 
 * If a class has configuration properties which you would like to be able to change on-the-fly, do
 * the following:
 * 

 * Implement the {@link ConfigurationObserver} interface. This would require you to implement
 * the {@link ConfigurationObserver#onConfigurationChange(Configuration)} method. This is a callback
 * that is used to notify your class' instance that the configuration has changed. In this method,
 * you need to check if the new values for the properties that are of interest to your class are
 * different from the cached values. If yes, update them. 

 * However, be careful with this. Certain properties might be trivially mutable online, but others
 * might not. Two properties might be trivially mutable by themselves, but not when changed
 * together. For example, if a method uses properties "a" and "b" to make some decision, and is
 * running in parallel when the notifyOnChange() method updates "a", but hasn't yet updated "b", it
 * might make a decision on the basis of a new value of "a", and an old value of "b". This might
 * introduce subtle bugs. This needs to be dealt on a case-by-case basis, and this class does not
 * provide any protection from such cases.
 * Register the appropriate instance of the class with the {@link ConfigurationManager}
 * instance, using the {@link ConfigurationManager#registerObserver(ConfigurationObserver)} method.
 * Be careful not to do this in the constructor, as you might cause the 'this' reference to escape.
 * Use a factory method, or an initialize() method which is called after the construction of the
 * object.
 * Deregister the instance using the
 * {@link ConfigurationManager#deregisterObserver(ConfigurationObserver)} method when it is going
 * out of scope. In case you are not able to do that for any reason, it is still okay, since entries
 * for dead observers are automatically collected during GC. But nonetheless, it is still a good
 * practice to deregister your observer, whenever possible.
 * 
 * 
 */
@InterfaceAudience.Private
@InterfaceStability.Evolving
public class ConfigurationManager {
  private static final Logger LOG = LoggerFactory.getLogger(ConfigurationManager.class);

  // The set of Configuration Observers. These classes would like to get
  // notified when the configuration is reloaded from disk. This is a set
  // constructed from a WeakHashMap, whose entries would be removed if the
  // observer classes go out of scope.
  private final Set configurationObservers =
    Collections.newSetFromMap(new WeakHashMap<>());

  /**
   * Register an observer class
   * @param observer observer to be registered.
   */
  public void registerObserver(ConfigurationObserver observer) {
    synchronized (configurationObservers) {
      configurationObservers.add(observer);
      if (observer instanceof PropagatingConfigurationObserver) {
        ((PropagatingConfigurationObserver) observer).registerChildren(this);
      }
    }
  }

  /**
   * Deregister an observer class
   * @param observer to be deregistered.
   */
  public void deregisterObserver(ConfigurationObserver observer) {
    synchronized (configurationObservers) {
      configurationObservers.remove(observer);
      if (observer instanceof PropagatingConfigurationObserver) {
        ((PropagatingConfigurationObserver) observer).deregisterChildren(this);
      }
    }
  }

  /**
   * The conf object has been repopulated from disk, and we have to notify all the observers that
   * are expressed interest to do that.
   */
  public void notifyAllObservers(Configuration conf) {
    LOG.info("Starting to notify all observers that config changed.");
    synchronized (configurationObservers) {
      for (ConfigurationObserver observer : configurationObservers) {
        try {
          if (observer != null) {
            observer.onConfigurationChange(conf);
          }
        } catch (Throwable t) {
          LOG.error("Encountered a throwable while notifying observers: of type : {}({})",
            observer.getClass().getCanonicalName(), observer, t);
        }
      }
    }
  }

  /** Returns the number of observers. */
  public int getNumObservers() {
    synchronized (configurationObservers) {
      return configurationObservers.size();
    }
  }
}