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1   /**
2    *
3    * Licensed to the Apache Software Foundation (ASF) under one
4    * or more contributor license agreements.  See the NOTICE file
5    * distributed with this work for additional information
6    * regarding copyright ownership.  The ASF licenses this file
7    * to you under the Apache License, Version 2.0 (the
8    * "License"); you may not use this file except in compliance
9    * with the License.  You may obtain a copy of the License at
10   *
11   *     http://www.apache.org/licenses/LICENSE-2.0
12   *
13   * Unless required by applicable law or agreed to in writing, software
14   * distributed under the License is distributed on an "AS IS" BASIS,
15   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16   * See the License for the specific language governing permissions and
17   * limitations under the License.
18   */
19  package org.apache.hadoop.hbase.master;
20  
21  import java.util.List;
22  import java.util.Map;
23  
24  import org.apache.hadoop.classification.InterfaceAudience;
25  import org.apache.hadoop.conf.Configurable;
26  import org.apache.hadoop.hbase.ClusterStatus;
27  import org.apache.hadoop.hbase.HBaseIOException;
28  import org.apache.hadoop.hbase.HRegionInfo;
29  import org.apache.hadoop.hbase.ServerName;
30  
31  /**
32   * Makes decisions about the placement and movement of Regions across
33   * RegionServers.
34   *
35   * <p>Cluster-wide load balancing will occur only when there are no regions in
36   * transition and according to a fixed period of a time using {@link #balanceCluster(Map)}.
37   *
38   * <p>Inline region placement with {@link #immediateAssignment} can be used when
39   * the Master needs to handle closed regions that it currently does not have
40   * a destination set for.  This can happen during master failover.
41   *
42   * <p>On cluster startup, bulk assignment can be used to determine
43   * locations for all Regions in a cluster.
44   *
45   * <p>This classes produces plans for the {@link AssignmentManager} to execute.
46   */
47  @InterfaceAudience.Public
48  public interface LoadBalancer extends Configurable {
49  
50    /**
51     * Set the current cluster status.  This allows a LoadBalancer to map host name to a server
52     * @param st
53     */
54    void setClusterStatus(ClusterStatus st);
55  
56  
57    /**
58     * Set the master service.
59     * @param masterServices
60     */
61    void setMasterServices(MasterServices masterServices);
62  
63    /**
64     * Perform the major balance operation
65     * @param clusterState
66     * @return List of plans
67     */
68    List<RegionPlan> balanceCluster(Map<ServerName,
69        List<HRegionInfo>> clusterState) throws HBaseIOException;
70  
71    /**
72     * Perform a Round Robin assignment of regions.
73     * @param regions
74     * @param servers
75     * @return Map of servername to regioninfos
76     */
77    Map<ServerName, List<HRegionInfo>> roundRobinAssignment(
78      List<HRegionInfo> regions,
79      List<ServerName> servers
80    ) throws HBaseIOException;
81  
82    /**
83     * Assign regions to the previously hosting region server
84     * @param regions
85     * @param servers
86     * @return List of plans
87     */
88    Map<ServerName, List<HRegionInfo>> retainAssignment(
89      Map<HRegionInfo, ServerName> regions,
90      List<ServerName> servers
91    ) throws HBaseIOException;
92  
93    /**
94     * Sync assign a region
95     * @param regions
96     * @param servers
97      * @return Map regioninfos to servernames
98     */
99    Map<HRegionInfo, ServerName> immediateAssignment(
100     List<HRegionInfo> regions,
101     List<ServerName> servers
102   ) throws HBaseIOException;
103 
104   /**
105    * Get a random region server from the list
106    * @param regionInfo Region for which this selection is being done.
107    * @param servers
108    * @return Servername
109    */
110   ServerName randomAssignment(
111     HRegionInfo regionInfo, List<ServerName> servers
112   ) throws HBaseIOException;
113 }