Class ELECTION2

java.lang.Object

org.jgroups.stack.Protocol

org.jgroups.protocols.raft.ELECTION2

All Implemented Interfaces:

org.jgroups.Lifecycle

public class ELECTION2
extends org.jgroups.stack.Protocol

A leader election protocol.

This implementation extends ELECTION with a pre-vote mechanism. The pre-vote always runs before starting the voting thread to define the leader. The pre-vote increases the delay in electing the leader but, in turn, covers more edge cases. As a rule of thumb, if deploying in an unstable network with frequent partitions, this protocol should give a more stable mechanism, avoid leader disruptions, and avoid possible liveness issues. Otherwise, ELECTION is the default choice.

Pre-Voting phase:

This extension includes the pre-voting mechanism proposed in Ongaro's dissertation (§9.6). In the current implementation, a pre-voting phase starts in case the current node is the new view coordinator and:

• The view coordinator changes and the computed update is Utils.Majority.no_change;
• The computed view update is Utils.Majority.reached or Utils.Majority.leader_lost

The process which executes the pre-voting mechanism sends a PreVoteRequest to all processes in the view. The recipients reply with a PreVoteResponse identifying the node they see as leader. Once the initiator receives the reply from all nodes in the view, it can start the voting process, resuming the work the same as ELECTION.

Since:

1.0.12

Author:

José Bolina

See Also:

• ELECTION
• Ongaro's dissertation
• Issue #221

Field Summary

Fields

Modifier and Type	Field	Description
static final short	ELECTION_ID
static final short	LEADER_ELECTED
protected int	num_voting_rounds
static final short	PRE_VOTE_REQ
static final short	PRE_VOTE_RSP
protected RAFT	raft
protected org.jgroups.View	view
static final short	VOTE_REQ
static final short	VOTE_RSP
protected long	vote_timeout

Fields inherited from class org.jgroups.stack.Protocol

after_creation_hook, down_prot, ergonomics, id, local_addr, log, policies, preview_warning, stack, stats, up_prot

Constructor Summary

Constructors

Constructor	Description
ELECTION2()

Method Summary

Modifier and Type	Method	Description
protected org.jgroups.Address	determineLeader(org.jgroups.Address exclude)
Object	down(org.jgroups.Event evt)
Instant	electionEnd()
LatencyMetrics	electionLatency()
double	electionMeanLatency()
Instant	electionStart()
org.jgroups.util.ResponseCollector<org.jgroups.protocols.raft.election.VoteResponse>	getVotes()
protected void	handleMessage(org.jgroups.Message msg, RaftHeader hdr)
protected void	handleView(org.jgroups.View v)
void	init()
protected final boolean	isMajorityAvailable()
boolean	isPreVoteThreadRunning()
protected boolean	isViewCoordinator()
boolean	isVotingThreadRunning()
RAFT	raft()
org.jgroups.protocols.raft.election.BaseElection	raft(RAFT r)
void	resetStats()
boolean	runVotingThread()
protected void	sendLeaderElectedMessage(org.jgroups.Address leader, long term)
protected void	sendVoteRequest(long new_term)
protected void	sendVoteResponse(org.jgroups.Address dest, long term, long last_log_term, long last_log_index)
void	start()
CompletionStage<org.jgroups.Address>	startForcedElection(org.jgroups.Address exclude)	Starts a forced leader election, excluding the given address from candidacy.
org.jgroups.protocols.raft.election.BaseElection	startVotingThread()
void	stop()
org.jgroups.protocols.raft.election.BaseElection	stopVotingThread()
long	timeSinceLastElection()
Object	up(org.jgroups.Event evt)
Object	up(org.jgroups.Message msg)
void	up(org.jgroups.util.MessageBatch batch)
long	voteTimeout()
org.jgroups.protocols.raft.election.BaseElection	voteTimeout(long timeoutMs)	Defines the default timeout in milliseconds to utilize during any election operation.

Methods inherited from class org.jgroups.stack.Protocol

accept, addPolicy, addr, addr, afterCreationHook, destroy, down, down, enableStats, getAddress, getComponents, getDownProtocol, getDownServices, getId, getIdsAbove, getLevel, getLog, getName, getPolicies, getProtocolStack, getSocketFactory, getThreadFactory, getTransport, getUpProtocol, getUpServices, getValue, isErgonomics, level, parse, policies, previewWarning, previewWarning, providedDownServices, providedUpServices, removePolicy, requiredDownServices, requiredUpServices, resetStatistics, setAddress, setDownProtocol, setErgonomics, setId, setLevel, setPolicies, setProtocolStack, setSocketFactory, setUpProtocol, setValue, statsEnabled, toString

Methods inherited from class Object

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

Field Details

ELECTION_ID

public static final short ELECTION_ID

See Also:

• Constant Field Values

VOTE_REQ

public static final short VOTE_REQ

See Also:

• Constant Field Values

VOTE_RSP

public static final short VOTE_RSP

See Also:

• Constant Field Values

LEADER_ELECTED

public static final short LEADER_ELECTED

See Also:

• Constant Field Values

PRE_VOTE_REQ

public static final short PRE_VOTE_REQ

See Also:

• Constant Field Values

PRE_VOTE_RSP

public static final short PRE_VOTE_RSP

See Also:

• Constant Field Values

raft

protected RAFT raft

vote_timeout

protected long vote_timeout

num_voting_rounds

protected int num_voting_rounds

view

protected volatile org.jgroups.View view

Constructor Details

ELECTION2

public ELECTION2()

Method Details

isPreVoteThreadRunning

public boolean isPreVoteThreadRunning()

handleView

protected void handleView(org.jgroups.View v)

handleMessage

protected void handleMessage(org.jgroups.Message msg,
 RaftHeader hdr)

voteTimeout

public long voteTimeout()

voteTimeout

public org.jgroups.protocols.raft.election.BaseElection voteTimeout(long timeoutMs)

Defines the default timeout in milliseconds to utilize during any election operation.

Parameters:

timeoutMs - Timeout value in milliseconds.

Returns:

This election instance.

Throws:

IllegalArgumentException - In case timeout is less than or equal to 0.

raft

public RAFT raft()

raft

public org.jgroups.protocols.raft.election.BaseElection raft(RAFT r)

getVotes

public org.jgroups.util.ResponseCollector<org.jgroups.protocols.raft.election.VoteResponse> getVotes()

isVotingThreadRunning

public boolean isVotingThreadRunning()

runVotingThread

public boolean runVotingThread()

electionStart

public Instant electionStart()

electionEnd

public Instant electionEnd()

timeSinceLastElection

public long timeSinceLastElection()

electionMeanLatency

public double electionMeanLatency()

resetStats

public void resetStats()

Overrides:

resetStats in class org.jgroups.stack.Protocol

init

public void init()
          throws Exception

Specified by:

init in interface org.jgroups.Lifecycle

Overrides:

init in class org.jgroups.stack.Protocol

Throws:

Exception

start

public void start()
           throws Exception

Specified by:

start in interface org.jgroups.Lifecycle

Overrides:

start in class org.jgroups.stack.Protocol

Throws:

Exception

stop

public void stop()

Specified by:

stop in interface org.jgroups.Lifecycle

Overrides:

stop in class org.jgroups.stack.Protocol

down

public Object down(org.jgroups.Event evt)

Overrides:

down in class org.jgroups.stack.Protocol

up

public Object up(org.jgroups.Event evt)

Overrides:

up in class org.jgroups.stack.Protocol

up

public Object up(org.jgroups.Message msg)

Overrides:

up in class org.jgroups.stack.Protocol

up

public void up(org.jgroups.util.MessageBatch batch)

Overrides:

up in class org.jgroups.stack.Protocol

electionLatency

public LatencyMetrics electionLatency()

Returns:

election latency metrics, or disabled metrics if stats are off.

isViewCoordinator

protected boolean isViewCoordinator()

determineLeader

protected org.jgroups.Address determineLeader(org.jgroups.Address exclude)

isMajorityAvailable

protected final boolean isMajorityAvailable()

startForcedElection

public CompletionStage<org.jgroups.Address> startForcedElection(org.jgroups.Address exclude)

Starts a forced leader election, excluding the given address from candidacy.

Must be called on the view coordinator. The excluded address still participates in quorum (votes are collected from it), but it is skipped when determining the leader.

Parameters:

exclude - the address to exclude from leader candidacy, or null for no exclusion.

Returns:

a stage that completes with the newly elected leader's address.

startVotingThread

public org.jgroups.protocols.raft.election.BaseElection startVotingThread()

sendVoteRequest

protected void sendVoteRequest(long new_term)

sendLeaderElectedMessage

protected void sendLeaderElectedMessage(org.jgroups.Address leader,
 long term)

sendVoteResponse

protected void sendVoteResponse(org.jgroups.Address dest,
 long term,
 long last_log_term,
 long last_log_index)

stopVotingThread

public org.jgroups.protocols.raft.election.BaseElection stopVotingThread()