org.apache.catalina.tribes.group.interceptors

Class NonBlockingCoordinator

java.lang.Object

org.apache.catalina.tribes.group.ChannelInterceptorBase

org.apache.catalina.tribes.group.interceptors.NonBlockingCoordinator

All Implemented Interfaces:

ChannelInterceptor, Heartbeat, MembershipListener

public class NonBlockingCoordinator
extends ChannelInterceptorBase

Title: Auto merging leader election algorithm

Description: Implementation of a simple coordinator algorithm that not only selects a coordinator, it also merges groups automatically when members are discovered that werent part of the

This algorithm is non blocking meaning it allows for transactions while the coordination phase is going on

This implementation is based on a home brewed algorithm that uses the AbsoluteOrder of a membership to pass a token ring of the current membership.
This is not the same as just using AbsoluteOrder! Consider the following scenario:
Nodes, A,B,C,D,E on a network, in that priority. AbsoluteOrder will only work if all nodes are receiving pings from all the other nodes. meaning, that node{i} receives pings from node{all}-node{i}
but the following could happen if a multicast problem occurs. A has members {B,C,D}
B has members {A,C}
C has members {D,E}
D has members {A,B,C,E}
E has members {A,C,D}
Because the default Tribes membership implementation, relies on the multicast packets to arrive at all nodes correctly, there is nothing guaranteeing that it will.

To best explain how this algorithm works, lets take the above example: For simplicity we assume that a send operation is O(1) for all nodes, although this algorithm will work where messages overlap, as they all depend on absolute order
Scenario 1: A,B,C,D,E all come online at the same time Eval phase, A thinks of itself as leader, B thinks of A as leader, C thinks of itself as leader, D,E think of A as leader
Token phase:
(1) A sends out a message X{A-ldr, A-src, mbrs-A,B,C,D} to B where X is the id for the message(and the view)
(1) C sends out a message Y{C-ldr, C-src, mbrs-C,D,E} to D where Y is the id for the message(and the view)
(2) B receives X{A-ldr, A-src, mbrs-A,B,C,D}, sends X{A-ldr, A-src, mbrs-A,B,C,D} to C
(2) D receives Y{C-ldr, C-src, mbrs-C,D,E} D is aware of A,B, sends Y{A-ldr, C-src, mbrs-A,B,C,D,E} to E
(3) C receives X{A-ldr, A-src, mbrs-A,B,C,D}, sends X{A-ldr, A-src, mbrs-A,B,C,D,E} to D
(3) E receives Y{A-ldr, C-src, mbrs-A,B,C,D,E} sends Y{A-ldr, C-src, mbrs-A,B,C,D,E} to A
(4) D receives X{A-ldr, A-src, mbrs-A,B,C,D,E} sends sends X{A-ldr, A-src, mbrs-A,B,C,D,E} to A
(4) A receives Y{A-ldr, C-src, mbrs-A,B,C,D,E}, holds the message, add E to its list of members
(5) A receives X{A-ldr, A-src, mbrs-A,B,C,D,E}
At this point, the state looks like
A - {A-ldr, mbrs-A,B,C,D,E, id=X}
B - {A-ldr, mbrs-A,B,C,D, id=X}
C - {A-ldr, mbrs-A,B,C,D,E, id=X}
D - {A-ldr, mbrs-A,B,C,D,E, id=X}
E - {A-ldr, mbrs-A,B,C,D,E, id=Y}

A message doesn't stop until it reaches its original sender, unless its dropped by a higher leader. As you can see, E still thinks the viewId=Y, which is not correct. But at this point we have arrived at the same membership and all nodes are informed of each other.
To synchronize the rest we simply perform the following check at A when A receives X:
Original X{A-ldr, A-src, mbrs-A,B,C,D} == Arrived X{A-ldr, A-src, mbrs-A,B,C,D,E}
Since the condition is false, A, will resend the token, and A sends X{A-ldr, A-src, mbrs-A,B,C,D,E} to B When A receives X again, the token is complete.
Optionally, A can send a message X{A-ldr, A-src, mbrs-A,B,C,D,E confirmed} to A,B,C,D,E who then install and accept the view.

Lets assume that C1 arrives, C1 has lower priority than C, but higher priority than D.
Lets also assume that C1 sees the following view {B,D,E}
C1 waits for a token to arrive. When the token arrives, the same scenario as above will happen.
In the scenario where C1 sees {D,E} and A,B,C cannot see C1, no token will ever arrive.
In this case, C1 sends a Z{C1-ldr, C1-src, mbrs-C1,D,E} to D
D receives Z{C1-ldr, C1-src, mbrs-C1,D,E} and sends Z{A-ldr, C1-src, mbrs-A,B,C,C1,D,E} to E
E receives Z{A-ldr, C1-src, mbrs-A,B,C,C1,D,E} and sends it to A
A sends Z{A-ldr, A-src, mbrs-A,B,C,C1,D,E} to B and the chain continues until A receives the token again. At that time A optionally sends out Z{A-ldr, A-src, mbrs-A,B,C,C1,D,E, confirmed} to A,B,C,C1,D,E

To ensure that the view gets implemented at all nodes at the same time, A will send out a VIEW_CONF message, this is the 'confirmed' message that is optional above.

Ideally, the interceptor below this one would be the TcpFailureDetector to ensure correct memberships

The example above, of course can be simplified with a finite statemachine:
But I suck at writing state machines, my head gets all confused. One day I will document this algorithm though.
Maybe I'll do a state diagram :)

State Diagrams

Initiate an election

Receive an election message

Version:

1.0

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	NonBlockingCoordinator.CoordinationEvent
static class	NonBlockingCoordinator.CoordinationMessage

Nested classes/interfaces inherited from interface org.apache.catalina.tribes.ChannelInterceptor

ChannelInterceptor.InterceptorEvent

Field Summary

Fields

Modifier and Type	Field and Description
protected static byte[]	COORD_ALIVE Alive message
protected static byte[]	COORD_CONF Coordination confirmation, for blocking installations
protected static byte[]	COORD_HEADER header for a coordination message
protected static byte[]	COORD_REQUEST Coordination request
protected AtomicBoolean	coordMsgReceived
protected Object	electionMutex
protected Membership	membership Our nonblocking membership
protected static StringManager	sm
protected boolean	started
protected int	startsvc
protected Membership	suggestedView
protected UniqueId	suggestedviewId indicates that we are running an election and this is the one we are running
protected Membership	view Our current view
protected UniqueId	viewId Out current viewId
protected long	waitForCoordMsgTimeout Time to wait for coordination timeout

Fields inherited from class org.apache.catalina.tribes.group.ChannelInterceptorBase

optionFlag

Constructor Summary

Constructors

Constructor and Description
NonBlockingCoordinator()

Method Summary

Modifier and Type	Method and Description
protected boolean	alive(Member mbr)
ChannelData	createData(NonBlockingCoordinator.CoordinationMessage msg, Member local)
void	fireInterceptorEvent(ChannelInterceptor.InterceptorEvent event)
Member	getCoordinator() Returns coordinator if one is available
Member	getLocalMember(boolean incAlive) Return the member that represents this node.
Member	getMember(Member mbr) Intercepts the Channel.getMember(Member) method
Member[]	getMembers() Get all current cluster members
Member[]	getView()
UniqueId	getViewId()
protected void	halt() Block in/out messages while a election is going on
protected void	handleMyToken(Member local, NonBlockingCoordinator.CoordinationMessage msg, Membership merged)
protected void	handleOtherToken(Member local, NonBlockingCoordinator.CoordinationMessage msg, Membership merged)
protected void	handleToken(NonBlockingCoordinator.CoordinationMessage msg, Membership merged)
protected void	handleViewConf(NonBlockingCoordinator.CoordinationMessage msg, Membership merged)
protected boolean	hasHigherPriority(Member[] complete, Member[] local)
boolean	hasMembers() has members
void	heartbeat() The heartbeat() method gets invoked periodically to allow interceptors to clean up resources, time out object and perform actions that are unrelated to sending/receiving data.
boolean	isCoordinator()
boolean	isHighest()
protected boolean	isViewConf(NonBlockingCoordinator.CoordinationMessage msg)
void	memberAdded(Member member) A member was added to the group
void	memberAdded(Member member, boolean elect)
protected boolean	memberAlive(Member mbr, long conTimeout)
void	memberDisappeared(Member member) A member was removed from the group If the member left voluntarily, the Member.getCommand will contain the Member.SHUTDOWN_PAYLOAD data
protected Membership	mergeOnArrive(NonBlockingCoordinator.CoordinationMessage msg)
void	messageReceived(ChannelMessage msg) the messageReceived is invoked when a message is received.
protected void	processCoordMessage(NonBlockingCoordinator.CoordinationMessage msg)
protected void	release() Release lock for in/out messages election is completed
protected void	sendElectionMsg(Member local, Member next, NonBlockingCoordinator.CoordinationMessage msg)
protected void	sendElectionMsgToNextInline(Member local, NonBlockingCoordinator.CoordinationMessage msg)
void	sendMessage(Member[] destination, ChannelMessage msg, InterceptorPayload payload) The sendMessage method is called when a message is being sent to one more destinations.
protected void	setupMembership()
void	start(int svc) Starts up the channel.
void	startElection(boolean force)
void	stop(int svc) Shuts down the channel.
protected void	waitForRelease() Wait for an election to end

Methods inherited from class org.apache.catalina.tribes.group.ChannelInterceptorBase

getChannel, getNext, getOptionFlag, getPrevious, okToProcess, setChannel, setNext, setOptionFlag, setPrevious

Methods inherited from class java.lang.Object

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

Field Detail

sm

protected static final StringManager sm

COORD_HEADER

protected static final byte[] COORD_HEADER

header for a coordination message

COORD_REQUEST

protected static final byte[] COORD_REQUEST

Coordination request

COORD_CONF

protected static final byte[] COORD_CONF

Coordination confirmation, for blocking installations

COORD_ALIVE

protected static final byte[] COORD_ALIVE

Alive message

waitForCoordMsgTimeout

protected final long waitForCoordMsgTimeout

Time to wait for coordination timeout

See Also:

Constant Field Values

view

protected volatile Membership view

Our current view

viewId

protected UniqueId viewId

Out current viewId

membership

protected Membership membership

Our nonblocking membership

suggestedviewId

protected UniqueId suggestedviewId

indicates that we are running an election and this is the one we are running

suggestedView

protected volatile Membership suggestedView

started

protected volatile boolean started

startsvc

protected final int startsvc

See Also:

Constant Field Values

electionMutex

protected final Object electionMutex

coordMsgReceived

protected final AtomicBoolean coordMsgReceived

Constructor Detail

NonBlockingCoordinator

public NonBlockingCoordinator()

Method Detail

startElection

public void startElection(boolean force)
                   throws ChannelException

Throws:

ChannelException

sendElectionMsg

protected void sendElectionMsg(Member local,
                               Member next,
                               NonBlockingCoordinator.CoordinationMessage msg)
                        throws ChannelException

Throws:

ChannelException

sendElectionMsgToNextInline

protected void sendElectionMsgToNextInline(Member local,
                                           NonBlockingCoordinator.CoordinationMessage msg)
                                    throws ChannelException

Throws:

ChannelException

createData

public ChannelData createData(NonBlockingCoordinator.CoordinationMessage msg,
                              Member local)

alive

protected boolean alive(Member mbr)

memberAlive

protected boolean memberAlive(Member mbr,
                              long conTimeout)

mergeOnArrive

protected Membership mergeOnArrive(NonBlockingCoordinator.CoordinationMessage msg)

processCoordMessage

protected void processCoordMessage(NonBlockingCoordinator.CoordinationMessage msg)
                            throws ChannelException

Throws:

ChannelException

handleToken

protected void handleToken(NonBlockingCoordinator.CoordinationMessage msg,
                           Membership merged)
                    throws ChannelException

Throws:

ChannelException

handleMyToken

protected void handleMyToken(Member local,
                             NonBlockingCoordinator.CoordinationMessage msg,
                             Membership merged)
                      throws ChannelException

Throws:

ChannelException

handleOtherToken

protected void handleOtherToken(Member local,
                                NonBlockingCoordinator.CoordinationMessage msg,
                                Membership merged)
                         throws ChannelException

Throws:

ChannelException

handleViewConf

protected void handleViewConf(NonBlockingCoordinator.CoordinationMessage msg,
                              Membership merged)
                       throws ChannelException

Throws:

ChannelException

isViewConf

protected boolean isViewConf(NonBlockingCoordinator.CoordinationMessage msg)

hasHigherPriority

protected boolean hasHigherPriority(Member[] complete,
                                    Member[] local)

getCoordinator

public Member getCoordinator()

Returns coordinator if one is available

Returns:

Member

getView

public Member[] getView()

getViewId

public UniqueId getViewId()

halt

protected void halt()

Block in/out messages while a election is going on

release

protected void release()

Release lock for in/out messages election is completed

waitForRelease

protected void waitForRelease()

Wait for an election to end

start

public void start(int svc)
           throws ChannelException

Description copied from class: ChannelInterceptorBase

Starts up the channel. This can be called multiple times for individual services to start The svc parameter can be the logical or value of any constants

Specified by:

start in interface ChannelInterceptor

Overrides:

start in class ChannelInterceptorBase

Parameters:

svc - int value of
DEFAULT - will start all services
MBR_RX_SEQ - starts the membership receiver
MBR_TX_SEQ - starts the membership broadcaster
SND_TX_SEQ - starts the replication transmitter
SND_RX_SEQ - starts the replication receiver

Throws:

ChannelException - if a startup error occurs or the service is already started.

See Also:

Channel

stop

public void stop(int svc)
          throws ChannelException

Description copied from class: ChannelInterceptorBase

Shuts down the channel. This can be called multiple times for individual services to shutdown The svc parameter can be the logical or value of any constants

Specified by:

stop in interface ChannelInterceptor

Overrides:

stop in class ChannelInterceptorBase

Parameters:

svc - int value of
DEFAULT - will shutdown all services
MBR_RX_SEQ - stops the membership receiver
MBR_TX_SEQ - stops the membership broadcaster
SND_TX_SEQ - stops the replication transmitter
SND_RX_SEQ - stops the replication receiver

Throws:

ChannelException - if a startup error occurs or the service is already started.

See Also:

Channel

sendMessage

public void sendMessage(Member[] destination,
                        ChannelMessage msg,
                        InterceptorPayload payload)
                 throws ChannelException

Description copied from interface: ChannelInterceptor

The sendMessage method is called when a message is being sent to one more destinations. The interceptor can modify any of the parameters and then pass on the message down the stack by invoking getNext().sendMessage(destination,msg,payload)
Alternatively the interceptor can stop the message from being sent by not invoking getNext().sendMessage(destination,msg,payload)
If the message is to be sent asynchronous the application can be notified of completion and errors by passing in an error handler attached to a payload object.
The ChannelMessage.getAddress contains Channel.getLocalMember, and can be overwritten to simulate a message sent from another node.

Specified by:

sendMessage in interface ChannelInterceptor

Overrides:

sendMessage in class ChannelInterceptorBase

Parameters:

destination - Member[] - the destination for this message

msg - ChannelMessage - the message to be sent

payload - InterceptorPayload - the payload, carrying an error handler and future useful data, can be null

Throws:

ChannelException - if a serialization error happens.

See Also:

ErrorHandler, InterceptorPayload

messageReceived

public void messageReceived(ChannelMessage msg)

Description copied from interface: ChannelInterceptor

the messageReceived is invoked when a message is received. ChannelMessage.getAddress() is the sender, or the reply-to address if it has been overwritten.

Specified by:

messageReceived in interface ChannelInterceptor

Overrides:

messageReceived in class ChannelInterceptorBase

Parameters:

msg - ChannelMessage

memberAdded

public void memberAdded(Member member)

Description copied from interface: MembershipListener

A member was added to the group

Specified by:

memberAdded in interface MembershipListener

Overrides:

memberAdded in class ChannelInterceptorBase

Parameters:

member - Member - the member that was added

memberAdded

public void memberAdded(Member member,
                        boolean elect)

memberDisappeared

public void memberDisappeared(Member member)

Description copied from interface: MembershipListener

A member was removed from the group
If the member left voluntarily, the Member.getCommand will contain the Member.SHUTDOWN_PAYLOAD data

Specified by:

memberDisappeared in interface MembershipListener

Overrides:

memberDisappeared in class ChannelInterceptorBase

Parameters:

member - Member

See Also:

Member.SHUTDOWN_PAYLOAD

isHighest

public boolean isHighest()

isCoordinator

public boolean isCoordinator()

heartbeat

public void heartbeat()

Description copied from interface: ChannelInterceptor

The heartbeat() method gets invoked periodically to allow interceptors to clean up resources, time out object and perform actions that are unrelated to sending/receiving data.

Specified by:

heartbeat in interface ChannelInterceptor

Specified by:

heartbeat in interface Heartbeat

Overrides:

heartbeat in class ChannelInterceptorBase

hasMembers

public boolean hasMembers()

has members

Specified by:

hasMembers in interface ChannelInterceptor

Overrides:

hasMembers in class ChannelInterceptorBase

Returns:

boolean - if the channel has members in its membership group

See Also:

Channel.hasMembers()

getMembers

public Member[] getMembers()

Get all current cluster members

Specified by:

getMembers in interface ChannelInterceptor

Overrides:

getMembers in class ChannelInterceptorBase

Returns:

all members or empty array

See Also:

Channel.getMembers()

getMember

public Member getMember(Member mbr)

Description copied from interface: ChannelInterceptor

Intercepts the Channel.getMember(Member) method

Specified by:

getMember in interface ChannelInterceptor

Overrides:

getMember in class ChannelInterceptorBase

Parameters:

mbr - Member

Returns:

Member

See Also:

Channel.getMember(Member)

getLocalMember

public Member getLocalMember(boolean incAlive)

Return the member that represents this node.

Specified by:

getLocalMember in interface ChannelInterceptor

Overrides:

getLocalMember in class ChannelInterceptorBase

Parameters:

incAlive - boolean

Returns:

Member

See Also:

Channel.getLocalMember(boolean)

setupMembership

protected void setupMembership()

fireInterceptorEvent

public void fireInterceptorEvent(ChannelInterceptor.InterceptorEvent event)

Specified by:

fireInterceptorEvent in interface ChannelInterceptor

Overrides:

fireInterceptorEvent in class ChannelInterceptorBase