HBase RegionServer详解

RegionServer组件介绍

RegionServer是HBase集群运行在每个工作节点上的服务。它是整个HBase系统的关键所在，一方面它维护了Region的状态，提供了对于Region的管理和服务；另一方面，它与Master交互，参与Master的分布式协调管理。

MemStoreFlusher

MemStoreFlusher主要功能是将MemStore刷新到文件中，当满足一下条件时会出发MemStore执行flush操作，最小的flush单元是region：

• 当一个MemStore的大小等于hbase.hregion.memstore.flush.size指定大小时，所有属于当前region的memstores都将写入到文件；
• 当MemStore使用内存总量达到hbase.regionserver.global.memstore.upperLimit指定值时，将会有多个MemStores flush到文件中，MemStore flush 顺序是按照大小降序执行的，直到刷新到MemStore使用内存略小于hbase.regionserver.global.memstore.lowerLimit。
• 当每一个region server WAL数量达到hbase.regionserver.max.logs指定的值时，不同的region将按照时间先后顺序flush memstores ,较早的将先被刷新，直到WAL数量低于hbase.regionserver.max.logs为止。

  MemStoreFlusher的主要成员变量:

  class MemStoreFlusher implements FlushRequester {
  static final Log LOG = LogFactory.getLog(MemStoreFlusher.class);
  
  // These two data members go together.  Any entry in the one must have
  // a corresponding entry in the other.
  private final BlockingQueue<FlushQueueEntry> flushQueue =
  new DelayQueue<FlushQueueEntry>();
  private final Map<HRegion, FlushRegionEntry> regionsInQueue =
  new HashMap<HRegion, FlushRegionEntry>();
  private AtomicBoolean wakeupPending = new AtomicBoolean();
  
  private final long threadWakeFrequency;
  private final HRegionServer server;
  private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
  private final Object blockSignal = new Object();
  
  protected long globalMemStoreLimit;
  protected float globalMemStoreLimitLowMarkPercent;
  protected long globalMemStoreLimitLowMark;
  
  private long blockingWaitTime;
  private final Counter updatesBlockedMsHighWater = new Counter();
  
  private final FlushHandler[] flushHandlers;
  private List<FlushRequestListener> flushRequestListeners = new ArrayList<FlushRequestListener>(1);

• flushQueue :代表某一个region 的Flush请求，Flusher线程不断地从该队列中获取 请求信息，完成Region的Flush操作；
• regionsInQueue :维护HRegion实例与请求FlushRegionEntry之间的对应关系;某一个FlushQueueEntry实例存在regionsInQueue 中也必然存在于flushQueue中 。
• threadWakeFrequency:用于flushQueue执行poll操作时，最大等待时间，配置项为hbase.server.thread.wakefrequency，默认值10000ms。

  MemStoreFlusher相关配置项：

HeapMemoryManager

CompactSplitThread

ZooKeeperWatcher

MasterAddressTracker

捕获Master服务节点的变化。HBase使用多Master来解决Master单点故障的问题，主Master服务故障时，它与ZooKeeper的心跳延迟超过阈值，ZooKeeeper路径下的数据被清理，备Master上的ActiveMaserManager服务会竞争该Master路径，成为主Master。MasterAddresTracker是RS内部监听Master节点变化的追踪器。

ClusterStatusTracker

HBase集群状态追踪器。该选项可以标识当前集群的状态，及它的启动时间。该设置选项有利于集群中的各个工作节点(RS)统一执行启动和退出操作。

SplitLogWorker

基于Region的HLog文件切分器。在RS宕机之后，RS上的保存的HLog文件，需要按照Region进行切分。HMaster会把这些文件作为任务放置到Zookeeper的splitlog路径下，RS上SplitLogWorker会尝试获取任务，对获取到的HLog文件按照Region进行分组，处理的结果保存到相应Region的recovered.edits目录下。

RegionServer启动过程分析

1.HRegionServer main方法

RegionServer是一个独立的服务，有一个mian方法在启动时被调用。mian方法内部调用HRegionServerCommandLine实现RegionServer的启动。

具体代码如下：

  public static void main(String[] args) throws Exception {
    VersionInfo.logVersion();
    Configuration conf = HBaseConfiguration.create();
    @SuppressWarnings("unchecked")
    Class<? extends HRegionServer> regionServerClass = (Class<? extends HRegionServer>) conf
        .getClass(HConstants.REGION_SERVER_IMPL, HRegionServer.class);

    new HRegionServerCommandLine(regionServerClass).doMain(args);
  }

1. HRegionServer run方法
2. HRegionServer preRegistrationInitialization方法

   此方法内部主要包括初始化zookeeper相关的服务以及RegionServer服务组件的初始化。

 private void preRegistrationInitialization(){
    try {
      setupClusterConnection();

      // Health checker thread.
      if (isHealthCheckerConfigured()) {
        int sleepTime = this.conf.getInt(HConstants.HEALTH_CHORE_WAKE_FREQ,
          HConstants.DEFAULT_THREAD_WAKE_FREQUENCY);
        healthCheckChore = new HealthCheckChore(sleepTime, this, getConfiguration());
      }
      this.pauseMonitor = new JvmPauseMonitor(conf);
      pauseMonitor.start();

      initializeZooKeeper();
      if (!isStopped() && !isAborted()) {
        initializeThreads();
      }
    } catch (Throwable t) {
      // Call stop if error or process will stick around for ever since server
      // puts up non-daemon threads.
      this.rpcServices.stop();
      abort("Initialization of RS failed.  Hence aborting RS.", t);
    }
  }

initializeThreads主要初始化RegionServer服务组件，主要包括初始化compactSplitThread，cacheFlusher，compactionChecker，以及Leases等。具体代码如下：

  private void initializeThreads() throws IOException {
    // Cache flushing thread.
    this.cacheFlusher = new MemStoreFlusher(conf, this);

    // Compaction thread
    this.compactSplitThread = new CompactSplitThread(this);

    // Background thread to check for compactions; needed if region has not gotten updates
    // in a while. It will take care of not checking too frequently on store-by-store basis.
    this.compactionChecker = new CompactionChecker(this, this.threadWakeFrequency, this);
    this.periodicFlusher = new PeriodicMemstoreFlusher(this.threadWakeFrequency, this);
    this.leases = new Leases(this.threadWakeFrequency);

    // Create the thread to clean the moved regions list
    movedRegionsCleaner = MovedRegionsCleaner.createAndStart(this);

    if (this.nonceManager != null) {
      // Create the chore that cleans up nonces.
      nonceManagerChore = this.nonceManager.createCleanupChore(this);
    }

    // Setup RPC client for master communication
    rpcClient = RpcClientFactory.createClient(conf, clusterId, new InetSocketAddress(
        rpcServices.isa.getAddress(), 0));

    int storefileRefreshPeriod = conf.getInt(
        StorefileRefresherChore.REGIONSERVER_STOREFILE_REFRESH_PERIOD
      , StorefileRefresherChore.DEFAULT_REGIONSERVER_STOREFILE_REFRESH_PERIOD);
    if (storefileRefreshPeriod > 0) {
      this.storefileRefresher = new StorefileRefresherChore(storefileRefreshPeriod, this, this);
    }
    registerConfigurationObservers();
  }