Java tutorial
/* * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance * with the License. You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.hadoop.hbase.zookeeper; import java.util.concurrent.atomic.AtomicBoolean; import org.apache.commons.logging.Log; import org.apache.commons.logging.LogFactory; import org.apache.hadoop.hbase.Stoppable; import org.apache.hadoop.hbase.classification.InterfaceAudience; import org.apache.hadoop.hbase.util.Bytes; import org.apache.zookeeper.KeeperException; /** * Handles coordination of a single "leader" instance among many possible * candidates. The first {@link ZKLeaderManager} to successfully create * the given znode becomes the leader, allowing the instance to continue * with whatever processing must be protected. Other {@link ZKLeaderManager} * instances will wait to be notified of changes to the leader znode. * If the current master instance fails, the ephemeral leader znode will * be removed, and all waiting instances will be notified, with the race * to claim the leader znode beginning all over again. * @deprecated Not used */ @Deprecated @InterfaceAudience.Private public class ZKLeaderManager extends ZooKeeperListener { private static Log LOG = LogFactory.getLog(ZKLeaderManager.class); private final AtomicBoolean leaderExists = new AtomicBoolean(); private String leaderZNode; private byte[] nodeId; private Stoppable candidate; public ZKLeaderManager(ZooKeeperWatcher watcher, String leaderZNode, byte[] identifier, Stoppable candidate) { super(watcher); this.leaderZNode = leaderZNode; this.nodeId = identifier; this.candidate = candidate; } public void start() { try { watcher.registerListener(this); String parent = ZKUtil.getParent(leaderZNode); if (ZKUtil.checkExists(watcher, parent) < 0) { ZKUtil.createWithParents(watcher, parent); } } catch (KeeperException ke) { watcher.abort("Unhandled zk exception when starting", ke); candidate.stop("Unhandled zk exception starting up: " + ke.getMessage()); } } @Override public void nodeCreated(String path) { if (leaderZNode.equals(path) && !candidate.isStopped()) { handleLeaderChange(); } } @Override public void nodeDeleted(String path) { if (leaderZNode.equals(path) && !candidate.isStopped()) { handleLeaderChange(); } } private void handleLeaderChange() { try { synchronized (leaderExists) { if (ZKUtil.watchAndCheckExists(watcher, leaderZNode)) { LOG.info("Found new leader for znode: " + leaderZNode); leaderExists.set(true); } else { LOG.info("Leader change, but no new leader found"); leaderExists.set(false); leaderExists.notifyAll(); } } } catch (KeeperException ke) { watcher.abort("ZooKeeper error checking for leader znode", ke); candidate.stop("ZooKeeper error checking for leader: " + ke.getMessage()); } } /** * Blocks until this instance has claimed the leader ZNode in ZooKeeper */ public void waitToBecomeLeader() { while (!candidate.isStopped()) { try { if (ZKUtil.createEphemeralNodeAndWatch(watcher, leaderZNode, nodeId)) { // claimed the leader znode leaderExists.set(true); if (LOG.isDebugEnabled()) { LOG.debug("Claimed the leader znode as '" + Bytes.toStringBinary(nodeId) + "'"); } return; } // if claiming the node failed, there should be another existing node byte[] currentId = ZKUtil.getDataAndWatch(watcher, leaderZNode); if (currentId != null && Bytes.equals(currentId, nodeId)) { // claimed with our ID, but we didn't grab it, possibly restarted? LOG.info("Found existing leader with our ID (" + Bytes.toStringBinary(nodeId) + "), removing"); ZKUtil.deleteNode(watcher, leaderZNode); leaderExists.set(false); } else { LOG.info("Found existing leader with ID: " + Bytes.toStringBinary(nodeId)); leaderExists.set(true); } } catch (KeeperException ke) { watcher.abort("Unexpected error from ZK, stopping candidate", ke); candidate.stop("Unexpected error from ZK: " + ke.getMessage()); return; } // wait for next chance synchronized (leaderExists) { while (leaderExists.get() && !candidate.isStopped()) { try { leaderExists.wait(); } catch (InterruptedException ie) { LOG.debug("Interrupted waiting on leader", ie); } } } } } /** * Removes the leader znode, if it is currently claimed by this instance. */ public void stepDownAsLeader() { try { synchronized (leaderExists) { if (!leaderExists.get()) { return; } byte[] leaderId = ZKUtil.getData(watcher, leaderZNode); if (leaderId != null && Bytes.equals(nodeId, leaderId)) { LOG.info("Stepping down as leader"); ZKUtil.deleteNodeFailSilent(watcher, leaderZNode); leaderExists.set(false); } else { LOG.info("Not current leader, no need to step down"); } } } catch (KeeperException ke) { watcher.abort("Unhandled zookeeper exception removing leader node", ke); candidate.stop("Unhandled zookeeper exception removing leader node: " + ke.getMessage()); } catch (InterruptedException e) { watcher.abort("Unhandled zookeeper exception removing leader node", e); candidate.stop("Unhandled zookeeper exception removing leader node: " + e.getMessage()); } } public boolean hasLeader() { return leaderExists.get(); } }