public static void setResourceObserver(ResourceObserver observer) {
    if (observer == null) {
      observer = new ResourceObserverAdapter();
    }
    InternalResourceManager.observer = observer;
  }
For testing only, an observer which is called when rebalancing is started and finished for a
particular region. This observer is called even the "rebalancing" is actually redundancy
recovery for a particular region.
geode-core/src/main/java/org/apache/geode/internal/cache/control/InternalResourceManager.java