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

LeadershipResolver.java

package org.neverfear.ha;

import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.Lock;

import org.jgroups.JChannel;
import org.jgroups.blocks.locking.LockService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Resolves who will be leader through the use of a distributed lock.
 * 
 * Should not be called from multiple threads at once (not thread safe).
 */
class LeadershipResolver {
  private static final Logger LOGGER = LoggerFactory.getLogger(LeadershipResolver.class);
  private static final String LOCK_NAME_FORMAT = "ha-%s";

  private final String lockName;
  private final Lock leaderLock;
  private final LeadershipListener leadershipListener;

  private final AtomicBoolean leader = new AtomicBoolean(false);

  public LeadershipResolver(
      final JChannel channel,
      final String name,
      final LeadershipListener leadershipListener) {
    this.lockName = String.format(LOCK_NAME_FORMAT, name);
    final LockService lockService = new LockService(channel);
    this.leaderLock = lockService.getLock(this.lockName);
    this.leadershipListener = leadershipListener;
  }

  public void tryLeadership() {
    LOGGER.info("Trying for lock");
    if (this.leaderLock.tryLock()) {
      LOGGER.info("Got lock");
      setLeader(true);
    } else {
      LOGGER.info("Failed");
      setLeader(false);
    }
  }

  public void surrenderLeadership() {
    this.leaderLock.unlock();
    setLeader(false);
  }

  public void close() {
    surrenderLeadership();
  }

  public boolean isLeader() {
    return this.leader.get();
  }

  private void setLeader(final boolean leader) {
    if (leader) {
      if (this.leader.compareAndSet(false, true)) {
        LOGGER.info("Acquired leadership");
        this.leadershipListener.onAcquired();
      }
    } else {
      if (this.leader.compareAndSet(true, false)) {
        LOGGER.info("Relinquishing leadership");
        this.leadershipListener.onRelinquished();
      }
    }
  }
}