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 can not 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

Version:

1.0

Author:

Filip Hanik

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 long waitForCoordMsgTimeout

Time to wait for coordination timeout

view

protected 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 Membership suggestedView

started

protected boolean started

startsvc

protected final int startsvc

See Also:

Constant Field Values

electionMutex

protected java.lang.Object electionMutex

coordMsgReceived

protected java.util.concurrent.atomic.AtomicBoolean coordMsgReceived

NonBlockingCoordinator

public NonBlockingCoordinator()

startElection

public void startElection(boolean force)
                   throws ChannelException

Throws:

ChannelException

sendElectionMsg

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

Throws:

ChannelException

sendElectionMsgToNextInline

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

Throws:

ChannelException

getNextInLine

public Member getNextInLine(MemberImpl local,
                            MemberImpl[] others)

createData

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

viewChange

protected void viewChange(UniqueId viewId,
                          Member[] view)

alive

protected boolean alive(Member mbr)

mergeOnArrive

protected Membership mergeOnArrive(NonBlockingCoordinator.CoordinationMessage msg,
                                   Member sender)

processCoordMessage

protected void processCoordMessage(NonBlockingCoordinator.CoordinationMessage msg,
                                   Member sender)
                            throws ChannelException

Throws:

ChannelException

handleToken

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

Throws:

ChannelException

handleMyToken

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

Throws:

ChannelException

handleOtherToken

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

Throws:

ChannelException

handleViewConf

protected void handleViewConf(NonBlockingCoordinator.CoordinationMessage msg,
                              Member sender,
                              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

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

accept

public boolean accept(ChannelMessage msg)

Overrides:

accept in class ChannelInterceptorBase

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 code>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