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HBASE-25872 Add documentation for LoadBalancer about synchronization #3267

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May 17, 2021
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HBASE-25872 Add documentation for LoadBalancer about synchronization #3267

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17 changes: 17 additions & 0 deletions hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/LoadBalancer.java
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Expand Up @@ -42,6 +42,23 @@
* cluster.
* <p/>
* This class produces plans for the {@code AssignmentManager} to execute.
* <p/>
* About locking:
* <ul>
* <li>We will first call {@link #setClusterInfoProvider(ClusterInfoProvider)} and then
* {@link #initialize()} to initialize the balancer, and before calling {@link #initialize()}, we
* will never call any methods of this balancer. So these two methods do not need to be
* synchronized.</li>
* <li>The {@link #balanceCluster(Map)} method will use the {@link ClusterMetrics} which is set by
* {@link #updateClusterMetrics(ClusterMetrics)}, and also lots of configurations, which could be
* changed by {@link #onConfigurationChange(Configuration)}, so the easier way is to make these
* three methods synchronized. And since there will be only one balancing thread, this will not
* impact performance too much.</li>
* <li>The {@link #roundRobinAssignment(List, List)}, {@link #retainAssignment(Map, List)} and
* {@link #randomAssignment(RegionInfo, List)} could be called from multiple threads concurrently,
* so these three methods should not be synchronized, the implementation classes need to make sure
* that they are thread safe.</li>
* </ul>
*/
@InterfaceAudience.Private
public interface LoadBalancer extends Stoppable, ConfigurationObserver {
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