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 weren't 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

    All Methods Instance Methods Concrete Methods
    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`

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