/**
* 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.zookeeper.server.quorum;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.zookeeper.jmx.MBeanRegistry;
import org.apache.zookeeper.server.quorum.QuorumCnxManager.Message;
import org.apache.zookeeper.server.quorum.QuorumPeer.LearnerType;
import org.apache.zookeeper.server.quorum.QuorumPeer.QuorumServer;
import org.apache.zookeeper.server.quorum.QuorumPeer.ServerState;
import org.apache.zookeeper.server.util.ZxidUtils;
/**
* Implementation of leader election using TCP. It uses an object of the class
* QuorumCnxManager to manage connections. Otherwise, the algorithm is push-based
* as with the other UDP implementations.
*
* There are a few parameters that can be tuned to change its behavior. First,
* finalizeWait determines the amount of time to wait until deciding upon a leader.
* This is part of the leader election algorithm.
*/
public class FastLeaderElection implements Election {
private static final Logger LOG = LoggerFactory.getLogger(FastLeaderElection.class);
/**
* Determine how much time a process has to wait
* once it believes that it has reached the end of
* leader election.
*/
final static int finalizeWait = 200;
/**
* Upper bound on the amount of time between two consecutive
* notification checks. This impacts the amount of time to get
* the system up again after long partitions. Currently 60 seconds.
*/
final static int maxNotificationInterval = 60000;
/**
* Connection manager. Fast leader election uses TCP for
* communication between peers, and QuorumCnxManager manages
* such connections.
*/
QuorumCnxManager manager;
/**
* Notifications are messages that let other peers know that
* a given peer has changed its vote, either because it has
* joined leader election or because it learned of another
* peer with higher zxid or same zxid and higher server id
*/
static public class Notification {
/*
* Proposed leader
*/
long leader;
/*
* zxid of the proposed leader
*/
long zxid;
/*
* Epoch
*/
long electionEpoch;
/*
* current state of sender
*/
QuorumPeer.ServerState state;
/*
* Address of sender
*/
long sid;
/*
* epoch of the proposed leader
*/
long peerEpoch;
}
/**
* Messages that a peer wants to send to other peers.
* These messages can be both Notifications and Acks
* of reception of notification.
*/
static public class ToSend {
static enum mType {crequest, challenge, notification, ack}
ToSend(mType type,
long leader,
long zxid,
long electionEpoch,
ServerState state,
long sid,
long peerEpoch) {
this.leader = leader;
this.zxid = zxid;
this.electionEpoch = electionEpoch;
this.state = state;
this.sid = sid;
this.peerEpoch = peerEpoch;
}
/*
* Proposed leader in the case of notification
*/
long leader;
/*
* id contains the tag for acks, and zxid for notifications
*/
long zxid;
/*
* Epoch
*/
long electionEpoch;
/*
* Current state;
*/
QuorumPeer.ServerState state;
/*
* Address of recipient
*/
long sid;
/*
* Leader epoch
*/
long peerEpoch;
}
LinkedBlockingQueue<ToSend> sendqueue;
LinkedBlockingQueue<Notification> recvqueue;
/**
* Multi-threaded implementation of message handler. Messenger
* implements two sub-classes: WorkReceiver and WorkSender. The
* functionality of each is obvious from the name. Each of these
* spawns a new thread.
*/
private class Messenger {
/**
* Receives messages from instance of QuorumCnxManager on
* method run(), and processes such messages.
*/
class WorkerReceiver implements Runnable {
volatile boolean stop;
QuorumCnxManager manager;
WorkerReceiver(QuorumCnxManager manager) {
this.stop = false;
this.manager = manager;
}
public void run() {
Message response;
while (!stop) {
// Sleeps on receive
try{
response = manager.pollRecvQueue(3000, TimeUnit.MILLISECONDS);
if(response == null) continue;
/*
* If it is from an observer, respond right away.
* Note that the following predicate assumes that
* if a server is not a follower, then it must be
* an observer. If we ever have any other type of
* learner in the future, we'll have to change the
* way we check for observers.
*/
if(!self.getVotingView().containsKey(response.sid)){
Vote current = self.getCurrentVote();
ToSend notmsg = new ToSend(ToSend.mType.notification,
current.getId(),
current.getZxid(),
logicalclock,
self.getPeerState(),
response.sid,
current.getPeerEpoch());
sendqueue.offer(notmsg);
} else {
// Receive new message
if (LOG.isDebugEnabled()) {
LOG.debug("Receive new notification message. My id = "
+ self.getId());
}
/*
* We check for 28 bytes for backward compatibility
*/
if (response.buffer.capacity() < 28) {
LOG.error("Got a short response: "
+ response.buffer.capacity());
continue;
}
boolean backCompatibility = (response.buffer.capacity() == 28);
response.buffer.clear();
// State of peer that sent this message
QuorumPeer.ServerState ackstate = QuorumPeer.ServerState.LOOKING;
switch (response.buffer.getInt()) {
case 0:
ackstate = QuorumPeer.ServerState.LOOKING;
break;
case 1:
ackstate = QuorumPeer.ServerState.FOLLOWING;
break;
case 2:
ackstate = QuorumPeer.ServerState.LEADING;
break;
case 3:
ackstate = QuorumPeer.ServerState.OBSERVING;
break;
}
// Instantiate Notification and set its attributes
Notification n = new Notification();
n.leader = response.buffer.getLong();
n.zxid = response.buffer.getLong();
n.electionEpoch = response.buffer.getLong();
n.state = ackstate;
n.sid = response.sid;
if(!backCompatibility){
n.peerEpoch = response.buffer.getLong();
} else {
if(LOG.isInfoEnabled()){
LOG.info("Backward compatibility mode, server id: " + n.sid);
}
n.peerEpoch = ZxidUtils.getEpochFromZxid(n.zxid);
}
/*
* Print notification info
*/
if(LOG.isInfoEnabled()){
printNotification(n);
}
/*
* If this server is looking, then send proposed leader
*/
if(self.getPeerState() == QuorumPeer.ServerState.LOOKING){
recvqueue.offer(n);
/*
* Send a notification back if the peer that sent this
* message is also looking and its logical clock is
* lagging behind.
*/
if((ackstate == QuorumPeer.ServerState.LOOKING)
&& (n.electionEpoch < logicalclock)){
Vote v = getVote();
ToSend notmsg = new ToSend(ToSend.mType.notification,
v.getId(),
v.getZxid(),
logicalclock,
self.getPeerState(),
response.sid,
v.getPeerEpoch());
sendqueue.offer(notmsg);
}
} else {
/*
* If this server is not looking, but the one that sent the ack
* is looking, then send back what it believes to be the leader.
*/
Vote current = self.getCurrentVote();
if(ackstate == QuorumPeer.ServerState.LOOKING){
if(LOG.isDebugEnabled()){
LOG.debug("Sending new notification. My id = " +
self.getId() + ", Recipient = " +
response.sid + " zxid =" +
current.getZxid() + " leader=" +
current.getId());
}
ToSend notmsg = new ToSend(
ToSend.mType.notification,
current.getId(),
current.getZxid(),
logicalclock,
self.getPeerState(),
response.sid,
current.getPeerEpoch());
sendqueue.offer(notmsg);
}
}
}
} catch (InterruptedException e) {
System.out.println("Interrupted Exception while waiting for new message" +
e.toString());
}
}
LOG.info("WorkerReceiver is down");
}
}
/**
* This worker simply dequeues a message to send and
* and queues it on the manager's queue.
*/
class WorkerSender implements Runnable {
volatile boolean stop;
QuorumCnxManager manager;
WorkerSender(QuorumCnxManager manager){
this.stop = false;
this.manager = manager;
}
public void run() {
while (!stop) {
try {
ToSend m = sendqueue.poll(3000, TimeUnit.MILLISECONDS);
if(m == null) continue;
process(m);
} catch (InterruptedException e) {
break;
}
}
LOG.info("WorkerSender is down");
}
/**
* Called by run() once there is a new message to send.
*
* @param m message to send
*/
private void process(ToSend m) {
byte requestBytes[] = new byte[36];
ByteBuffer requestBuffer = ByteBuffer.wrap(requestBytes);
/*
* Building notification packet to send
*/
requestBuffer.clear();
requestBuffer.putInt(m.state.ordinal());
requestBuffer.putLong(m.leader);
requestBuffer.putLong(m.zxid);
requestBuffer.putLong(m.electionEpoch);
requestBuffer.putLong(m.peerEpoch);
manager.toSend(m.sid, requestBuffer);
}
}
/**
* Test if both send and receive queues are empty.
*/
public boolean queueEmpty() {
return (sendqueue.isEmpty() || recvqueue.isEmpty());
}
WorkerSender ws;
WorkerReceiver wr;
/**
* Constructor of class Messenger.
*
* @param manager Connection manager
*/
Messenger(QuorumCnxManager manager) {
this.ws = new WorkerSender(manager);
Thread t = new Thread(this.ws,
"WorkerSender[myid=" + self.getId() + "]");
t.setDaemon(true);
t.start();
this.wr = new WorkerReceiver(manager);
t = new Thread(this.wr,
"WorkerReceiver[myid=" + self.getId() + "]");
t.setDaemon(true);
t.start();
}
/**
* Stops instances of WorkerSender and WorkerReceiver
*/
void halt(){
this.ws.stop = true;
this.wr.stop = true;
}
}
QuorumPeer self;
Messenger messenger;
volatile long logicalclock; /* Election instance */
long proposedLeader;
long proposedZxid;
long proposedEpoch;
/**
* Returns the current vlue of the logical clock counter
*/
public long getLogicalClock(){
return logicalclock;
}
/**
* Constructor of FastLeaderElection. It takes two parameters, one
* is the QuorumPeer object that instantiated this object, and the other
* is the connection manager. Such an object should be created only once
* by each peer during an instance of the ZooKeeper service.
*
* @param self QuorumPeer that created this object
* @param manager Connection manager
*/
public FastLeaderElection(QuorumPeer self, QuorumCnxManager manager){
this.stop = false;
this.manager = manager;
starter(self, manager);
}
/**
* This method is invoked by the constructor. Because it is a
* part of the starting procedure of the object that must be on
* any constructor of this class, it is probably best to keep as
* a separate method. As we have a single constructor currently,
* it is not strictly necessary to have it separate.
*
* @param self QuorumPeer that created this object
* @param manager Connection manager
*/
private void starter(QuorumPeer self, QuorumCnxManager manager) {
this.self = self;
proposedLeader = -1;
proposedZxid = -1;
sendqueue = new LinkedBlockingQueue<ToSend>();
recvqueue = new LinkedBlockingQueue<Notification>();
this.messenger = new Messenger(manager);
}
private void leaveInstance(Vote v) {
if(LOG.isDebugEnabled()){
LOG.debug("About to leave FLE instance: Leader= "
+ v.getId() + ", Zxid = " +
v.getZxid() + ", My id = " + self.getId()
+ ", My state = " + self.getPeerState());
}
recvqueue.clear();
}
public QuorumCnxManager getCnxManager(){
return manager;
}
volatile boolean stop;
public void shutdown(){
stop = true;
LOG.debug("Shutting down connection manager");
manager.halt();
LOG.debug("Shutting down messenger");
messenger.halt();
LOG.debug("FLE is down");
}
/**
* Send notifications to all peers upon a change in our vote
*/
private void sendNotifications() {
for (QuorumServer server : self.getVotingView().values()) {
long sid = server.id;
ToSend notmsg = new ToSend(ToSend.mType.notification,
proposedLeader,
proposedZxid,
logicalclock,
QuorumPeer.ServerState.LOOKING,
sid,
proposedEpoch);
if(LOG.isDebugEnabled()){
LOG.debug("Sending Notification: " + proposedLeader + " (n.leader), " +
proposedZxid + " (n.zxid), " + logicalclock +
" (n.round), " + sid + " (recipient), " + self.getId() +
" (myid), " + proposedEpoch + " (n.peerEpoch)");
}
sendqueue.offer(notmsg);
}
}
private void printNotification(Notification n){
LOG.info("Notification: " + n.leader + " (n.leader), " + n.zxid +
" (n.zxid), " + n.electionEpoch + " (n.round), " + n.state +
" (n.state), " + n.sid + " (n.sid), " + n.peerEpoch + " (n.peerEPoch), " +
self.getPeerState() + " (my state)");
}
/**
* Check if a pair (server id, zxid) succeeds our
* current vote.
*
* @param id Server identifier
* @param zxid Last zxid observed by the issuer of this vote
*/
private boolean totalOrderPredicate(long newId, long newZxid, long newEpoch, long curId, long curZxid, long curEpoch) {
LOG.debug("id: " + newId + ", proposed id: " + curId + ", zxid: " +
newZxid + ", proposed zxid: " + curZxid);
if(self.getQuorumVerifier().getWeight(newId) == 0){
return false;
}
return ((newEpoch > curEpoch) ||
((newEpoch == curEpoch) && (newZxid > curZxid)) ||
((newZxid == curZxid) && (newId > curId)));
}
/**
* Termination predicate. Given a set of votes, determines if
* have sufficient to declare the end of the election round.
*
* @param votes Set of votes
* @param l Identifier of the vote received last
* @param zxid zxid of the the vote received last
*/
private boolean termPredicate(
HashMap<Long, Vote> votes,
Vote vote) {
HashSet<Long> set = new HashSet<Long>();
/*
* First make the views consistent. Sometimes peers will have
* different zxids for a server depending on timing.
*/
for (Map.Entry<Long,Vote> entry : votes.entrySet()) {
if (vote.equals(entry.getValue())){
set.add(entry.getKey());
}
}
return self.getQuorumVerifier().containsQuorum(set);
}
/**
* In the case there is a leader elected, and a quorum supporting
* this leader, we have to check if the leader has voted and acked
* that it is leading. We need this check to avoid that peers keep
* electing over and over a peer that has crashed and it is no
* longer leading.
*
* @param votes set of votes
* @param leader leader id
* @param electionEpoch epoch id
*/
private boolean checkLeader(
HashMap<Long, Vote> votes,
long leader,
long electionEpoch){
boolean predicate = true;
/*
* If everyone else thinks I'm the leader, I must be the leader.
* The other two checks are just for the case in which I'm not the
* leader. If I'm not the leader and I haven't received a message
* from leader stating that it is leading, then predicate is false.
*/
if(leader != self.getId()){
if(votes.get(leader) == null) predicate = false;
else if(votes.get(leader).getState() != ServerState.LEADING) predicate = false;
}
return predicate;
}
synchronized void updateProposal(long leader, long zxid, long epoch){
if(LOG.isDebugEnabled()){
LOG.debug("Updating proposal: " + leader + " (newleader), " + zxid +
" (newzxid), " + proposedLeader + " (oldleader), " +
proposedZxid + " (oldzxid)");
}
proposedLeader = leader;
proposedZxid = zxid;
proposedEpoch = epoch;
}
synchronized Vote getVote(){
return new Vote(proposedLeader, proposedZxid, proposedEpoch);
}
/**
* A learning state can be either FOLLOWING or OBSERVING.
* This method simply decides which one depending on the
* role of the server.
*
* @return ServerState
*/
private ServerState learningState(){
if(self.getLearnerType() == LearnerType.PARTICIPANT){
LOG.debug("I'm a participant: " + self.getId());
return ServerState.FOLLOWING;
}
else{
LOG.debug("I'm an observer: " + self.getId());
return ServerState.OBSERVING;
}
}
/**
* Returns the initial vote value of server identifier.
*
* @return long
*/
private long getInitId(){
if(self.getLearnerType() == LearnerType.PARTICIPANT)
return self.getId();
else return Long.MIN_VALUE;
}
/**
* Returns initial last logged zxid.
*
* @return long
*/
private long getInitLastLoggedZxid(){
if(self.getLearnerType() == LearnerType.PARTICIPANT)
return self.getLastLoggedZxid();
else return Long.MIN_VALUE;
}
/**
* Returns the initial vote value of the peer epoch.
*
* @return long
*/
private long getPeerEpoch(){
if(self.getLearnerType() == LearnerType.PARTICIPANT)
try {
return self.getCurrentEpoch();
} catch(IOException e) {
RuntimeException re = new RuntimeException(e.getMessage());
re.setStackTrace(e.getStackTrace());
throw re;
}
else return Long.MIN_VALUE;
}
/**
* Starts a new round of leader election. Whenever our QuorumPeer
* changes its state to LOOKING, this method is invoked, and it
* sends notifications to all other peers.
*/
public Vote lookForLeader() throws InterruptedException {
try {
self.jmxLeaderElectionBean = new LeaderElectionBean();
MBeanRegistry.getInstance().register(
self.jmxLeaderElectionBean, self.jmxLocalPeerBean);
} catch (Exception e) {
LOG.warn("Failed to register with JMX", e);
self.jmxLeaderElectionBean = null;
}
if (self.start_fle == 0) {
self.start_fle = System.currentTimeMillis();
}
try {
HashMap<Long, Vote> recvset = new HashMap<Long, Vote>();
HashMap<Long, Vote> outofelection = new HashMap<Long, Vote>();
int notTimeout = finalizeWait;
synchronized(this){
logicalclock++;
updateProposal(getInitId(), getInitLastLoggedZxid(), getPeerEpoch());
}
LOG.info("New election. My id = " + self.getId() +
", Proposed zxid = " + proposedZxid);
sendNotifications();
/*
* Loop in which we exchange notifications until we find a leader
*/
while ((self.getPeerState() == ServerState.LOOKING) &&
(!stop)){
/*
* Remove next notification from queue, times out after 2 times
* the termination time
*/
Notification n = recvqueue.poll(notTimeout,
TimeUnit.MILLISECONDS);
/*
* Sends more notifications if haven't received enough.
* Otherwise processes new notification.
*/
if(n == null){
if(manager.haveDelivered()){
sendNotifications();
} else {
manager.connectAll();
}
/*
* Exponential backoff
*/
int tmpTimeOut = notTimeout*2;
notTimeout = (tmpTimeOut < maxNotificationInterval?
tmpTimeOut : maxNotificationInterval);
LOG.info("Notification time out: " + notTimeout);
}
else if(self.getVotingView().containsKey(n.sid)) {
/*
* Only proceed if the vote comes from a replica in the
* voting view.
*/
switch (n.state) {
case LOOKING:
// If notification > current, replace and send messages out
if (n.electionEpoch > logicalclock) {
logicalclock = n.electionEpoch;
recvset.clear();
if(totalOrderPredicate(n.leader, n.zxid, n.peerEpoch,
getInitId(), getInitLastLoggedZxid(), getPeerEpoch())) {
updateProposal(n.leader, n.zxid, n.peerEpoch);
} else {
updateProposal(getInitId(),
getInitLastLoggedZxid(),
getPeerEpoch());
}
sendNotifications();
} else if (n.electionEpoch < logicalclock) {
if(LOG.isDebugEnabled()){
LOG.debug("Notification election epoch is smaller than logicalclock. n.electionEpoch = " + n.electionEpoch
+ ", Logical clock" + logicalclock);
}
break;
} else if (totalOrderPredicate(n.leader, n.zxid, n.peerEpoch,
proposedLeader, proposedZxid, proposedEpoch)) {
updateProposal(n.leader, n.zxid, n.peerEpoch);
sendNotifications();
}
if(LOG.isDebugEnabled()){
LOG.debug("Adding vote: From = " + n.sid +
", Proposed leader = " + n.leader +
", Proposed zxid = " + n.zxid +
", Proposed election epoch = " + n.electionEpoch);
}
recvset.put(n.sid, new Vote(n.leader, n.zxid, n.electionEpoch, n.peerEpoch));
if (termPredicate(recvset,
new Vote(proposedLeader, proposedZxid,
logicalclock, proposedEpoch))) {
// Verify if there is any change in the proposed leader
while((n = recvqueue.poll(finalizeWait,
TimeUnit.MILLISECONDS)) != null){
if(totalOrderPredicate(n.leader, n.zxid, n.peerEpoch,
proposedLeader, proposedZxid, proposedEpoch)){
recvqueue.put(n);
break;
}
}
/*
* This predicate is true once we don't read any new
* relevant message from the reception queue
*/
if (n == null) {
self.setPeerState((proposedLeader == self.getId()) ?
ServerState.LEADING: learningState());
Vote endVote = new Vote(proposedLeader,
proposedZxid, proposedEpoch);
leaveInstance(endVote);
return endVote;
}
}
break;
case OBSERVING:
LOG.debug("Notification from observer: " + n.sid);
break;
case FOLLOWING:
case LEADING:
/*
* Consider all notifications from the same epoch
* together.
*/
if(n.electionEpoch == logicalclock){
recvset.put(n.sid, new Vote(n.leader, n.zxid, n.electionEpoch, n.peerEpoch));
if(termPredicate(recvset, new Vote(n.leader,
n.zxid, n.electionEpoch, n.peerEpoch, n.state))
&& checkLeader(outofelection, n.leader, n.electionEpoch)) {
self.setPeerState((n.leader == self.getId()) ?
ServerState.LEADING: learningState());
Vote endVote = new Vote(n.leader, n.zxid, n.peerEpoch);
leaveInstance(endVote);
return endVote;
}
}
/**
* Before joining an established ensemble, verify that
* a majority are following the same leader.
*/
outofelection.put(n.sid, new Vote(n.leader, n.zxid,
n.electionEpoch, n.peerEpoch, n.state));
if (termPredicate(outofelection, new Vote(n.leader,
n.zxid, n.electionEpoch, n.peerEpoch, n.state))
&& checkLeader(outofelection, n.leader, n.electionEpoch)) {
synchronized(this){
logicalclock = n.electionEpoch;
self.setPeerState((n.leader == self.getId()) ?
ServerState.LEADING: learningState());
}
Vote endVote = new Vote(n.leader, n.zxid, n.peerEpoch);
leaveInstance(endVote);
return endVote;
}
break;
default:
LOG.warn("Notification state unrecoginized: " + n.state
+ " (n.state), " + n.sid + " (n.sid)");
break;
}
} else {
LOG.warn("Ignoring notification from non-cluster member " + n.sid);
}
}
return null;
} finally {
try {
if(self.jmxLeaderElectionBean != null){
MBeanRegistry.getInstance().unregister(
self.jmxLeaderElectionBean);
}
} catch (Exception e) {
LOG.warn("Failed to unregister with JMX", e);
}
self.jmxLeaderElectionBean = null;
}
}
}