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.HRegionInfo;
28  import org.apache.hadoop.hbase.ServerName;
29  
30  /**
31   * Makes decisions about the placement and movement of Regions across
32   * RegionServers.
33   *
34   * <p>Cluster-wide load balancing will occur only when there are no regions in
35   * transition and according to a fixed period of a time using {@link #balanceCluster(Map)}.
36   *
37   * <p>Inline region placement with {@link #immediateAssignment} can be used when
38   * the Master needs to handle closed regions that it currently does not have
39   * a destination set for.  This can happen during master failover.
40   *
41   * <p>On cluster startup, bulk assignment can be used to determine
42   * locations for all Regions in a cluster.
43   *
44   * <p>This classes produces plans for the {@link AssignmentManager} to execute.
45   */
46  @InterfaceAudience.Public
47  public interface LoadBalancer extends Configurable {
48  
49    /**
50     * Set the current cluster status.  This allows a LoadBalancer to map host name to a server
51     * @param st
52     */
53    public void setClusterStatus(ClusterStatus st);
54  
55  
56    /**
57     * Set the master service.
58     * @param masterServices
59     */
60    public void setMasterServices(MasterServices masterServices);
61  
62    /**
63     * Perform the major balance operation
64     * @param clusterState
65     * @return List of plans
66     */
67    public List<RegionPlan> balanceCluster(Map<ServerName, List<HRegionInfo>> clusterState);
68  
69    /**
70     * Perform a Round Robin assignment of regions.
71     * @param regions
72     * @param servers
73     * @return Map of servername to regioninfos
74     */
75    public Map<ServerName, List<HRegionInfo>> roundRobinAssignment(List<HRegionInfo> regions, List<ServerName> servers);
76  
77    /**
78     * Assign regions to the previously hosting region server
79     * @param regions
80     * @param servers
81     * @return List of plans
82     */
83    public Map<ServerName, List<HRegionInfo>> retainAssignment(Map<HRegionInfo, ServerName> regions, List<ServerName> servers);
84  
85    /**
86     * Sync assign a region
87     * @param regions
88     * @param servers
89      * @return Map regioninfos to servernames
90     */
91    public Map<HRegionInfo, ServerName> immediateAssignment(List<HRegionInfo> regions, List<ServerName> servers);
92  
93    /**
94     * Get a random region server from the list
95     * @param regionInfo Region for which this selection is being done.
96     * @param servers
97     * @return Servername
98     */
99    public ServerName randomAssignment(HRegionInfo regionInfo, 
100 		  List<ServerName> servers);
101 }