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HBASE-25872 Add documentation for LoadBalancer about synchronization (#3267)

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Signed-off-by: Xin Sun <ddupgs@gmail.com>
This commit is contained in:
Duo Zhang 2021-05-17 10:17:24 +08:00
parent d5d972f92e
commit 439fd4aff2
1 changed files with 26 additions and 12 deletions
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38
hbase-server/src/main/java/org/apache/hadoop/hbase/master/LoadBalancer.java
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@ -33,18 +33,32 @@ import org.apache.hadoop.hbase.conf.ConfigurationObserver;
import org.apache.yetus.audience.InterfaceAudience;
/**
* Makes decisions about the placement and movement of Regions across
* RegionServers.
*
* <p>Cluster-wide load balancing will occur only when there are no regions in
* transition and according to a fixed period of a time using {@link #balanceCluster(Map)}.
*
* <p>On cluster startup, bulk assignment can be used to determine
* locations for all Regions in a cluster.
*
* <p>This class produces plans for the
* {@link org.apache.hadoop.hbase.master.assignment.AssignmentManager}
* to execute.
* Makes decisions about the placement and movement of Regions across RegionServers.
* <p/>
* Cluster-wide load balancing will occur only when there are no regions in transition and according
* to a fixed period of a time using {@link #balanceCluster(Map)}.
* <p/>
* On cluster startup, bulk assignment can be used to determine locations for all Regions in a
* cluster.
* <p/>
* This class produces plans for the {@code AssignmentManager} to execute.
* <p/>
* About locking:
* <ul>
* <li>We will first call {@link #setMasterServices(MasterServices)} 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 {