1. 简介

在Flink部署架构中，JobManager负责协调Flink任务调度和资源管理。

默认情况下，每个Flink集群都有一个JobManager实例，这会产生单点故障（SPOF single point of failure），如果JobManager进程崩溃，则无法提交新任务且运行中的任务也会失败。

启用JobManager HA后，就可以JobManager崩溃故障中恢复，消除单点故障。目前Flink支持独立集群和YARN集群两种HA配置方式。

YARN集群模式主要依赖YARN的HA机制，具体的行为还依赖YARN版本，本文将主要介绍独立集群的HA机制。

2. 独立集群HA

独立集群的JobManager的机制是，一个leader JobManager和多个standby JobManager，当leader JobManager崩溃后，多个standby JobManager选举后产生新的leader JobManager。

leader JobManager和standby JobManager之间没有区别，任何JobManager都可以承担leader或standby角色。

3. 源码分析

3.1 HighAvailabilityMode

public enum HighAvailabilityMode {
	NONE(false),
	ZOOKEEPER(true),
	FACTORY_CLASS(true);

	private final boolean haActive;

	HighAvailabilityMode(boolean haActive) {
		this.haActive = haActive;
	}
}

Flink定义了三种高可用性模式枚举

NONE：非HA模式
ZOOKEEPER：基于ZK实现HA
FACTORY_CLASS：自定义HA工厂类，该类需要实现HighAvailabilityServicesFactory接口

3.2 HighAvailabilityOptions

HighAvailabilityOptions定义了HA相关配置项

HA_MODE：HA模式，可以设置为NONE、ZOOKEEPER，或设置为自定义HA工厂类名
HA_CLUSTER_ID：Flink集群id，区分多个Flink集群
HA_STORAGE_PATH：Flink储存HA元数据的文件系统路径
HA_JOB_MANAGER_PORT_RANGE：JobManager监听端口范围，JobManager在此范围内随机选择一个端口号
HA_ZOOKEEPER_QUORUM：ZK集群节点的地址和端口号
HA_ZOOKEEPER_ROOT：Flink在ZK中持久化数据的根目录

3.3 HighAvailabilityServicesUtils

HighAvailabilityServicesUtils主要用于创建具体HighAvailabilityServices实例对象

3.3.1 createHighAvailabilityServices

创建HighAvailabilityServices实例

	public static HighAvailabilityServices createHighAvailabilityServices(
		Configuration configuration,
		Executor executor,
		AddressResolution addressResolution) throws Exception {

		HighAvailabilityMode highAvailabilityMode = LeaderRetrievalUtils.getRecoveryMode(configuration);

		switch (highAvailabilityMode) {
			case NONE:
				// 省略部分代码

				// 返回非HA服务类实例
				return new StandaloneHaServices(
					resourceManagerRpcUrl,
					dispatcherRpcUrl,
					jobManagerRpcUrl,
					String.format("%s%s:%s", protocol, address, port));
			case ZOOKEEPER:
				BlobStoreService blobStoreService = BlobUtils.createBlobStoreFromConfig(configuration);
				
				// 返回ZK HA服务类实例
				return new ZooKeeperHaServices(
					ZooKeeperUtils.startCuratorFramework(configuration),
					executor,
					configuration,
					blobStoreService);

			case FACTORY_CLASS:
				// 返回自定义HA服务类实例
				return createCustomHAServices(configuration, executor);

			default:
				throw new Exception("Recovery mode " + highAvailabilityMode + " is not supported.");
		}
	}

3.3.2 createCustomHAServices

该方法用于创建自定义HighAvailabilityServices实现类

	private static HighAvailabilityServices createCustomHAServices(Configuration config, Executor executor) throws FlinkException {
		// 获取当前线程的上下文类加载器
		final ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
		// 获取工厂类类名
		final String haServicesClassName = config.getString(HighAvailabilityOptions.HA_MODE);

		final HighAvailabilityServicesFactory highAvailabilityServicesFactory;

		try {
			// 根据类名、类加载器实例化工厂类
			highAvailabilityServicesFactory = InstantiationUtil.instantiate(
				haServicesClassName,
				HighAvailabilityServicesFactory.class,
				classLoader);
		} catch (Exception e) {
			throw new FlinkException(
				String.format(
					"Could not instantiate the HighAvailabilityServicesFactory '%s'. Please make sure that this class is on your class path.",
					haServicesClassName),
				e);
		}

		try {
			// 调用工厂类的createHAServices方法，返回HighAvailabilityServices实例
			return highAvailabilityServicesFactory.createHAServices(config, executor);
		} catch (Exception e) {
			throw new FlinkException(
				String.format(
					"Could not create the ha services from the instantiated HighAvailabilityServicesFactory %s.",
					haServicesClassName),
				e);
		}
	}

3.4 HighAvailabilityServices

HighAvailabilityServices接口定义了HA服务类应当实现的方法，它继承了AutoCloseable接口。

3.5 HighAvailabilityServices的实现类

HighAvailabilityServices的实现类主要有StandaloneHaServices（非HA）、ZooKeeperHaServices、YarnHighAvailabilityServices。

3.6 LeaderElectionService

LeaderElectionService接口定义了leader选举和获取leader的方法

public interface LeaderElectionService {
	// 启动leader选举服务
	void start(LeaderContender contender) throws Exception;

	// 停止leader选举服务
	void stop() throws Exception;

	// 获取新的leader session ID
	void confirmLeaderSessionID(UUID leaderSessionID);

	// 是否拥有leader
	boolean hasLeadership(@Nonnull UUID leaderSessionId);
}

4. 总结

借助ZK的临时节点机制，Flink实现了Job Manager独立集群的高可用性。但是由于ZK是CP，并不保证每次可用性，实际使用中应当予以考虑。

5. 引用

[https://ci.apache.org/projects/flink/flink-docs-release-1.8/ops/jobmanager_high_availability.html]
[https://cwiki.apache.org/confluence/display/FLINK/JobManager+High+Availability]

Apache Flink JobManager HA机制