1 /**
2 * Licensed to the Apache Software Foundation (ASF) under one
3 * or more contributor license agreements. See the NOTICE file
4 * distributed with this work for additional information
5 * regarding copyright ownership. The ASF licenses this file
6 * to you under the Apache License, Version 2.0 (the
7 * "License"); you may not use this file except in compliance
8 * with the License. You may obtain a copy of the License at
9 *
10 * http://www.apache.org/licenses/LICENSE-2.0
11 *
12 * Unless required by applicable law or agreed to in writing, software
13 * distributed under the License is distributed on an "AS IS" BASIS,
14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15 * See the License for the specific language governing permissions and
16 * limitations under the License.
17 */
18 package org.apache.hadoop.hbase.conf;
19
20 import org.apache.commons.logging.Log;
21 import org.apache.commons.logging.LogFactory;
22 import org.apache.hadoop.conf.Configuration;
23 import org.apache.hadoop.hbase.classification.InterfaceAudience;
24 import org.apache.hadoop.hbase.classification.InterfaceStability;
25
26 import java.util.Collections;
27 import java.util.Set;
28 import java.util.WeakHashMap;
29
30 /**
31 * Maintains the set of all the classes which would like to get notified
32 * when the Configuration is reloaded from the disk in the Online Configuration
33 * Change mechanism, which lets you update certain configuration properties
34 * on-the-fly, without having to restart the cluster.
35 *
36 * If a class has configuration properties which you would like to be able to
37 * change on-the-fly, do the following:
38 * 1. Implement the {@link ConfigurationObserver} interface. This would require
39 * you to implement the
40 * {@link ConfigurationObserver#onConfigurationChange(Configuration)}
41 * method. This is a callback that is used to notify your class' instance
42 * that the configuration has changed. In this method, you need to check
43 * if the new values for the properties that are of interest to your class
44 * are different from the cached values. If yes, update them.
45 *
46 * However, be careful with this. Certain properties might be trivially
47 * mutable online, but others might not. Two properties might be trivially
48 * mutable by themselves, but not when changed together. For example, if a
49 * method uses properties "a" and "b" to make some decision, and is running
50 * in parallel when the notifyOnChange() method updates "a", but hasn't
51 * yet updated "b", it might make a decision on the basis of a new value of
52 * "a", and an old value of "b". This might introduce subtle bugs. This
53 * needs to be dealt on a case-by-case basis, and this class does not provide
54 * any protection from such cases.
55 *
56 * 2. Register the appropriate instance of the class with the
57 * {@link ConfigurationManager} instance, using the
58 * {@link ConfigurationManager#registerObserver(ConfigurationObserver)}
59 * method. For the RS side of things, the ConfigurationManager is a static
60 * member of the {@link org.apache.hadoop.hbase.regionserver.HRegionServer}
61 * class. Be careful not to do this in the constructor, as you might cause
62 * the 'this' reference to escape. Use a factory method, or an initialize()
63 * method which is called after the construction of the object.
64 *
65 * 3. Deregister the instance using the
66 * {@link ConfigurationManager#deregisterObserver(ConfigurationObserver)}
67 * method when it is going out of scope. In case you are not able to do that
68 * for any reason, it is still okay, since entries for dead observers are
69 * automatically collected during GC. But nonetheless, it is still a good
70 * practice to deregister your observer, whenever possible.
71 *
72 */
73 @InterfaceAudience.Private
74 @InterfaceStability.Evolving
75 public class ConfigurationManager {
76 private static final Log LOG = LogFactory.getLog(ConfigurationManager.class);
77
78 // The set of Configuration Observers. These classes would like to get
79 // notified when the configuration is reloaded from disk. This is a set
80 // constructed from a WeakHashMap, whose entries would be removed if the
81 // observer classes go out of scope.
82 private Set<ConfigurationObserver> configurationObservers =
83 Collections.newSetFromMap(new WeakHashMap<ConfigurationObserver,
84 Boolean>());
85
86 /**
87 * Register an observer class
88 * @param observer
89 */
90 public void registerObserver(ConfigurationObserver observer) {
91 synchronized (configurationObservers) {
92 configurationObservers.add(observer);
93 if (observer instanceof PropagatingConfigurationObserver) {
94 ((PropagatingConfigurationObserver) observer).registerChildren(this);
95 }
96 }
97 }
98
99 /**
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 }