FailoverManager.java Used in http://seekingconsistency.com/2013-05-26/simple-cheap-and-effective-high-availability/

FailoverManager.java

public class FailoverManager implements AutoCloseable, ChannelListener, Receiver {
  private static final Logger LOGGER = LoggerFactory.getLogger(FailoverManager.class);

  private static final String HA_CLUSTER = "ha-cluster";

  private final JChannel channel;
  private final int maximumClusterSize;

  private final LeadershipResolver decider;

  public FailoverManager(final String configName,
      final String applicationGroup,
      final int maximumClusterSize,
      final LeadershipListener leadershipListener)
      throws HighAvailabilityException {
    if (maximumClusterSize < 3) {
      throw new IllegalArgumentException("Cluster must contain at least 3 members");
    }

    this.maximumClusterSize = maximumClusterSize;

    try {
      this.channel = new JChannel(configName);
      this.channel.addChannelListener(this);
      this.channel.setReceiver(this);

      this.decider = new LeadershipResolver(this.channel, applicationGroup, leadershipListener);

      this.channel.connect(HA_CLUSTER);

    } catch (final Exception e) {
      throw new HighAvailabilityException("Failed to initialise channel '" + applicationGroup
          + "' and connect to '" + HA_CLUSTER + "'", e);
    }
  }

  private boolean isMajority(final int currentClusterSize) {
    final int majorityThreshold = (this.maximumClusterSize / 2) + 1;
    return (currentClusterSize >= majorityThreshold);
  }

  @Override
  public void viewAccepted(final View newView) {
    LOGGER.info("View accepted {}", newView);

    if (isMajority(newView.getMembers().size())) {
      this.decider.tryLeadership();
    } else {
      this.decider.surrenderLeadership();
    }
  }

  @Override
  public void close() {
    this.channel.close();
    this.decider.close();
  }
 
  /* Other methods omitted for brevity */
}