016 Hadoop NameNode Automatic Failover
1. Objective
1. 目标
This article is all about Hadoop NameNode Automatic failover. Here we will cover Introduction to failover in Hadoop. It also covers types of Failover i.e. Graceful Failover and Automatic Failover. The components add by failover in Hadoop HDFS- ZooKeeper quorum, ZKFailoverController Process (ZKFC) are also covered on this blog in detail.
这篇文章是关于自动故障切换.在这里，我们将介绍Hadoop.它还涵盖了故障切换的类型，即优雅故障切换和自动故障切换.这部分增加的故障转移 Hadoop HDFS-管理员的法定人数，ZKFailoverController 过程 (ZKFC) 也包含在本博客的细节.
Hadoop NameNode Automatic Failover
Hadoop NameNode Automatic Failover
2. What is Failover?
2. 什么是失败转移？
Failover is the process in which system transfers its control to the secondary system when it detects a fault or failure.
There are two types of Failover:
故障切换是系统在检测到故障或故障时将其控制转移到辅助系统的过程.
故障切换有两种类型:
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Graceful Failover- Administrator manually initiates Graceful Failover, for example, in the case of routine maintenance. The system will not trigger a failover automatically from the active to standby namenode, even if the active node has failed in Graceful Failover.
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Automatic Failover- In the case of NameNode failure, Failover will start automatically. This failover is known as Automatic failover.
Hadoop Quiz
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优雅的故障切换例如，在例行维护的情况下，管理员手动启动优雅的故障切换.即使活动节点在优雅的故障切换中失败，系统也不会自动触发从活动到备用名称节点的故障切换.
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自动故障切换在 NameNode 失败的情况下，故障切换将自动启动.此故障切换称为自动故障切换.
Hadoop Quiz
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3. Apache Hadoop NameNode Automatic Failover
3. Apache Hadoop 、复制指令自动转移
Automatic failover adds below components to an HDFS deployment:
自动故障切换将以下组件添加到HDFS 部署:
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ZooKeeper quorum.
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ZKFailoverController Process (ZKFC).
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动物园管理员人数
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处理 (ZKFC).
3.1. Zookeeper Quorum
3.1.动物园管理员人数
It is a basic service for maintaining small amounts of coordination data, configuration information, and naming. And it provides distributed synchronization and group services. It informs the client of changes in the data, and track clients for failures.
Implementation of automatic HDFS failover relies on Zookeeper for:
它是维护少量协调数据、配置信息和命名的基本服务.提供分布式同步和组服务.它将数据的变化通知客户端，并跟踪客户端是否出现故障.
执行自动 HDFS 失败转移功能依赖于管理员的:
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Failure detection- Zookeeper maintains a session with the namenodes. At the time of failure, the session will expire. And the Zookeeper will inform other namenodes to start the failover process.
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Active NameNode election- Zookeeper provides a mechanism to only elect a node as active. As a result, when active namenode fails, other namenode can take an exclusive lock in Zookeeper. It states that it wants to become the next active namenode.
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故障检测-动物园管理员拥有 namenodes 会话.失败时，会话将过期.和管理员会通知其他 namenodes 启动失败转移过程.
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南德选举活动-Zookeeper 提供了一种只选择活动节点的机制.因此，不活跃、复制指令时，其他、复制指令可以在动物园管理员专用锁.它说，它想成为下一个活跃的 namenode.
Also see Top 50 Hadoop Interview Questions and Answers
3.2. ZKFailoverController process (ZKFC)
3.2.ZKFailoverController 过程 (ZKFC)
ZKFC is a client of Zookeeper that monitors and manages the namenode status. So, Each of the machines which run namenode service also runs a ZKFC.
ZKFC handles:
ZKFC是一个客户管理员监督和管理、复制指令的情况.因此，运行 namenode 服务的每台机器也都运行 ZKFC.
ZKFC 手柄:
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Health monitoring- ZKFS pings its local namenode with a health check command. The ZKFC considers the namenode healthy, as long as the namenode responds at a time. The health check will mark a node as unhealthy, when the node has crashed or entered an unhealthy state.
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Zookeeper session management- ZKFC holds a session open in Zookeeper when the local namenode is healthy. It also holds a special “lock” znode, if the local namenode is active. If the session expires, then the lock will be automatically deleted.
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Zookeeper-based election- In HDFS, if the local namenode is healthy, and the ZKFC sees that no other node currently holds the lock znode, it will itself try to get the lock. So, If it succeeds, then it has won the election, and it handles running a failover to make its local namenode active.
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健康监测-用健康检查命令 ping 其本地名称节点.ZKFC 认为、复制指令的健康，只要、复制指令做出反应的时间.当节点崩溃或进入不健康状态时，健康检查会将节点标记为不健康.
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会话管理-举办一届 ZKFC 开的管理员在本地、复制指令是健康的.如果本地名称节点处于活动状态，它还持有一个特殊的 “锁” znode.如果会话过期，则会自动删除锁.
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动物园管理员的选举在 HDFS 中，如果本地 namenode 是健康的，并且 ZKFC 发现当前没有其他节点持有锁 znode，它本身会尝试获取锁.因此，如果成功，那么它就赢得了选举，并处理运行故障切换以使其本地名称节点处于活动状态.
Also see Distributed Cache in Hadoop: Most Comprehensive Guide
4. Conclusion
结论 4.
In conclusion, we can say that it starts automatically in case of NameNode failure. Automatic failover adds ZooKeeper quorum and ZKFC components to an HDFS deployment. Hence, Zookeeper is a service that provides the automatic failover capability in HDFS.
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See Also-
总之，我们可以说，在 NameNode 失败的情况下，它会自动启动.自动故障转移增加管理员组成法定人数和 ZKFC 到 HDFS 部署.因此，动物园管理员是一种服务，提供了自动 HDFS 转移能力.
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另见-
Reference:
https://hadoop.apache.org
https://data-flair.training/blogs/hadoop-namenode-automatic-failover
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