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Common functionality for HBase
/*
* 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();
}
}
/** Returns true if contains the observer, for unit test only */
public boolean containsObserver(ConfigurationObserver observer) {
synchronized (configurationObservers) {
return configurationObservers.contains(observer);
}
}
}