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*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
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*/
package org.apache.hadoop.hbase.util;
import org.apache.yetus.audience.InterfaceAudience;
/**
* Manages a singleton instance of the environment edge. This class shall implement static versions
* of the interface {@link EnvironmentEdge}, then defer to the delegate on invocation.
* Original Motivation: The main purpose of the Environment Edge Manager was to have better
* control over the tests so that they behave the same when run in any system. (Refer:
* HBASE-2578 - The issue which added
* the {@link org.apache.hadoop.hbase.util.EnvironmentEdgeManager}). The idea is to have a central
* place where time can be assigned in HBase. That makes it easier to inject different
* implementations of time. The default environment edge is the Java Current Time in millis. The
* environment edge manager class is designed to be able to plug in a new implementation of time by
* simply injecting an implementation of {@link org.apache.hadoop.hbase.util.EnvironmentEdge}
* interface to {@link org.apache.hadoop.hbase.util.EnvironmentEdgeManager}
*
* Problems with Environment Edge:
* 1. One of the major problems is the side effects of injecting an Environment Edge into
* Environment Edge Manager.
* For example, A test could inject an edge to fast forward time in order to avoid thread sleep to
* save time, but it could trigger a premature waking up of another thread waiting on a condition
* dependent on time lapse, which could potentially affect the normal working of the system leading
* to failure of tests.
* 2. Every test should ensure it is setting the Environment Edge it needs for the test to perform
* in an expected way. Because another test which might have run before the current test could have
* injected its own custom Environment Edge which may not be applicable to this test. This is still
* solvable but the problem is that the tests can run in parallel leading to different combinations
* of environment edges being injected causing unexpected results.
* 3. Another important issue with respect to injecting time through Environment Edge is that the
* milliseconds unit of time is ingrained throughout the codebase in the form of hardcoded sleep
* time or timeouts that any change of time unit or making it fast or slow can potentially trigger
* unexpected failures due to timeout or unintended flow of execution.
*
* Because of the above issues, only {@link org.apache.hadoop.hbase.util.DefaultEnvironmentEdge} is
* being used, whose implementation of time returns the {@link System#currentTimeMillis()}. It is
* advised not to inject any other {@link org.apache.hadoop.hbase.util.EnvironmentEdge}.
*/
@InterfaceAudience.Private
public class EnvironmentEdgeManager {
private static volatile EnvironmentEdge delegate = new DefaultEnvironmentEdge();
private EnvironmentEdgeManager() {
}
/**
* Retrieves the singleton instance of the {@link EnvironmentEdge} that is being managed.
* @return the edge.
*/
public static EnvironmentEdge getDelegate() {
return delegate;
}
/**
* Resets the managed instance to the default instance: {@link DefaultEnvironmentEdge}.
*/
public static void reset() {
injectEdge(new DefaultEnvironmentEdge());
}
/**
* Injects the given edge such that it becomes the managed entity. If null is passed to this
* method, the default type is assigned to the delegate.
* @param edge the new edge.
*/
public static void injectEdge(EnvironmentEdge edge) {
if (edge == null) {
reset();
} else {
delegate = edge;
}
}
/**
* Defers to the delegate and calls the {@link EnvironmentEdge#currentTime()} method.
* @return current time in millis according to the delegate.
*/
public static long currentTime() {
return getDelegate().currentTime();
}
}