ZooKeeper源码解析:Leader选举

Leader选举又称为master选举是zookeeper中最为经典的应用场景了。那为什么需要leader 选举呢。
ZooKeeper需要在所有的服务(可理解为服务器)中选举出一个Leader，然后让这个Leader来负责管理集群。此时，集群中的其他服务器则成了此Leader的follower。并且，当Leader出现故障的时候，ZooKeeper要能够快速地在Follower中选举出下一个Leader。这就是ZooKeeper的Leader机制，下面我们将简单介绍如何使用ZooKeeper实现Leader选举(Leader Election)。

此操作实现的核心思想是：首先创建一个EPHEMERAL的节点，例如"/election"。然后每一个ZooKeeper服务器在此目录下创建一个SEQUENCE|EPHEMERAL类型的节点，例如“/election/n_”。在SEQUENCE标志下，ZooKeeper将自动地为每一个ZooKeeper服务分配一个比前面所分配的序号要大的序号。此时创建节点ZooKeeper服务器中拥有最小编号的服务器将成为Leader。

在实际的操作中，还需要保证：当Leader服务器发生故障的时候，系统能够快速地选出下一个ZooKeeper服务器作为Leader。一个简单的方案是，让所有的Follower监视leader所对应的节点。当Leader发生故障时，Leader所对应的临时节点会被自动删除，此操作将会触发所有监视Leader的服务器的watch。这样这些服务器就会收到Leader故障的消息，进而进行下一次的Leader选举操作。但是，这种操作将会导致“从众效应”的发生，尤其是当集群中服务器众多并且宽带延迟比较大的时候更为明显。在ZooKeeper中，为了避免从众效应的发生，它是这样来实现的：每一个Follower为Follower集群中对应着比自己节点序号小的节点中x序号最大的节点设置一个watch。只有当Followers所设置的watch被触发时，它才惊醒Leader选举操作，一般情况下它将成为集群中的下一个Leader。很明显，此Leader选举操作的速度是很快的。因为每一次Leader选举几乎只涉及单个Follower的操作。
下面我们看下源码是怎么实现的 org.apache.zookeeper.recipes.leader.LeaderElectionSupport
具体的实现逻辑在这个类中。首先有一个start 方法我们来看下在这个方法中
可以看到首先调用了 makeOffer();然后是 determineElectionStatus();

 /**
     *选举的开始方法
     */
    public synchronized void start() {
        state = State.START;
        // 广播选举开始
        dispatchEvent(EventType.START);

        LOG.info("Starting leader election support");

        if (zooKeeper == null) {
            throw new IllegalStateException(
                "No instance of zookeeper provided. Hint: use setZooKeeper()");
        }

        if (hostName == null) {
            throw new IllegalStateException(
                "No hostname provided. Hint: use setHostName()");
        }

        try {
            makeOffer();
            determineElectionStatus();
        } catch (KeeperException | InterruptedException e) {
            becomeFailed(e);
        }
    }

我们一起来看下 makeOffer()方法，这个方法主要就是创建临时节点


    /**
     * 真正开始选举的方法 在root 目录下创建节点
     * @throws KeeperException
     * @throws InterruptedException
     */
    private void makeOffer() throws KeeperException, InterruptedException {
        state = State.OFFER;
        dispatchEvent(EventType.OFFER_START);

        LeaderOffer newLeaderOffer = new LeaderOffer();
        byte[] hostnameBytes;
        synchronized (this) {
            newLeaderOffer.setHostName(hostName);
            hostnameBytes = hostName.getBytes();
            newLeaderOffer.setNodePath(zooKeeper.create(rootNodeName + "/" + "n_",
                                                        hostnameBytes, ZooDefs.Ids.OPEN_ACL_UNSAFE,
                                                        // 零时节点
                                                        CreateMode.EPHEMERAL_SEQUENTIAL));
            leaderOffer = newLeaderOffer;
        }
        LOG.debug("Created leader offer {}", leaderOffer);

        dispatchEvent(EventType.OFFER_COMPLETE);
    }

然后就是 determineElectionStatus（） 这个方法获取文件列表下面所有的文件最小的那个设置为leader 其他的节点添加对上一个的监听


    /**
     * 
     * 选出最小序号的文件 对应的机器就是leader
     * @throws KeeperException
     * @throws InterruptedException
     */
    private void determineElectionStatus() throws KeeperException, InterruptedException {

        state = State.DETERMINE;
        dispatchEvent(EventType.DETERMINE_START);

        LeaderOffer currentLeaderOffer = getLeaderOffer();

        String[] components = currentLeaderOffer.getNodePath().split("/");

        currentLeaderOffer.setId(Integer.valueOf(components[components.length - 1].substring("n_".length())));

        List<LeaderOffer> leaderOffers = toLeaderOffers(zooKeeper.getChildren(rootNodeName, false));

        /*
         * For each leader offer, find out where we fit in. If we're first, we
         * become the leader. If we're not elected the leader, attempt to stat the
         * offer just less than us. If they exist, watch for their failure, but if
         * they don't, become the leader.
         */
        for (int i = 0; i < leaderOffers.size(); i++) {
            LeaderOffer leaderOffer = leaderOffers.get(i);

            if (leaderOffer.getId().equals(currentLeaderOffer.getId())) {
                LOG.debug("There are {} leader offers. I am {} in line.", leaderOffers.size(), i);

                dispatchEvent(EventType.DETERMINE_COMPLETE);

                if (i == 0) {
                    // 最小的那个变成leader
                    becomeLeader();
                } else {
                    // 其他的是非leader
                    becomeReady(leaderOffers.get(i - 1));
                }

                /* Once we've figured out where we are, we're done. */
                break;
            }
        }
    }

如果没有成为leader 的节点监听上一个节点如果上一个节点故障了则重新执行上面的方法


    private void becomeReady(LeaderOffer neighborLeaderOffer)
        throws KeeperException, InterruptedException {

        LOG.info(
            "{} not elected leader. Watching node: {}",
            getLeaderOffer().getNodePath(),
            neighborLeaderOffer.getNodePath());

        /*
         * Make sure to pass an explicit Watcher because we could be sharing this
         * zooKeeper instance with someone else.
         */
        /**
         *
         * 进行watch，监视上一个节点 如果上一个节点删除了 就重新掉用determineElectionStatus
         */
        Stat stat = zooKeeper.exists(neighborLeaderOffer.getNodePath(), this);

        if (stat != null) {
            dispatchEvent(EventType.READY_START);
            LOG.debug(
                "We're behind {} in line and they're alive. Keeping an eye on them.",
                neighborLeaderOffer.getNodePath());
            state = State.READY;
            dispatchEvent(EventType.READY_COMPLETE);
        } else {
            /*
             * If the stat fails, the node has gone missing between the call to
             * getChildren() and exists(). We need to try and become the leader.
             */
            LOG.info(
                "We were behind {} but it looks like they died. Back to determination.",
                neighborLeaderOffer.getNodePath());
            determineElectionStatus();
        }

    }

CXHC

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ZooKeeper源码解析:Leader选举

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