2024-10-07 15:19:35 +00:00
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# 简介
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为了解决HDFS的水平扩展性问题，社区从Apache Hadoop 0.23.0版本开始引入了HDFS federation。HDFS Federation是指 HDFS集群可同时存在多个NameNode/Namespace，每个Namespace之间是互相独立的；
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单独的一个Namespace里面包含多个 NameNode，其中一个是主，剩余的是备，这个和上面我们介绍的单Namespace里面的架构是一样的。这些Namespace共同管理整个集群的数据，每个Namespace只管理一部分数据，之间互不影响。
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集群中的DataNode向所有的NameNode注册，并定期向这些NameNode发送心跳和块信息，同时DataNode也会执行NameNode发送过来的命令。集群中的NameNodes共享所有DataNode的存储资源。HDFS Federation的架构如下图所示：
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2024-10-07 15:26:19 +00:00
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2024-10-07 15:19:35 +00:00
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# 子模块
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## State Store模块
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### 初始化
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初始化是从类Router的serviceInit函数触发的。
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提供开关：dfs.federation.router.store.enable，默认开启。核心实现类是StateStoreService。
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在serviceInit的时候初始化。
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初始化store driver，通过配置dfs.federation.router.store.driver.class，默认为StateStoreZooKeeperImpl.class,通过反射机制初始化。目前默认支持的有：
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- StateStoreFileImpl
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- StateStoreFileSystemImpl
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- StateStoreMySQLImpl
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- StateStoreZooKeeperImpl
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注册record stores，目前支持的有：
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- MembershipStoreImpl
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- MountTableStoreImpl
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- RouterStoreImpl
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- DisabledNameserviceStoreImpl
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所有的record stores都保存在recordStores当中。
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```java
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// Add supported record stores
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addRecordStore(MembershipStoreImpl.class);
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addRecordStore(MountTableStoreImpl.class);
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addRecordStore(RouterStoreImpl.class);
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addRecordStore(DisabledNameserviceStoreImpl.class);
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```
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初始化定期检查任务
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```java
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// Check the connection to the State Store periodicallythis
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this.monitorService = new StateStoreConnectionMonitorService(this);
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this.addService(monitorService);
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```
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初始化缓存跟新服务
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```java
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// Cache update service
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this.cacheUpdater = new StateStoreCacheUpdateService(this);
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addService(this.cacheUpdater);
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```
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最后是初始化监控信息，核心的监控实现bean是StateStoreMBean。
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### 启动
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启动主要是Router的serviceStart函数触发，最终调用StateStoreDriver的init函数，用于初始化driver。核心函数为initDriver和initRecordStorage。
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#### StateStoreFileImpl
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##### initDriver
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##### initRecordStorage
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#### StateStoreFileSystemImpl
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initDriver
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initRecordStorage
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#### StateStoreMySQLImpl
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initDriver
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initRecordStorage
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#### StateStoreZooKeeperImpl
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initDriver
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initRecordStorage
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## ActiveNamenodeResolver
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## subclusterResolver
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## RPC
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## adminServer
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## httpServer
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## NameNode Heartbeat
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## Router metrics system
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## quota relevant service
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## Safemode
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## mount table cache update
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## quota manager
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