Zookeeper HA相关配置

## 使用zk做HA
high-availability: zookeeper
## zk地址
high-availability.zookeeper.quorum: node1:2181,node2:2181,node3:2181
## flink在zk下的工作路径
high-availability.zookeeper.path.root: /flink
## 任务所在的HA路径
high-availability.cluster-id: /default
## 保存元数据到文件系统
high-availability.storageDir: hdfs:///flink/recovery## --任务运行在YARN上的配置--
## applicationMaster重试的次数，默认为1，当application master失败的时候，该任务不会重启。
## 设置一个比较大的值的话，yarn会尝试重启applicationMaster。
yarn.application-attempts: 10
## flink是否应该重新分配失败的taskmanager容器。默认是true。
yarn.reallocate-failed：true
## applicationMaster可以接受的容器最大失败次数，达到这个参数，就会认为yarn job失败。
## 默认这个次数和初始化请求的taskmanager数量相等(-n 参数指定的)。
yarn.maximum-failed-containers：1

flink使用Zookeeper做HA

flink的ResourceManager、Dispatcher、JobManager、WebServer组件都需要高可用保证，同时flink高可用还需要持久化checkpoint的元数据信息，保留最近一次已经完成的checkpoint等工作，其中最重要的就是组件的leader选举、leader状态跟踪。本次抽取出Flink使用zk实现leader选举、leader状态跟踪代码，学习下flink是如何使用curator的。类之间的关系如下：

ZooKeeperHaServices是HighAvailabilityServices基于zookeeper的实现，通过使用ZooKeeperUtils类来创建组件的LeaderRetrievalService以及LeaderElectionService。

LeaderRetrievalService用来跟踪leader的变化，当发现leader地址变化时，要通知依赖它的组件去依赖新的leader。比如getResourceManagerLeaderRetriever方法，flink会监听zk的/leader/resource_manager_lock节点内容变化，内容是rm的leader地址和leaderUUID,而taskmanger调用该服务的start方法传递了一个LeaderRetrievalListener。如果节点内容发生变化，意味着rm的leader地址发生变化,那么的LeaderRetrievalListener的notifyLeaderAddress就会通知taskmanger去新的ResourceManager地址进行注册。zk实现该功能使用的是curator的NodeCache并重写了nodeChanged方法。

LeaderElectionService用来进行leader选举工作，当节点成为leader后会调用LeaderContender的grantLeadership方法。以ResourceManagerLeaderElection为例，flink会在zk的/leaderlatch/resource_manager_lock路径下创建临时节点，创建成功的rm节点成为leader触发rm的grantLeadership，最终将当前地址和UUID写入/leader/resource_manager_lock中，这样就触发了LeaderRetrievalService服务。zk实现leader选举使用的是curator的LeaderLatch并重写了isLeader和notLeader方法。同时使用NodeCache监听/leader/resource_manager_lock内容变化，确保新leader地址和UUID成功写入节点。

LeaderRetrievalListener对LeaderRetrievalService的leader地址变化做出响应，通过notifyLeaderAddress传递新leader地址。

LeaderContender对LeaderElectionService的节点角色发生变化做出响应，通过grantLeadership和revokeLeadership进行leader的授权和撤销工作。

一个集群目录下的zk结构如下图所示：

zk文件系统目录

flink相关源码

简单的走一下流程，看看集群启动时是如何创建ZooKeeperHaServices的。

集群启动入口ClusterEntrypoint

• 根据集群的部署模式session or perjob由对应的子类调用ClusterEntrypoint的startCluster方法启动集群，接着会先调用initializeServices方法，启动集群相关的组件信息。这里只看启动haServices部分。

public void startCluster() throws ClusterEntrypointException {SecurityContext securityContext = installSecurityContext(configuration);securityContext.runSecured((Callable<Void>) () -> {runCluster(configuration);return null;});
}protected void initializeServices(Configuration configuration) {ioExecutor = Executors.newFixedThreadPool(Hardware.getNumberCPUCores(),new ExecutorThreadFactory("cluster-io"));haServices = createHaServices(configuration, ioExecutor);blobServer = new BlobServer(configuration, haServices.createBlobStore());blobServer.start();}
}

• 根据high-availability配置创建ZooKeeperHaServices,默认情况下为NONE。

protected HighAvailabilityServices createHaServices(Configuration configuration,Executor executor) throws Exception {//创建HA服务时不需要地址解析return HighAvailabilityServicesUtils.createHighAvailabilityServices(configuration,executor,HighAvailabilityServicesUtils.AddressResolution.NO_ADDRESS_RESOLUTION);
}//根据传递的high-availability配置，选择创建哪种HA服务，默认为NONE
public static HighAvailabilityServices createHighAvailabilityServices(Configuration configuration,Executor executor,AddressResolution addressResolution) throws Exception {//获取high-availability配置 如：zookeeperHighAvailabilityMode highAvailabilityMode = LeaderRetrievalUtils.getRecoveryMode(configuration);switch (highAvailabilityMode) {case NONE:final Tuple2<String, Integer> hostnamePort = getJobManagerAddress(configuration);final String jobManagerRpcUrl = AkkaRpcServiceUtils.getRpcUrl(hostnamePort.f0,hostnamePort.f1,JobMaster.JOB_MANAGER_NAME,addressResolution,configuration);final String resourceManagerRpcUrl = AkkaRpcServiceUtils.getRpcUrl(hostnamePort.f0,hostnamePort.f1,ResourceManager.RESOURCE_MANAGER_NAME,addressResolution,configuration);final String dispatcherRpcUrl = AkkaRpcServiceUtils.getRpcUrl(hostnamePort.f0,hostnamePort.f1,Dispatcher.DISPATCHER_NAME,addressResolution,configuration);final String address = checkNotNull(configuration.getString(RestOptions.ADDRESS),"%s must be set",RestOptions.ADDRESS.key());final int port = configuration.getInteger(RestOptions.PORT);final boolean enableSSL = SSLUtils.isRestSSLEnabled(configuration);final String protocol = enableSSL ? "https://" : "http://";return new StandaloneHaServices(resourceManagerRpcUrl,dispatcherRpcUrl,jobManagerRpcUrl,String.format("%s%s:%s", protocol, address, port));case ZOOKEEPER://元数据存储服务  我们通常使用FileSystemBlobStore 路径就是 high-availability.storageDir: hdfs:///flink/recoveryBlobStoreService blobStoreService = BlobUtils.createBlobStoreFromConfig(configuration);//  使用 ZooKeeper做HA服务return new ZooKeeperHaServices(ZooKeeperUtils.startCuratorFramework(configuration),executor,configuration,blobStoreService);case FACTORY_CLASS:return createCustomHAServices(configuration, executor);default:throw new Exception("Recovery mode " + highAvailabilityMode + " is not supported.");}
}

• ZooKeeperHaServices主要提供了创建LeaderRetrievalService和LeaderElectionService方法，并给出了各个服务组件使用的ZK节点名称。别看是以_lock结尾，这个节点名称既在leaderlatcher做leader选举的分布式锁产生的路径，又在leader目录下用来存放leader的地址信息。

private static final String RESOURCE_MANAGER_LEADER_PATH = "/resource_manager_lock";
private static final String DISPATCHER_LEADER_PATH = "/dispatcher_lock";
private static final String JOB_MANAGER_LEADER_PATH = "/job_manager_lock";
//  web展示服务
private static final String REST_SERVER_LEADER_PATH = "/rest_server_lock";// 创建ResourceManagerLeaderRetriever，对RM的leader地址变化进行跟踪
public LeaderRetrievalService getResourceManagerLeaderRetriever() {return ZooKeeperUtils.createLeaderRetrievalService(client, configuration, RESOURCE_MANAGER_LEADER_PATH);
}// 创建ResourceManagerLeaderElectionService，对RMleader挂掉后重新进行选举
public LeaderElectionService getResourceManagerLeaderElectionService() {return ZooKeeperUtils.createLeaderElectionService(client, configuration, RESOURCE_MANAGER_LEADER_PATH);
}

• ZooKeeperUtils创建LeaderRetrievalService流程。

1. 接收curator客户端以及服务在zk下的节点路径，创建出ZooKeeperLeaderRetrievalService(ZKLRS)对象。
2. ZKLRS这个对象就是对zk节点的内容进行了监听，当内容发生变化时，通知给通过start方法传递过来的LeaderRetrievalListener。

public void start(LeaderRetrievalListener listener) throws Exception {synchronized (lock) {//leader发生变化时，通知对应的LeaderRetrievalListenerleaderListener = listener;// 异常时调用当前对象的unhandledError方法client.getUnhandledErrorListenable().addListener(this);// 使用NodeCache监听节点内容变化cache.getListenable().addListener(this);cache.start();// 对会话连接状态进行跟踪client.getConnectionStateListenable().addListener(connectionStateListener);running = true;}
}

1. 通过重写nodeChanged方法，来获取Leader变更后的地址，并传递新的地址

public void nodeChanged() throws Exception {synchronized (lock) {if (running) {try {LOG.debug("Leader node has changed.");ChildData childData = cache.getCurrentData();String leaderAddress;UUID leaderSessionID;if (childData == null) {leaderAddress = null;leaderSessionID = null;} else {byte[] data = childData.getData();if (data == null || data.length == 0) {leaderAddress = null;leaderSessionID = null;} else {ByteArrayInputStream bais = new ByteArrayInputStream(data);ObjectInputStream ois = new ObjectInputStream(bais);// leader 地址leaderAddress = ois.readUTF();// leader uuidleaderSessionID = (UUID) ois.readObject();}}// leader 地址发生变化if (!(Objects.equals(leaderAddress, lastLeaderAddress) &&Objects.equals(leaderSessionID, lastLeaderSessionID))) {lastLeaderAddress = leaderAddress;lastLeaderSessionID = leaderSessionID;//  传递新的leaderAddress和leaderSessionIDleaderListener.notifyLeaderAddress(leaderAddress, leaderSessionID);}} catch (Exception e) {leaderListener.handleError(new Exception("Could not handle node changed event.", e));throw e;}} else {LOG.debug("Ignoring node change notification since the service has already been stopped.");}}
}

• ZooKeeperUtils创建ZooKeeperLeaderElectionService流程。

1. 传递leader所在的zk路径、选举时临时节点创建的zk路径。之所以要传递leader节点是要在新leader产生时，将新leader的地址和uuid写入。

public static ZooKeeperLeaderElectionService createLeaderElectionService(final CuratorFramework client,final Configuration configuration,final String pathSuffix) {// 在leaderlatch节点下进行选举final String latchPath = configuration.getString(HighAvailabilityOptions.HA_ZOOKEEPER_LATCH_PATH) + pathSuffix;// leader节点    final String leaderPath = configuration.getString(HighAvailabilityOptions.HA_ZOOKEEPER_LEADER_PATH) + pathSuffix;return new ZooKeeperLeaderElectionService(client, latchPath, leaderPath);
}

1. 通过调用start方法传递LeaderContender，并开启leader选举。

public void start(LeaderContender contender) throws Exception {synchronized (lock) {// 绑定异常处理监听器client.getUnhandledErrorListenable().addListener(this);// 传递Contender竞争者leaderContender = contender;//开启leader选举服务，成为leader的节点会触发isleaderleaderLatch.addListener(this);leaderLatch.start();//监听leader节点内容变化cache.getListenable().addListener(this);cache.start();client.getConnectionStateListenable().addListener(listener);running = true;}
}

1. 当某一Contender成为leader后，会触发grantLeadership传递新leader的uuid进行授权，并调用LeaderElectionService的confirmLeaderSessionID，将新leader地址写入leader节点。

public void confirmLeaderSessionID(UUID leaderSessionID) {// 是Leaderif (leaderLatch.hasLeadership()) {// check if this is an old confirmation callsynchronized (lock) {if (running) {if (leaderSessionID.equals(this.issuedLeaderSessionID)) {confirmedLeaderSessionID = leaderSessionID;// 将confirmLeaderSessionID写到 leader目录下writeLeaderInformation(confirmedLeaderSessionID);}}}} 
}

1. 写入时会触发当前对象的nodeChanged方法，该方法用来确保新leader地址和uuid成功写入。

public void nodeChanged() throws Exception {try {// leaderSessionID is null if the leader contender has not yet confirmed the session IDif (leaderLatch.hasLeadership()) { // leadersynchronized (lock) {if (running) {// 当选为leader 已经被确认if (confirmedLeaderSessionID != null) {ChildData childData = cache.getCurrentData();//  没写进去，再写一次if (childData == null) {if (LOG.isDebugEnabled()) {LOG.debug("Writing leader information into empty node by {}.",leaderContender.getAddress());}writeLeaderInformation(confirmedLeaderSessionID);} else {byte[] data = childData.getData();if (data == null || data.length == 0) {// the data field seems to be empty, rewrite informationwriteLeaderInformation(confirmedLeaderSessionID);} else {ByteArrayInputStream bais = new ByteArrayInputStream(data);ObjectInputStream ois = new ObjectInputStream(bais);String leaderAddress = ois.readUTF();UUID leaderSessionID = (UUID) ois.readObject();if (!leaderAddress.equals(this.leaderContender.getAddress()) ||(leaderSessionID == null || !leaderSessionID.equals(confirmedLeaderSessionID))) {writeLeaderInformation(confirmedLeaderSessionID);}}}}} else {// leader未确认confirmedLeaderSessionIDLOG.debug("Ignoring node change notification since the service has already been stopped.");}}}} catch (Exception e) {...}
}

writeLeaderInformation用来写入leader地址和uuid,写入时先判断leader节点是否由当前leader会话创建的，如果不是则删除后重写创建。

protected void writeLeaderInformation(UUID leaderSessionID) {try {ByteArrayOutputStream baos = new ByteArrayOutputStream();ObjectOutputStream oos = new ObjectOutputStream(baos);//  leader 地址oos.writeUTF(leaderContender.getAddress());//  leader的 UUIDoos.writeObject(leaderSessionID);oos.close();boolean dataWritten = false;while (!dataWritten && leaderLatch.hasLeadership()) {Stat stat = client.checkExists().forPath(leaderPath);if (stat != null) {long owner = stat.getEphemeralOwner();long sessionID = client.getZookeeperClient().getZooKeeper().getSessionId();//节点由当前会话创建if (owner == sessionID) {try {client.setData().forPath(leaderPath, baos.toByteArray());dataWritten = true;} catch (KeeperException.NoNodeException noNode) {// node was deleted in the meantime}} else {try {//  不是当前节点创建则先删除client.delete().forPath(leaderPath);} catch (KeeperException.NoNodeException noNode) {// node was deleted in the meantime --> try again}}} else {try {client.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath(leaderPath,baos.toByteArray());dataWritten = true;} catch (KeeperException.NodeExistsException nodeExists) {// node has been created in the meantime --> try again}}}} 
}

本次学习了flink如何使用curator来操作zk节点，实现leader选举和leader状态跟踪。LeaderRetrievalListener和LeaderContender两个接口更像是这一部分功能的输入和输出，来跟踪leader的变化情况。而中间部分对zk节点的操作和状态监听，则可以抽取出来在自己的项目中使用。