hadoop-3

结合https://blog.csdn.net/zhangjun5965/article/details/76796998，自己过一遍感受下

public class DFSZKFailoverController extends ZKFailoverController 模板调用

  public static void main(String args[])
      throws Exception {
    StringUtils.startupShutdownMessage(DFSZKFailoverController.class,
        args, LOG);
    if (DFSUtil.parseHelpArgument(args,
        ZKFailoverController.USAGE, System.out, true)) {
      System.exit(0);
    }

    GenericOptionsParser parser = new GenericOptionsParser(
        new HdfsConfiguration(), args);
    DFSZKFailoverController zkfc = DFSZKFailoverController.create(
        parser.getConfiguration());
    try {
      System.exit(zkfc.run(parser.getRemainingArgs()));
    } catch (Throwable t) {
      LOG.fatal("DFSZKFailOverController exiting due to earlier exception "
          + t);
      terminate(1, t);
    }
  }

抽象类中run方法

  public int run(final String[] args) throws Exception {
    if (!localTarget.isAutoFailoverEnabled()) {
      LOG.error("Automatic failover is not enabled for " + localTarget + "." +
          " Please ensure that automatic failover is enabled in the " +
          "configuration before running the ZK failover controller.");
      return ERR_CODE_AUTO_FAILOVER_NOT_ENABLED;
    }
    loginAsFCUser();
    try {
      return SecurityUtil.doAsLoginUserOrFatal(new PrivilegedAction<Integer>() {
        @Override
        public Integer run() {
          try {
            return doRun(args);
          } catch (Exception t) {
            throw new RuntimeException(t);
          } finally {
            if (elector != null) {
              elector.terminateConnection();
            }
          }
        }
      });
    } catch (RuntimeException rte) {
      throw (Exception)rte.getCause();
    }
  }

下面这个注释说明了选举的机制，就是利用创建zk临时节点机制， atomically creating an ephemeral lock file (znode) onZookeeper. The service instance that successfully creates the znode becomesactive and the rest become standbys，之中还定义了回调接口，包含当节点变为主节点或者从节点的通知，这里注意，存在一个防止脑裂的enterNeutralMode，在状态不确定的情况下通知到节点

public class ActiveStandbyElector implements StatCallback, StringCallback {

  /**
   * Callback interface to interact with the ActiveStandbyElector object. <br/>
   * The application will be notified with a callback only on state changes
   * (i.e. there will never be successive calls to becomeActive without an
   * intermediate call to enterNeutralMode). <br/>
   * The callbacks will be running on Zookeeper client library threads. The
   * application should return from these callbacks quickly so as not to impede
   * Zookeeper client library performance and notifications. The app will
   * typically remember the state change and return from the callback. It will
   * then proceed with implementing actions around that state change. It is
   * possible to be called back again while these actions are in flight and the
   * app should handle this scenario.
   */
  public interface ActiveStandbyElectorCallback {
    /**
     * This method is called when the app becomes the active leader.
     * If the service fails to become active, it should throw
     * ServiceFailedException. This will cause the elector to
     * sleep for a short period, then re-join the election.
     *
     * Callback implementations are expected to manage their own
     * timeouts (e.g. when making an RPC to a remote node).
     */
    void becomeActive() throws ServiceFailedException;

    /**
     * This method is called when the app becomes a standby
     */
    void becomeStandby();

    /**
     * If the elector gets disconnected from Zookeeper and does not know about
     * the lock state, then it will notify the service via the enterNeutralMode
     * interface. The service may choose to ignore this or stop doing state
     * changing operations. Upon reconnection, the elector verifies the leader
     * status and calls back on the becomeActive and becomeStandby app
     * interfaces. <br/>
     * Zookeeper disconnects can happen due to network issues or loss of
     * Zookeeper quorum. Thus enterNeutralMode can be used to guard against
     * split-brain issues. In such situations it might be prudent to call
     * becomeStandby too. However, such state change operations might be
     * expensive and enterNeutralMode can help guard against doing that for
     * transient issues.
     */
    void enterNeutralMode();

    void notifyFatalError(String errorMessage);
    void fenceOldActive(byte[] oldActiveData);
  }

定义了一个watcher，监听zk相关事件

  /**
   * Watcher implementation which keeps a reference around to the
   * original ZK connection, and passes it back along with any
   * events.
   */
  private final class WatcherWithClientRef implements Watcher {
    private ZooKeeper zk;

    /**
     * Latch fired whenever any event arrives. This is used in order
     * to wait for the Connected event when the client is first created.
     */
    private CountDownLatch hasReceivedEvent = new CountDownLatch(1);

    /**
     * Latch used to wait until the reference to ZooKeeper is set.
     */
    private CountDownLatch hasSetZooKeeper = new CountDownLatch(1);

    /**
     * Waits for the next event from ZooKeeper to arrive.
     *
     * @param connectionTimeoutMs zookeeper connection timeout in milliseconds
     * @throws KeeperException if the connection attempt times out. This will
     * be a ZooKeeper ConnectionLoss exception code.
     * @throws IOException if interrupted while connecting to ZooKeeper
     */
    private void waitForZKConnectionEvent(int connectionTimeoutMs)
        throws KeeperException, IOException {
      try {
        if (!hasReceivedEvent.await(connectionTimeoutMs, TimeUnit.MILLISECONDS)) {
          LOG.error("Connection timed out: couldn't connect to ZooKeeper in " +
              "{} milliseconds", connectionTimeoutMs);
          zk.close();
          throw KeeperException.create(Code.CONNECTIONLOSS);
        }
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new IOException(
            "Interrupted when connecting to zookeeper server", e);
      }
    }

    private void setZooKeeperRef(ZooKeeper zk) {
      Preconditions.checkState(this.zk == null,
          "zk already set -- must be set exactly once");
      this.zk = zk;
      hasSetZooKeeper.countDown();
    }

    @Override
    public void process(WatchedEvent event) {
      hasReceivedEvent.countDown();
      try {
        if (!hasSetZooKeeper.await(zkSessionTimeout, TimeUnit.MILLISECONDS)) {
          LOG.debug("Event received with stale zk");
        }
        ActiveStandbyElector.this.processWatchEvent(
            zk, event);
      } catch (Throwable t) {
        fatalError(
            "Failed to process watcher event " + event + ": " +
            StringUtils.stringifyException(t));
      }
    }
  }

具体处理逻辑

  synchronized void processWatchEvent(ZooKeeper zk, WatchedEvent event) {
    Event.EventType eventType = event.getType();
    if (isStaleClient(zk)) return;
    if (LOG.isDebugEnabled()) {
      LOG.debug("Watcher event type: " + eventType + " with state:"
          + event.getState() + " for path:" + event.getPath()
          + " connectionState: " + zkConnectionState
          + " for " + this);
    }

    if (eventType == Event.EventType.None) {
      // the connection state has changed
      switch (event.getState()) {
      case SyncConnected:
        LOG.info("Session connected.");
        // if the listener was asked to move to safe state then it needs to
        // be undone
        ConnectionState prevConnectionState = zkConnectionState;
        zkConnectionState = ConnectionState.CONNECTED;
        if (prevConnectionState == ConnectionState.DISCONNECTED &&
            wantToBeInElection) {
          monitorActiveStatus();
        }
        break;
      case Disconnected:
        LOG.info("Session disconnected. Entering neutral mode...");

        // ask the app to move to safe state because zookeeper connection
        // is not active and we dont know our state
        zkConnectionState = ConnectionState.DISCONNECTED;
        enterNeutralMode();
        break;
      case Expired:
        // the connection got terminated because of session timeout
        // call listener to reconnect
        LOG.info("Session expired. Entering neutral mode and rejoining...");
        enterNeutralMode();
        reJoinElection(0);
        break;
      case SaslAuthenticated:
        LOG.info("Successfully authenticated to ZooKeeper using SASL.");
        break;
      default:
        fatalError("Unexpected Zookeeper watch event state: "
            + event.getState());
        break;
      }

      return;
    }

    // a watch on lock path in zookeeper has fired. so something has changed on
    // the lock. ideally we should check that the path is the same as the lock
    // path but trusting zookeeper for now
    String path = event.getPath();
    if (path != null) {
      switch (eventType) {
      case NodeDeleted:
        if (state == State.ACTIVE) {
          enterNeutralMode();
        }
        joinElectionInternal();
        break;
      case NodeDataChanged:
        monitorActiveStatus();
        break;
      default:
        if (LOG.isDebugEnabled()) {
          LOG.debug("Unexpected node event: " + eventType + " for path: " + path);
        }
        monitorActiveStatus();
      }

      return;
    }

    // some unexpected error has occurred
    fatalError("Unexpected watch error from Zookeeper");
  }

看下加入选举，起始非常简单，就是创建节点

  private void joinElectionInternal() {
    Preconditions.checkState(appData != null,
        "trying to join election without any app data");
    if (zkClient == null) {
      if (!reEstablishSession()) {
        fatalError("Failed to reEstablish connection with ZooKeeper");
        return;
      }
    }

    createRetryCount = 0;
    wantToBeInElection = true;
    createLockNodeAsync();
  }

  private void createLockNodeAsync() {
    zkClient.create(zkLockFilePath, appData, zkAcl, CreateMode.EPHEMERAL,
        this, zkClient);
  }

一个变为不确定状态的代码

  @Override
  public void enterNeutralMode() {
    LOG.warn("Lost contact with Zookeeper. Transitioning to standby in "
        + zkSessionTimeout + " ms if connection is not reestablished.");

    // If we've just become disconnected, start a timer.  When the time's up,
    // we'll transition to standby.
    synchronized (zkDisconnectLock) {
      if (zkDisconnectTimer == null) {
        zkDisconnectTimer = new Timer("Zookeeper disconnect timer");
        zkDisconnectTimer.schedule(new TimerTask() {
          @Override
          public void run() {
            synchronized (zkDisconnectLock) {
              // Only run if the timer hasn't been cancelled
              if (zkDisconnectTimer != null) {
                becomeStandby();
              }
            }
          }
        }, zkSessionTimeout);
      }
    }
  }

  @Override
  public void becomeStandby() {
    cancelDisconnectTimer();

    try {
      rm.getRMContext().getRMAdminService().transitionToStandby(req);
    } catch (Exception e) {
      LOG.error("RM could not transition to Standby", e);
    }
  }

线索非常清晰