/jboss-as-7.1.1.Final/clustering/singleton/src/main/java/org/jboss/as/clustering/singleton/election/SimpleSingletonElectionPolicy.java

 1/*
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 3 * Copyright 2008, Red Hat Middleware LLC, and individual contributors
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 9 * published by the Free Software Foundation; either version 2.1 of
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11 *
12 * This software is distributed in the hope that it will be useful,
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14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 * Lesser General Public License for more details.
16 *
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19 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
20 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
21 */
22package org.jboss.as.clustering.singleton.election;
23
24import java.util.List;
25
26import org.jboss.as.clustering.ClusterNode;
27import org.jboss.as.clustering.singleton.SingletonElectionPolicy;
28
29/**
30 * A simple concrete policy service that decides which node in the cluster should be the master node to run certain HASingleton
31 * service based on attribute "Position". The value will be divided by partition size and only remainder will be used.
32 *
33 * Let's say partition size is n: 0 means the first oldest node. 1 means the 2nd oldest node. ... n-1 means the nth oldest node.
34 *
35 * -1 means the youngest node. -2 means the 2nd youngest node. ... -n means the nth youngest node.
36 *
37 * E.g. the following attribute says the singleton will be running on the 3rd oldest node of the current partition: <attribute
38 * name="Position">2</attribute>
39 *
40 * If no election policy is defined, the oldest node in the cluster runs the singleton. This behaivour can be achieved with this
41 * policy when "position" is set to 0.
42 *
43 * @author <a href="mailto:Alex.Fu@novell.com">Alex Fu</a>
44 * @author <a href="mailto:galder.zamarreno@jboss.com">Galder Zamarreno</a>
45 * @author Paul Ferraro
46 */
47public class SimpleSingletonElectionPolicy implements SingletonElectionPolicy {
48
49    private final int position;
50
51    public SimpleSingletonElectionPolicy() {
52        this(0);
53    }
54
55    public SimpleSingletonElectionPolicy(int position) {
56        this.position = position;
57    }
58
59    @Override
60    public ClusterNode elect(List<ClusterNode> candidates) {
61        int size = candidates.size();
62        return (size > 0) ? candidates.get(((this.position % size) + size) % size) : null;
63    }
64}