| /** |
| * 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 org.apache.commons.logging.Log; |
| import org.apache.commons.logging.LogFactory; |
| import org.apache.hadoop.conf.Configuration; |
| import org.apache.hadoop.hbase.classification.InterfaceAudience; |
| import org.apache.hadoop.hbase.classification.InterfaceStability; |
| |
| import java.util.Collections; |
| import java.util.Set; |
| import java.util.WeakHashMap; |
| |
| /** |
| * 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: |
| * 1. 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. |
| * |
| * 2. Register the appropriate instance of the class with the |
| * {@link ConfigurationManager} instance, using the |
| * {@link ConfigurationManager#registerObserver(ConfigurationObserver)} |
| * method. For the RS side of things, the ConfigurationManager is a static |
| * member of the {@link org.apache.hadoop.hbase.regionserver.HRegionServer} |
| * class. 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. |
| * |
| * 3. 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 Log LOG = LogFactory.getLog(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 Set<ConfigurationObserver> configurationObservers = |
| Collections.newSetFromMap(new WeakHashMap<ConfigurationObserver, |
| Boolean>()); |
| |
| /** |
| * Register an observer class |
| * @param observer |
| */ |
| public void registerObserver(ConfigurationObserver observer) { |
| synchronized (configurationObservers) { |
| configurationObservers.add(observer); |
| if (observer instanceof PropagatingConfigurationObserver) { |
| ((PropagatingConfigurationObserver) observer).registerChildren(this); |
| } |
| } |
| } |
| |
| /** |
| * Deregister an observer class |
| * @param observer |
| */ |
| 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); |
| } |
| } |
| } |
| } |
| |
| /** |
| * @return the number of observers. |
| */ |
| public int getNumObservers() { |
| synchronized (configurationObservers) { |
| return configurationObservers.size(); |
| } |
| } |
| } |