Java tutorial
/** * Copyright 2016 Yahoo Inc. * * Licensed 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.yahoo.pulsar.broker.loadbalance; import static com.google.common.base.Preconditions.checkState; import java.util.concurrent.ExecutorService; import java.util.concurrent.atomic.AtomicBoolean; import java.util.concurrent.atomic.AtomicReference; import org.apache.bookkeeper.util.ZkUtils; import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.KeeperException.NoNodeException; import org.apache.zookeeper.KeeperException.NodeExistsException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.apache.zookeeper.ZooDefs.Ids; import org.apache.zookeeper.ZooKeeper; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import com.fasterxml.jackson.databind.ObjectMapper; import com.yahoo.pulsar.broker.PulsarService; /** * A class that provides way to elect the leader among brokers. * * */ public class LeaderElectionService { private static final Logger log = LoggerFactory.getLogger(LeaderElectionService.class); private static final String ELECTION_ROOT = "/loadbalance/leader"; private final PulsarService pulsar; private final ExecutorService executor; private boolean stopped = true; private final ZooKeeper zkClient; private final AtomicReference<LeaderBroker> currentLeader = new AtomicReference<LeaderBroker>(); private final AtomicBoolean isLeader = new AtomicBoolean(); private final ObjectMapper jsonMapper; /** * Interface which should be implemented by classes which are interested in the leader election. The listener gets * called when current broker becomes the leader. */ public static interface LeaderListener { void brokerIsTheLeaderNow(); void brokerIsAFollowerNow(); } private final LeaderListener leaderListener; public LeaderElectionService(PulsarService pulsar, LeaderListener leaderListener) { this.pulsar = pulsar; this.zkClient = pulsar.getZkClient(); this.executor = pulsar.getExecutor(); this.leaderListener = leaderListener; this.jsonMapper = new ObjectMapper(); } /** * We try to get the data in the ELECTION_ROOT node. If the node is present (i.e. leader is present), we store it in * the currentLeader and keep a watch on the election node. If we lose the leader, then watch gets triggered and we * do the election again. If the node does not exist while getting the data, we get NoNodeException. This means, * there is no leader and we create the node at ELECTION_ROOT and write the leader broker's service URL in the node. * Once the leader is known, we call the listener method so that leader can take further actions. */ private void elect() { try { byte[] data = zkClient.getData(ELECTION_ROOT, new Watcher() { @Override public void process(WatchedEvent event) { log.warn("Type of the event is [{}] and path is [{}]", event.getType(), event.getPath()); switch (event.getType()) { case NodeDeleted: log.warn("Election node {} is deleted, attempting re-election...", event.getPath()); if (event.getPath().equals(ELECTION_ROOT)) { log.info("This should call elect again..."); executor.execute(new Runnable() { @Override public void run() { // If the node is deleted, attempt the re-election log.info("Broker [{}] is calling re-election from the thread", pulsar.getWebServiceAddress()); elect(); } }); } break; default: log.warn("Got something wrong on watch: {}", event); break; } } }, null); LeaderBroker leaderBroker = jsonMapper.readValue(data, LeaderBroker.class); currentLeader.set(leaderBroker); isLeader.set(false); leaderListener.brokerIsAFollowerNow(); // If broker comes here it is a follower. Do nothing, wait for the watch to trigger log.info("Broker [{}] is the follower now. Waiting for the watch to trigger...", pulsar.getWebServiceAddress()); } catch (NoNodeException nne) { // There's no leader yet... try to become the leader try { // Create the root node and add current broker's URL as its contents LeaderBroker leaderBroker = new LeaderBroker(pulsar.getWebServiceAddress()); ZkUtils.createFullPathOptimistic(pulsar.getLocalZkCache().getZooKeeper(), ELECTION_ROOT, jsonMapper.writeValueAsBytes(leaderBroker), Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL); // Update the current leader and set the flag to true currentLeader.set(new LeaderBroker(leaderBroker.getServiceUrl())); isLeader.set(true); // Notify the listener that this broker is now the leader so that it can collect usage and start load // manager. log.info("Broker [{}] is the leader now, notifying the listener...", pulsar.getWebServiceAddress()); leaderListener.brokerIsTheLeaderNow(); } catch (NodeExistsException nee) { // Re-elect the new leader log.warn( "Got exception [{}] while creating election node because it already exists. Attempting re-election...", nee.getMessage()); executor.execute(new Runnable() { @Override public void run() { elect(); } }); } catch (Exception e) { // Kill the broker because this broker's session with zookeeper might be stale. Killing the broker will // make sure that we get the fresh zookeeper session. log.error("Got exception [{}] while creating the election node", e.getMessage()); pulsar.getShutdownService().shutdown(-1); } } catch (Exception e) { // Kill the broker log.error("Could not get the content of [{}], got exception [{}]. Shutting down the broker...", ELECTION_ROOT, e); pulsar.getShutdownService().shutdown(-1); } } public void start() { checkState(stopped); stopped = false; log.info("LeaderElectionService started"); elect(); } public void stop() { if (stopped) { return; } stopped = true; log.info("LeaderElectionService stopped"); } public LeaderBroker getCurrentLeader() { return currentLeader.get(); } public boolean isLeader() { return isLeader.get(); } }