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/*
* 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.flink.runtime.leaderelection;
import org.apache.flink.util.Preconditions;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.recipes.cache.ChildData;
import org.apache.curator.framework.recipes.cache.NodeCache;
import org.apache.curator.framework.recipes.cache.NodeCacheListener;
import org.apache.curator.framework.recipes.leader.LeaderLatch;
import org.apache.curator.framework.recipes.leader.LeaderLatchListener;
import org.apache.curator.framework.state.ConnectionState;
import org.apache.curator.framework.state.ConnectionStateListener;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.data.Stat;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.UUID;
/**
* Leader election service for multiple JobManager. The leading JobManager is elected using
* ZooKeeper. The current leader's address as well as its leader session ID is published via
* ZooKeeper as well.
*/
public class ZooKeeperLeaderElectionService implements LeaderElectionService, LeaderLatchListener, NodeCacheListener {
private static final Logger LOG = LoggerFactory.getLogger(ZooKeeperLeaderElectionService.class);
/** Client to the ZooKeeper quorum */
private final CuratorFramework client;
/** Curator recipe for leader election */
private final LeaderLatch leaderLatch;
/** Curator recipe to watch a given ZooKeeper node for changes */
private final NodeCache cache;
/** ZooKeeper path of the node which stores the current leader information */
private final String leaderPath;
private UUID issuedLeaderSessionID;
private UUID confirmedLeaderSessionID;
/** The leader contender which applies for leadership */
private volatile LeaderContender leaderContender;
private final Object lock = new Object();
private final ConnectionStateListener listener = new ConnectionStateListener() {
@Override
public void stateChanged(CuratorFramework client, ConnectionState newState) {
handleStateChange(newState);
}
};
/**
* Creates a ZooKeeperLeaderElectionService object.
*
* @param client Client which is connected to the ZooKeeper quorum
* @param latchPath ZooKeeper node path for the leader election latch
* @param leaderPath ZooKeeper node path for the node which stores the current leader information
*/
public ZooKeeperLeaderElectionService(CuratorFramework client, String latchPath, String leaderPath) {
this.client = client;
this.leaderPath = leaderPath;
leaderLatch = new LeaderLatch(client, latchPath);
cache = new NodeCache(client, leaderPath);
}
/**
* Returns the current leader session ID or null, if the contender is not the leader.
*
* @return The last leader session ID or null, if the contender is not the leader
*/
public UUID getLeaderSessionID() {
return confirmedLeaderSessionID;
}
@Override
public void start(LeaderContender contender) throws Exception {
Preconditions.checkNotNull(contender, "Contender must not be null.");
Preconditions.checkState(leaderContender == null, "Contender was already set.");
LOG.info("Starting ZooKeeperLeaderElectionService.");
leaderContender = contender;
leaderLatch.addListener(this);
leaderLatch.start();
cache.getListenable().addListener(this);
cache.start();
client.getConnectionStateListenable().addListener(listener);
}
@Override
public void stop() throws Exception{
LOG.info("Stopping ZooKeeperLeaderElectionService.");
client.getConnectionStateListenable().removeListener(listener);
cache.close();
leaderLatch.close();
client.close();
synchronized (lock) {
confirmedLeaderSessionID = null;
issuedLeaderSessionID = null;
}
}
@Override
public void confirmLeaderSessionID(UUID leaderSessionID) {
if (LOG.isDebugEnabled()) {
LOG.debug(
"Confirm leader session ID {} for leader {}.",
leaderSessionID,
leaderContender.getAddress());
}
Preconditions.checkNotNull(leaderSessionID);
if(leaderLatch.hasLeadership()) {
// check if this is an old confirmation call
synchronized (lock) {
if (leaderSessionID.equals(this.issuedLeaderSessionID)) {
confirmedLeaderSessionID = leaderSessionID;
writeLeaderInformation(confirmedLeaderSessionID);
}
}
} else {
LOG.warn("The leader session ID {} was confirmed even though the" +
"corresponding JobManager was not elected as the leader.", leaderSessionID);
}
}
@Override
public boolean hasLeadership() {
return leaderLatch.hasLeadership();
}
@Override
public void isLeader() {
synchronized (lock) {
issuedLeaderSessionID = UUID.randomUUID();
confirmedLeaderSessionID = null;
if (LOG.isDebugEnabled()) {
LOG.debug(
"Grant leadership to contender {} with session ID {}.",
leaderContender.getAddress(),
issuedLeaderSessionID);
}
leaderContender.grantLeadership(issuedLeaderSessionID);
}
}
@Override
public void notLeader() {
synchronized (lock) {
issuedLeaderSessionID = null;
confirmedLeaderSessionID = null;
if (LOG.isDebugEnabled()) {
LOG.debug("Revoke leadership of {}.", leaderContender.getAddress());
}
leaderContender.revokeLeadership();
}
}
@Override
public void nodeChanged() throws Exception {
try {
// leaderSessionID is null if the leader contender has not yet confirmed the session ID
if (leaderLatch.hasLeadership()) {
synchronized (lock) {
if (LOG.isDebugEnabled()) {
LOG.debug(
"Leader node changed while {} is the leader with session ID {}.",
leaderContender.getAddress(),
confirmedLeaderSessionID);
}
if (confirmedLeaderSessionID != null) {
ChildData childData = cache.getCurrentData();
if (childData == null) {
if (LOG.isDebugEnabled()) {
LOG.debug(
"Writing leader information into empty node by {}.",
leaderContender.getAddress());
}
writeLeaderInformation(confirmedLeaderSessionID);
} else {
byte[] data = childData.getData();
if (data == null || data.length == 0) {
// the data field seems to be empty, rewrite information
if (LOG.isDebugEnabled()) {
LOG.debug(
"Writing leader information into node with empty data field by {}.",
leaderContender.getAddress());
}
writeLeaderInformation(confirmedLeaderSessionID);
} else {
ByteArrayInputStream bais = new ByteArrayInputStream(data);
ObjectInputStream ois = new ObjectInputStream(bais);
String leaderAddress = ois.readUTF();
UUID leaderSessionID = (UUID) ois.readObject();
if (!leaderAddress.equals(this.leaderContender.getAddress()) ||
(leaderSessionID == null || !leaderSessionID.equals(confirmedLeaderSessionID))) {
// the data field does not correspond to the expected leader information
if (LOG.isDebugEnabled()) {
LOG.debug(
"Correcting leader information by {}.",
leaderContender.getAddress());
}
writeLeaderInformation(confirmedLeaderSessionID);
}
}
}
}
}
}
} catch (Exception e) {
leaderContender.handleError(new Exception("Could not handle node changed event.", e));
throw e;
}
}
/**
* Writes the current leader's address as well the given leader session ID to ZooKeeper.
*
* @param leaderSessionID Leader session ID which is written to ZooKeeper
*/
protected void writeLeaderInformation(UUID leaderSessionID) {
// this method does not have to be synchronized because the curator framework client
// is thread-safe
try {
if (LOG.isDebugEnabled()) {
LOG.debug(
"Write leader information: Leader={}, session ID={}.",
leaderContender.getAddress(),
leaderSessionID);
}
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeUTF(leaderContender.getAddress());
oos.writeObject(leaderSessionID);
oos.close();
boolean dataWritten = false;
while(!dataWritten && leaderLatch.hasLeadership()) {
Stat stat = client.checkExists().forPath(leaderPath);
if (stat != null) {
long owner = stat.getEphemeralOwner();
long sessionID = client.getZookeeperClient().getZooKeeper().getSessionId();
if(owner == sessionID) {
try {
client.setData().forPath(leaderPath, baos.toByteArray());
dataWritten = true;
} catch (KeeperException.NoNodeException noNode) {
// node was deleted in the meantime
}
} else {
try {
client.delete().forPath(leaderPath);
} catch (KeeperException.NoNodeException noNode) {
// node was deleted in the meantime --> try again
}
}
} else {
try {
client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(
leaderPath,
baos.toByteArray());
dataWritten = true;
} catch (KeeperException.NodeExistsException nodeExists) {
// node has been created in the meantime --> try again
}
}
}
if (LOG.isDebugEnabled()) {
LOG.debug(
"Successfully wrote leader information: Leader={}, session ID={}.",
leaderContender.getAddress(),
leaderSessionID);
}
} catch (Exception e) {
leaderContender.handleError(
new Exception("Could not write leader address and leader session ID to " +
"ZooKeeper.", e));
}
}
protected void handleStateChange(ConnectionState newState) {
switch (newState) {
case CONNECTED:
LOG.debug("Connected to ZooKeeper quorum. Leader election can start.");
break;
case SUSPENDED:
LOG.warn("Connection to ZooKeeper suspended. The contender " + leaderContender.getAddress()
+ " no longer participates in the leader election.");
break;
case RECONNECTED:
LOG.info("Connection to ZooKeeper was reconnected. Leader election can be restarted.");
break;
case LOST:
// Maybe we have to throw an exception here to terminate the JobManager
LOG.warn("Connection to ZooKeeper lost. The contender " + leaderContender.getAddress()
+ " no longer participates in the leader election.");
break;
}
}
}
