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 com.dang.crawler.conlony.zookeeper.election; import java.util.ArrayList; import java.util.Collections; import java.util.HashSet; import java.util.List; import java.util.Set; import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooDefs; import org.apache.zookeeper.ZooKeeper; import org.apache.zookeeper.data.Stat; import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** * <p> * A leader election support library implementing the ZooKeeper election recipe. * </p> * <p> * This support library is meant to simplify the construction of an exclusive * leader system on top of Apache ZooKeeper. Any application that can become the * leader (usually a process that provides a service, exclusively) would * configure an instance of this class with their hostname, at least one * listener (an implementation of {@link LeaderElectionAware}), and either an * instance of {@link ZooKeeper} or the proper connection information. Once * configured, invoking {@link #start()} will cause the client to connect to * ZooKeeper and create a leader offer. The library then determines if it has * been elected the leader using the algorithm described below. The client * application can follow all state transitions via the listener callback. * </p> * <p> * Leader election algorithm * </p> * <p> * The library starts in a START state. Through each state transition, a state * start and a state complete event are sent to all listeners. When * {@link #start()} is called, a leader offer is created in ZooKeeper. A leader * offer is an ephemeral sequential node that indicates a process that can act * as a leader for this service. A read of all leader offers is then performed. * The offer with the lowest sequence number is said to be the leader. The * process elected leader will transition to the leader state. All other * processes will transition to a ready state. Internally, the library creates a * ZooKeeper watch on the leader offer with the sequence ID of N - 1 (where N is * the process's sequence ID). If that offer disappears due to a process * failure, the watching process will run through the election determination * process again to see if it should become the leader. Note that sequence ID * may not be contiguous due to failed processes. A process may revoke its offer * to be the leader at any time by calling {@link #stop()}. * </p> * <p> * Guarantees (not) Made and Caveats * </p> * <p> * <ul> * <li>It is possible for a (poorly implemented) process to create a leader * offer, get the lowest sequence ID, but have something terrible occur where it * maintains its connection to ZK (and thus its ephemeral leader offer node) but * doesn't actually provide the service in question. It is up to the user to * ensure any failure to become the leader - and whatever that means in the * context of the user's application - results in a revocation of its leader * offer (i.e. that {@link #stop()} is called).</li> * <li>It is possible for ZK timeouts and retries to play a role in service * liveliness. In other words, if process A has the lowest sequence ID but * requires a few attempts to read the other leader offers' sequence IDs, * election can seem slow. Users should apply timeouts during the determination * process if they need to hit a specific SLA.</li> * <li>The library makes a "best effort" to detect catastrophic failures of the * process. It is possible that an unforeseen event results in (for instance) an * unchecked exception that propagates passed normal error handling code. This * normally doesn't matter as the same exception would almost certain destroy * the entire process and thus the connection to ZK and the leader offer * resulting in another round of leader determination.</li> * </ul> * </p> */ public class LeaderElectionSupport implements Watcher { private static final Logger logger = LoggerFactory.getLogger(LeaderElectionSupport.class); private ZooKeeper zooKeeper; private State state; private Set<LeaderElectionAware> listeners; private String rootNodeName; private LeaderOffer leaderOffer; private String hostName; public LeaderElectionSupport() { state = State.STOP; listeners = Collections.synchronizedSet(new HashSet<LeaderElectionAware>()); } /** * <p> * Start the election process. This method will create a leader offer, * determine its status, and either become the leader or become ready. If an * instance of {@link ZooKeeper} has not yet been configured by the user, a * new instance is created using the connectString and sessionTime specified. * </p> * <p> * Any (anticipated) failures result in a failed event being sent to all * listeners. * </p> */ public synchronized void start() { state = State.START; dispatchEvent(EventType.START); logger.info("Starting leader election support"); if (zooKeeper == null) { throw new IllegalStateException("No instance of zookeeper provided. Hint: use setZooKeeper()"); } if (hostName == null) { throw new IllegalStateException("No hostname provided. Hint: use setHostName()"); } try { makeOffer(); determineElectionStatus(); } catch (KeeperException e) { becomeFailed(e); return; } catch (InterruptedException e) { becomeFailed(e); return; } } /** * Stops all election services, revokes any outstanding leader offers, and * disconnects from ZooKeeper. */ public synchronized void stop() { state = State.STOP; dispatchEvent(EventType.STOP_START); logger.info("Stopping leader election support"); if (leaderOffer != null) { try { zooKeeper.delete(leaderOffer.getNodePath(), -1); logger.info("Removed leader offer {}", leaderOffer.getNodePath()); } catch (InterruptedException e) { becomeFailed(e); } catch (KeeperException e) { becomeFailed(e); } } dispatchEvent(EventType.STOP_COMPLETE); } private void makeOffer() throws KeeperException, InterruptedException { state = State.OFFER; dispatchEvent(EventType.OFFER_START); leaderOffer = new LeaderOffer(); leaderOffer.setHostName(hostName); leaderOffer.setNodePath(zooKeeper.create(rootNodeName + "/" + "n_", hostName.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL)); logger.debug("Created leader offer {}", leaderOffer); dispatchEvent(EventType.OFFER_COMPLETE); } private void determineElectionStatus() throws KeeperException, InterruptedException { state = State.DETERMINE; dispatchEvent(EventType.DETERMINE_START); String[] components = leaderOffer.getNodePath().split("/"); leaderOffer.setId(Integer.valueOf(components[components.length - 1].substring("n_".length()))); List<LeaderOffer> leaderOffers = toLeaderOffers(zooKeeper.getChildren(rootNodeName, false)); /* * For each leader offer, find out where we fit in. If we're first, we * become the leader. If we're not elected the leader, attempt to stat the * offer just less than us. If they exist, watch for their failure, but if * they don't, become the leader. */ for (int i = 0; i < leaderOffers.size(); i++) { LeaderOffer leaderOffer = leaderOffers.get(i); if (leaderOffer.getId().equals(this.leaderOffer.getId())) { logger.debug("There are {} leader offers. I am {} in line.", leaderOffers.size(), i); dispatchEvent(EventType.DETERMINE_COMPLETE); if (i == 0) { becomeLeader(); } else { becomeReady(leaderOffers.get(i - 1)); } /* Once we've figured out where we are, we're done. */ break; } } } private void becomeReady(LeaderOffer neighborLeaderOffer) throws KeeperException, InterruptedException { dispatchEvent(EventType.READY_START); logger.info("{} not elected leader. Watching node:{}", leaderOffer.getNodePath(), neighborLeaderOffer.getNodePath()); /* * Make sure to pass an explicit Watcher because we could be sharing this * zooKeeper instance with someone else. */ Stat stat = zooKeeper.exists(neighborLeaderOffer.getNodePath(), this); if (stat != null) { logger.debug("We're behind {} in line and they're alive. Keeping an eye on them.", neighborLeaderOffer.getNodePath()); state = State.READY; dispatchEvent(EventType.READY_COMPLETE); } else { /* * If the stat fails, the node has gone missing between the call to * getChildren() and exists(). We need to try and become the leader. */ logger.info("We were behind {} but it looks like they died. Back to determination.", neighborLeaderOffer.getNodePath()); determineElectionStatus(); } } private void becomeLeader() { state = State.ELECTED; dispatchEvent(EventType.ELECTED_START); logger.info("Becoming leader with node:{}", leaderOffer.getNodePath()); dispatchEvent(EventType.ELECTED_COMPLETE); } private void becomeFailed(Exception e) { logger.error("Failed in state {} - Exception:{}", state, e); state = State.FAILED; dispatchEvent(EventType.FAILED); } /** * Fetch the (user supplied) hostname of the current leader. Note that by the * time this method returns, state could have changed so do not depend on this * to be strongly consistent. This method has to read all leader offers from * ZooKeeper to deterime who the leader is (i.e. there is no caching) so * consider the performance implications of frequent invocation. If there are * no leader offers this method returns null. * * @return hostname of the current leader * @throws KeeperException * @throws InterruptedException */ public String getLeaderHostName() throws KeeperException, InterruptedException { List<LeaderOffer> leaderOffers = toLeaderOffers(zooKeeper.getChildren(rootNodeName, false)); if (leaderOffers.size() > 0) { return leaderOffers.get(0).getHostName(); } return null; } private List<LeaderOffer> toLeaderOffers(List<String> strings) throws KeeperException, InterruptedException { List<LeaderOffer> leaderOffers = new ArrayList<LeaderOffer>(strings.size()); /* * Turn each child of rootNodeName into a leader offer. This is a tuple of * the sequence number and the node name. */ for (String offer : strings) { String hostName = new String(zooKeeper.getData(rootNodeName + "/" + offer, false, null)); leaderOffers.add(new LeaderOffer(Integer.valueOf(offer.substring("n_".length())), rootNodeName + "/" + offer, hostName)); } /* * We sort leader offers by sequence number (which may not be zero-based or * contiguous) and keep their paths handy for setting watches. */ Collections.sort(leaderOffers, new LeaderOffer.IdComparator()); return leaderOffers; } @Override public void process(WatchedEvent event) { if (event.getType().equals(Event.EventType.NodeDeleted)) { if (!event.getPath().equals(leaderOffer.getNodePath()) && state != State.STOP) { logger.debug("Node {} deleted. Need to run through the election process.", event.getPath()); try { determineElectionStatus(); } catch (KeeperException e) { becomeFailed(e); } catch (InterruptedException e) { becomeFailed(e); } } } } private void dispatchEvent(EventType eventType) { logger.debug("Dispatching event:{}", eventType); synchronized (listeners) { if (listeners.size() > 0) { for (LeaderElectionAware observer : listeners) { observer.onElectionEvent(eventType); } } } } /** * Adds {@code listener} to the list of listeners who will receive events. * * @param listener */ public void addListener(LeaderElectionAware listener) { listeners.add(listener); } /** * Remove {@code listener} from the list of listeners who receive events. * * @param listener */ public void removeListener(LeaderElectionAware listener) { listeners.remove(listener); } @Override public String toString() { return "{ state:" + state + " leaderOffer:" + leaderOffer + " zooKeeper:" + zooKeeper + " hostName:" + hostName + " listeners:" + listeners + " }"; } /** * <p> * Gets the ZooKeeper root node to use for this service. * </p> * <p> * For instance, a root node of {@code /mycompany/myservice} would be the * parent of all leader offers for this service. Obviously all processes that * wish to contend for leader status need to use the same root node. Note: We * assume this node already exists. * </p> * * @return a znode path */ public String getRootNodeName() { return rootNodeName; } /** * <p> * Sets the ZooKeeper root node to use for this service. * </p> * <p> * For instance, a root node of {@code /mycompany/myservice} would be the * parent of all leader offers for this service. Obviously all processes that * wish to contend for leader status need to use the same root node. Note: We * assume this node already exists. * </p> */ public void setRootNodeName(String rootNodeName) { this.rootNodeName = rootNodeName; } /** * The {@link ZooKeeper} instance to use for all operations. Provided this * overrides any connectString or sessionTimeout set. */ public ZooKeeper getZooKeeper() { return zooKeeper; } public void setZooKeeper(ZooKeeper zooKeeper) { this.zooKeeper = zooKeeper; } /** * The hostname of this process. Mostly used as a convenience for logging and * to respond to {@link #getLeaderHostName()} requests. */ public String getHostName() { return hostName; } public void setHostName(String hostName) { this.hostName = hostName; } /** * The type of event. */ public static enum EventType { START, OFFER_START, OFFER_COMPLETE, DETERMINE_START, DETERMINE_COMPLETE, ELECTED_START, ELECTED_COMPLETE, READY_START, READY_COMPLETE, FAILED, STOP_START, STOP_COMPLETE, } /** * The internal state of the election support service. */ public static enum State { START, OFFER, DETERMINE, ELECTED, READY, FAILED, STOP } }