narendly commented on a change in pull request #690: Reset the WAGED rebalancer
once the controller newly acquires leadership.
URL: https://github.com/apache/helix/pull/690#discussion_r371440400
##########
File path:
helix-core/src/main/java/org/apache/helix/controller/GenericHelixController.java
##########
@@ -1258,4 +1293,57 @@ private void initPipeline(Thread eventThread,
BaseControllerDataProvider cache)
eventThread.setDaemon(true);
eventThread.start();
}
+
+ /**
+ * A wrapper class for the stateful rebalancer instance that will be tracked
in the
+ * GenericHelixController.
+ */
+ private abstract class StatefulRebalancerRef<T extends StatefulRebalancer> {
+ private T _rebalancer = null;
+ private boolean _isRebalancerValid = true;
+
+ /**
+ * @param helixManager
+ * @return A new stateful rebalancer instance with initial state.
+ */
+ protected abstract T createRebalancer(HelixManager helixManager);
+
+ /**
+ * Mark the current rebalancer object to be invalid, which indicates it
needs to be reset before
+ * the next usage.
+ */
+ synchronized void invalidateRebalancer() {
+ _isRebalancerValid = false;
+ }
+
+ /**
+ * @return A valid rebalancer object.
+ * If the rebalancer is no longer valid, it will be reset before
returning.
+ */
+ synchronized T getRebalancer(HelixManager helixManager) {
+ // Lazily initialize the stateful rebalancer instance since the
GenericHelixController
+ // instance is instantiated without the HelixManager information that is
required.
+ if (_rebalancer == null) {
+ _rebalancer = createRebalancer(helixManager);
+ _isRebalancerValid = true;
+ }
+ // If the rebalance exists but has been marked as invalid (due to
leadership switch), it needs
+ // to be reset before return.
+ if (!_isRebalancerValid) {
+ _rebalancer.reset();
+ _isRebalancerValid = true;
+ }
+ return _rebalancer;
Review comment:
Should we follow the lazy-initialization idiom (using volatile keyword on
the singleton objects)?
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