聊聊artemis的QuorumVote

序

本文主要研究一下artemis的QuorumVote

QuorumVote

activemq-artemis-2.11.0/artemis-server/src/main/java/org/apache/activemq/artemis/core/server/cluster/qourum/QuorumVote.java

public abstract class QuorumVote<V extends Vote, T> {

   private SimpleString name;

   public QuorumVote(SimpleString name) {
      this.name = name;
   }

   /**
    * called by the {@link org.apache.activemq.artemis.core.server.cluster.qourum.QuorumManager} when one of the nodes in the quorum is
    * successfully connected to. The QuorumVote can then decide whether or not a decision can be made with just that information.
    *
    * @return the vote to use
    */
   public abstract Vote connected();

   /**
    * called by the {@link org.apache.activemq.artemis.core.server.cluster.qourum.QuorumManager} fails to connect to a node in the quorum.
    * The QuorumVote can then decide whether or not a decision can be made with just that information however the node
    * cannot cannot be asked.
    *
    * @return the vote to use
    */
   public abstract Vote notConnected();

   /**
    * called by the {@link org.apache.activemq.artemis.core.server.cluster.qourum.QuorumManager} when a vote can be made, either from the
    * cluster or decided by itself.
    *
    * @param vote the vote to make.
    */
   public abstract void vote(V vote);

   /**
    * get the decion of the vote
    *
    * @return the voting decision
    */
   public abstract T getDecision();

   /**
    * called by the {@link org.apache.activemq.artemis.core.server.cluster.qourum.QuorumManager} when all the votes have been cast and received.
    *
    * @param voteTopology the topology of where the votes were sent.
    */
   public abstract void allVotesCast(Topology voteTopology);

   /**
    * the name of this quorum vote, used for identifying the correct {@link org.apache.activemq.artemis.core.server.cluster.qourum.QuorumVoteHandler}
    *
    * @return the name of the wuorum vote
    */
   public SimpleString getName() {
      return name;
   }
}

• QuorumVote是个抽象类，定义了connected、notConnected、vote、getDecision、allVotesCast抽象方法

QuorumVoteServerConnect

activemq-artemis-2.11.0/artemis-server/src/main/java/org/apache/activemq/artemis/core/server/cluster/qourum/QuorumVoteServerConnect.java

public class QuorumVoteServerConnect extends QuorumVote<ServerConnectVote, Boolean> {

   public static final SimpleString LIVE_FAILOVER_VOTE = new SimpleString("LiveFailoverQuorumVote");
   private final CountDownLatch latch;
   private final String targetNodeId;
   private final String liveConnector;

   private int votesNeeded;

   private int total = 0;

   private boolean decision = false;

   // Is this the live requesting to stay live, or a backup requesting to become live.
   private boolean requestToStayLive = false;

   /**
    * live nodes | remaining nodes |  majority   | votes needed
    * 1      |       0         |     0       |      0
    * 2      |       1         |     1       |      1
    * n      |    r = n-1      |   n/2 + 1   |   n/2 + 1 rounded
    * 3      |       2         |     2.5     |      2
    * 4      |       3         |      3      |      3
    * 5      |       4         |     3.5     |      3
    * 6      |       5         |      4      |      4
    */
   public QuorumVoteServerConnect(int size, String targetNodeId, boolean requestToStayLive, String liveConnector) {
      super(LIVE_FAILOVER_VOTE);
      this.targetNodeId = targetNodeId;
      this.liveConnector = liveConnector;
      double majority;
      if (size <= 2) {
         majority = ((double) size) / 2;
      } else {
         //even
         majority = ((double) size) / 2 + 1;
      }
      //votes needed could be say 2.5 so we add 1 in this case
      votesNeeded = (int) majority;
      latch = new CountDownLatch(votesNeeded);
      if (votesNeeded == 0) {
         decision = true;
      }
      this.requestToStayLive = requestToStayLive;
   }

   public QuorumVoteServerConnect(int size, String targetNodeId) {
      this(size, targetNodeId, false, null);
   }
   /**
    * if we can connect to a node
    *
    * @return
    */
   @Override
   public Vote connected() {
      return new ServerConnectVote(targetNodeId, requestToStayLive, null);
   }
   /**
    * if we cant connect to the node
    *
    * @return
    */
   @Override
   public Vote notConnected() {
      return new BooleanVote(false);
   }

   /**
    * live nodes | remaining nodes |  majority   | votes needed
    * 1      |       0         |     0       |      0
    * 2      |       1         |     1       |      1
    * n      |    r = n-1      |   n/2 + 1   |   n/2 + 1 rounded
    * 3      |       2         |     2.5     |      2
    * 4      |       3         |      3      |      3
    * 5      |       4         |     3.5     |      3
    * 6      |       5         |      4      |      4
    *
    * @param vote the vote to make.
    */
   @Override
   public synchronized void vote(ServerConnectVote vote) {
      if (decision)
         return;
      if (!requestToStayLive && vote.getVote()) {
         total++;
         latch.countDown();
         if (total >= votesNeeded) {
            decision = true;
         }//do the opposite, if it says there is a node connected it means the backup has come live
      } else if (requestToStayLive && vote.getVote()) {
         total++;
         latch.countDown();
         if (liveConnector != null && !liveConnector.equals(vote.getTransportConfiguration())) {
            ActiveMQServerLogger.LOGGER.qourumBackupIsLive(liveConnector);
            return;
         }
         if (total >= votesNeeded) {
            decision = true;
         }
      }
   }

   @Override
   public void allVotesCast(Topology voteTopology) {
      while (latch.getCount() > 0) {
         latch.countDown();
      }
   }

   @Override
   public Boolean getDecision() {
      return decision;
   }

   public void await(int latchTimeout, TimeUnit unit) throws InterruptedException {
      ActiveMQServerLogger.LOGGER.waitingForQuorumVoteResults(latchTimeout, unit.toString().toLowerCase());
      if (latch.await(latchTimeout, unit))
         ActiveMQServerLogger.LOGGER.receivedAllQuorumVotes();
      else
         ActiveMQServerLogger.LOGGER.timeoutWaitingForQuorumVoteResponses();
   }

   public boolean isRequestToStayLive() {
      return requestToStayLive;
   }
}

• QuorumVoteServerConnect继承了QuorumVote，其构造器根据size初始化votesNeeded及decision；其connected方法返回ServerConnectVote；其notConnected方法返回BooleanVote(false)；其vote方法对于ServerConnectVote的vote为true的递增total，同时latch.countDown()，对于total大于等于votesNeeded的更新decision为true；其allVotesCast方法则循环latch.countDown()

小结

QuorumVote是个抽象类，定义了connected、notConnected、vote、getDecision、allVotesCast抽象方法；QuorumVoteServerConnect继承了QuorumVote，其构造器根据size初始化votesNeeded及decision；其connected方法返回ServerConnectVote；其notConnected方法返回BooleanVote(false)；其vote方法对于ServerConnectVote的vote为true的递增total，同时latch.countDown()，对于total大于等于votesNeeded的更新decision为true；其allVotesCast方法则循环latch.countDown()

doc

• QuorumVoteServerConnect