@InterfaceAudience.Private
public class EnvironmentEdgeManager
extends Object

Manages a singleton instance of the environment edge. This class shall implement static versions of the interface 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 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 EnvironmentEdge interface to 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 DefaultEnvironmentEdge is being used, whose implementation of time returns the System.currentTimeMillis(). It is advised not to inject any other EnvironmentEdge.