Package org.apache.zookeeper.server.quorum

Class FastLeaderElection

java.lang.Object

org.apache.zookeeper.server.quorum.FastLeaderElection

All Implemented Interfaces:

Election

public class FastLeaderElection
extends java.lang.Object
implements Election

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.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
protected class	FastLeaderElection.Messenger	Multi-threaded implementation of message handler.
static class	FastLeaderElection.Notification	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 class	FastLeaderElection.ToSend	Messages that a peer wants to send to other peers.

Constructor Summary

Constructors

Constructor	Description
FastLeaderElection(QuorumPeer self, QuorumCnxManager manager)	Constructor of FastLeaderElection.

Method Summary

Modifier and Type	Method	Description
protected boolean	checkLeader(java.util.HashMap<java.lang.Long,Vote> votes, long leader, long electionEpoch)	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.
QuorumCnxManager	getCnxManager()
long	getLogicalClock()	Returns the current vlue of the logical clock counter
Vote	lookForLeader()	Starts a new round of leader election.
protected boolean	ooePredicate(java.util.HashMap<java.lang.Long,Vote> recv, java.util.HashMap<java.lang.Long,Vote> ooe, FastLeaderElection.Notification n)	This predicate checks that a leader has been elected.
void	shutdown()
protected boolean	termPredicate(java.util.HashMap<java.lang.Long,Vote> votes, Vote vote)	Termination predicate.
protected boolean	totalOrderPredicate(long newId, long newZxid, long newEpoch, long curId, long curZxid, long curEpoch)	Check if a pair (server id, zxid) succeeds our current vote.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

FastLeaderElection

public FastLeaderElection(QuorumPeer self,
                          QuorumCnxManager manager)

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.

Parameters:

self - QuorumPeer that created this object

manager - Connection manager

Method Detail

getLogicalClock

public long getLogicalClock()

Returns the current vlue of the logical clock counter

getCnxManager

public QuorumCnxManager getCnxManager()

shutdown

public void shutdown()

Specified by:

shutdown in interface Election

totalOrderPredicate

protected boolean totalOrderPredicate(long newId,
                                      long newZxid,
                                      long newEpoch,
                                      long curId,
                                      long curZxid,
                                      long curEpoch)

Check if a pair (server id, zxid) succeeds our current vote.

Parameters:

id - Server identifier

zxid - Last zxid observed by the issuer of this vote

termPredicate

protected boolean termPredicate(java.util.HashMap<java.lang.Long,Vote> votes,
                                Vote vote)

Termination predicate. Given a set of votes, determines if have sufficient to declare the end of the election round.

Parameters:

votes - Set of votes

l - Identifier of the vote received last

zxid - zxid of the the vote received last

checkLeader

protected boolean checkLeader(java.util.HashMap<java.lang.Long,Vote> votes,
                              long leader,
                              long electionEpoch)

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.

Parameters:

votes - set of votes

leader - leader id

electionEpoch - epoch id

ooePredicate

protected boolean ooePredicate(java.util.HashMap<java.lang.Long,Vote> recv,
                               java.util.HashMap<java.lang.Long,Vote> ooe,
                               FastLeaderElection.Notification n)

This predicate checks that a leader has been elected. It doesn't make a lot of sense without context (check lookForLeader) and it has been separated for testing purposes.

Parameters:

recv - map of received votes

ooe - map containing out of election votes (LEADING or FOLLOWING)

n - Notification

Returns:

lookForLeader

public Vote lookForLeader()
                   throws java.lang.InterruptedException

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.

Specified by:

lookForLeader in interface Election

Throws:

java.lang.InterruptedException