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HBASE-16256 Purpose of EnvironmentEdge, EnvironmentEdgeManager (Sai Teja Ranuva)

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stack 2016-07-29 16:34:22 -07:00
parent 3729320099
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hbase-common/src/main/java/org/apache/hadoop/hbase/util/EnvironmentEdgeManager.java
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@ -24,6 +24,40 @@ import org.apache.hadoop.hbase.classification.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.
* <br>
* <b>Original Motivation:</b>
* 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: <a href="https://issues.apache.org/jira/browse/HBASE-2578">HBASE-2578</a> - 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}
<p>
<b>Problems with Environment Edge:</b><br>
1. One of the major problems is the side effects of injecting an Environment Edge into
Environment Edge Manager.<br>
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.<br>
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.<br>
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.<br>
</p>
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 {