001/**
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018package org.apache.hadoop.hbase.conf;
019
020import org.apache.hadoop.conf.Configuration;
021import org.apache.yetus.audience.InterfaceAudience;
022import org.apache.yetus.audience.InterfaceStability;
023import org.slf4j.Logger;
024import org.slf4j.LoggerFactory;
025
026import java.util.Collections;
027import java.util.Set;
028import java.util.WeakHashMap;
029
030/**
031 * Maintains the set of all the classes which would like to get notified
032 * when the Configuration is reloaded from the disk in the Online Configuration
033 * Change mechanism, which lets you update certain configuration properties
034 * on-the-fly, without having to restart the cluster.
035 *
036 * If a class has configuration properties which you would like to be able to
037 * change on-the-fly, do the following:
038 * 1. Implement the {@link ConfigurationObserver} interface. This would require
039 *    you to implement the
040 *    {@link ConfigurationObserver#onConfigurationChange(Configuration)}
041 *    method.  This is a callback that is used to notify your class' instance
042 *    that the configuration has changed. In this method, you need to check
043 *    if the new values for the properties that are of interest to your class
044 *    are different from the cached values. If yes, update them.
045 *
046 *    However, be careful with this. Certain properties might be trivially
047 *    mutable online, but others might not. Two properties might be trivially
048 *    mutable by themselves, but not when changed together. For example, if a
049 *    method uses properties "a" and "b" to make some decision, and is running
050 *    in parallel when the notifyOnChange() method updates "a", but hasn't
051 *    yet updated "b", it might make a decision on the basis of a new value of
052 *    "a", and an old value of "b". This might introduce subtle bugs. This
053 *    needs to be dealt on a case-by-case basis, and this class does not provide
054 *    any protection from such cases.
055 *
056 * 2. Register the appropriate instance of the class with the
057 *    {@link ConfigurationManager} instance, using the
058 *    {@link ConfigurationManager#registerObserver(ConfigurationObserver)}
059 *    method. For the RS side of things, the ConfigurationManager is a static
060 *    member of the {@link org.apache.hadoop.hbase.regionserver.HRegionServer}
061 *    class. Be careful not to do this in the constructor, as you might cause
062 *    the 'this' reference to escape. Use a factory method, or an initialize()
063 *    method which is called after the construction of the object.
064 *
065 * 3. Deregister the instance using the
066 *    {@link ConfigurationManager#deregisterObserver(ConfigurationObserver)}
067 *    method when it is going out of scope. In case you are not able to do that
068 *    for any reason, it is still okay, since entries for dead observers are
069 *    automatically collected during GC. But nonetheless, it is still a good
070 *    practice to deregister your observer, whenever possible.
071 *
072 */
073@InterfaceAudience.Private
074@InterfaceStability.Evolving
075public class ConfigurationManager {
076  private static final Logger LOG = LoggerFactory.getLogger(ConfigurationManager.class);
077
078  // The set of Configuration Observers. These classes would like to get
079  // notified when the configuration is reloaded from disk. This is a set
080  // constructed from a WeakHashMap, whose entries would be removed if the
081  // observer classes go out of scope.
082  private final Set<ConfigurationObserver> configurationObservers =
083    Collections.newSetFromMap(new WeakHashMap<ConfigurationObserver,
084                                              Boolean>());
085
086  /**
087   * Register an observer class
088   * @param observer
089   */
090  public void registerObserver(ConfigurationObserver observer) {
091    synchronized (configurationObservers) {
092      configurationObservers.add(observer);
093      if (observer instanceof PropagatingConfigurationObserver) {
094        ((PropagatingConfigurationObserver) observer).registerChildren(this);
095      }
096    }
097  }
098
099  /**
100   * Deregister an observer class
101   * @param observer
102   */
103  public void deregisterObserver(ConfigurationObserver observer) {
104    synchronized (configurationObservers) {
105      configurationObservers.remove(observer);
106      if (observer instanceof PropagatingConfigurationObserver) {
107        ((PropagatingConfigurationObserver) observer).deregisterChildren(this);
108      }
109    }
110  }
111
112  /**
113   * The conf object has been repopulated from disk, and we have to notify
114   * all the observers that are expressed interest to do that.
115   */
116  public void notifyAllObservers(Configuration conf) {
117    LOG.info("Starting to notify all observers that config changed.");
118    synchronized (configurationObservers) {
119      for (ConfigurationObserver observer : configurationObservers) {
120        try {
121          if (observer != null) {
122            observer.onConfigurationChange(conf);
123          }
124        } catch (Throwable t) {
125          LOG.error("Encountered a throwable while notifying observers: " + " of type : " +
126              observer.getClass().getCanonicalName() + "(" + observer + ")", t);
127        }
128      }
129    }
130  }
131
132  /**
133   * @return the number of observers. 
134   */
135  public int getNumObservers() {
136    synchronized (configurationObservers) {
137      return configurationObservers.size();
138    }
139  }
140}