hbase源码之 compact源码（一）

hbase compact流程较多，这里分章节介绍，首先介绍compact在regionserver中的调用流程，并不会涉及真正的compact读取合并文件的流程，后续介绍。

在regionserver启动时，会初始化compactsplitthread以及CompactionChecker。

/*
   * Check for compactions requests.
   * 检查合并请求
   */
  ScheduledChore compactionChecker;

  // Compactions
  public CompactSplitThread compactSplitThread;

　　其中compactsplitthread是用来实现时机的compact以及split流程的类，而compactchecker是用来周期性检查是否执行compact的。以下首先介绍compactchecker线程。

　　compactionChecker是ScheduledChore类型，而ScheduledChore是hbase定期执行的一个task，如下所示，由注释可知，是hbase周期性执行的一个task。在Regionserver中可以看到flushChecker成员变量也是ScheduledChore类型的。ScheduledChore继承自Runable，因此是一个线程，主要逻辑在其run方法中。

/**
 * ScheduledChore is a task performed on a period in hbase. ScheduledChores become active once
 * scheduled with a {@link ChoreService} via {@link ChoreService#scheduleChore(ScheduledChore)}. The
 * chore is run in a {@link ScheduledThreadPoolExecutor} and competes with other ScheduledChores for
 * access to the threads in the core thread pool. If an unhandled exception occurs, the chore
 * cancellation is logged. Implementers should consider whether or not the Chore will be able to
 * execute within the defined period. It is bad practice to define a ScheduledChore whose execution
 * time exceeds its period since it will try to hog one of the threads in the {@link ChoreService}'s
 * thread pool.
 * <p>
 * Don't subclass ScheduledChore if the task relies on being woken up for something to do, such as
 * an entry being added to a queue, etc.
 */
//scheduledChore继承自Runnable  所以Chore是一个线程
//1. 是hbase定期执行的一个task, 2.在它所在的线程内执行  3.提供了loop循环和sleep机制
@InterfaceAudience.Private
public abstract class ScheduledChore implements Runnable {
}

　　ScheduledChore中的比较重要和成员变量稍作说明，如下：

 //睡眠周期
  private final int period;
//上一次执行改task的时间
  private long timeOfLastRun = -1;
//本次执行的时间
  private long timeOfThisRun = -1;
//该ScheduledChore是否完成初始化，在第一次执行该check时会执行，调用的是initChore()方法，该方法直接返回true，不做任何逻辑处理。
  private boolean initialChoreComplete = false

　　其中还有一个重要的成员变量stopper，stopper是实现了Stopper接口的任意一个对象。根据注释可知，stopper是停止ScheduledChore的一种方式，一旦chore察觉到已经stopped了，会cancel它自己。在Regionserver初始化实例化compactionChecker的时候，会将该stopper设置为this，因此，此处以为这当RS stop时，该chore会感知到，自动cancel其compact。具体的代码：

在ScheduledChore中
/**
   * A means by which a ScheduledChore can be stopped. Once a chore recognizes that it has been
   * stopped, it will cancel itself. This is particularly useful in the case where a single stopper
   * instance is given to multiple chores. In such a case, a single {@link Stoppable#stop(String)}
   * command can cause many chores to stop together.
   */
  private final Stoppable stopper;

在RegionServer中
this.compactionChecker = new CompactionChecker(this,this.frequency, stopper: this)

　

　　ScheduledChore中最核心的部分，即其run方法，run()方法通过一系列的判断 然后周期性执行chore()方法。下面我们一行行解释。

public void run() {
    //将timeOfLastRun设置为当前timeOfThisRun，同时将timeOfThisRun设置为当前时间
    updateTimeTrackingBeforeRun();
    if (missedStartTime() && isScheduled()) {
      onChoreMissedStartTime();
      if (LOG.isInfoEnabled()) LOG.info("Chore: " + getName() + " missed its start time");
    } else if (stopper.isStopped() || !isScheduled()) {
      cancel(false);
      cleanup();
      if (LOG.isInfoEnabled()) LOG.info("Chore: " + getName() + " was stopped");
    } else {
      try {
        if (!initialChoreComplete) {
          initialChoreComplete = initialChore();
        } else {
          chore();
        }
      } catch (Throwable t) {
        if (LOG.isErrorEnabled()) LOG.error("Caught error", t);
        if (this.stopper.isStopped()) {
          cancel(false);
          cleanup();
        }
      }
    }
  }

　　在run方法中，首先调用updateTimeTrackingBeforeRun()方法，该方法很简单，只是简单的update timeOfLastRun和timeOfthsiRun（这两个变量初始化为-1）。每次周期性执行时都会更新。

/**
   * Update our time tracking members. Called at the start of an execution of this chore's run()
   * method so that a correct decision can be made as to whether or not we missed the start time
   */
  private synchronized void updateTimeTrackingBeforeRun() {
    timeOfLastRun = timeOfThisRun;
    timeOfThisRun = System.currentTimeMillis();
  }

　　然后对时间进行判断missedStartTime() && isScheduled()，在compact中isScheduled返回fasle。跳到else if分支，当该chore所依托的载体（此处即为RS）stop了，该chore会自动退出。最终会进入
最后的else分支。在第一次运行时，initialChoreComplete是false，因此会执行initialChore方法，该方法直接返回true，不会做任何处理。
　　在一切都准备好后，会周期执行chore方法，在Regionserver中有CompactionChecker，继承自ScheduledChore, 实现了自己的chore方法，在该方法中会根据判断执行具体的requestCompact方法，下次介绍，
逻辑中也可以看到，首先是判断是否需要compact，如果需要则不会再判断是否需要majorcompact。如下：

/*
   * Inner class that runs on a long period checking if regions need compaction.
   */
  private static class CompactionChecker extends ScheduledChore {
    private final HRegionServer instance;
    private final int majorCompactPriority;
    private final static int DEFAULT_PRIORITY = Integer.MAX_VALUE;
    private long iteration = 0;

    CompactionChecker(final HRegionServer h, final int sleepTime,
        final Stoppable stopper) {
      //调用父类的构造方法
      super("CompactionChecker", stopper, sleepTime);
      //将载体h赋值给instance
      this.instance = h;
      LOG.info(this.getName() + " runs every " + StringUtils.formatTime(sleepTime));

      /* MajorCompactPriority is configurable.
       * If not set, the compaction will use default priority.
       */
      //设置major合并优先级，取参数hbase.regionserver.compactionChecker.majorCompactPriority，默认为Integer.MAX_VALUE
      this.majorCompactPriority = this.instance.conf.
        getInt("hbase.regionserver.compactionChecker.majorCompactPriority",
        DEFAULT_PRIORITY);
    }

    //ScheduledChore的run方法会一直调用chore函数
    @Override
    protected void chore() {
      //遍历instance下的所有online的region  进行循环检测
      //onlineRegions是HRegionServer上存储的所有能够提供有效服务的在线Region集合；
      for (HRegion r : this.instance.onlineRegions.values()) {
        if (r == null)
          continue;
        //取出每个region的store
        for (Store s : r.getStores().values()) {
          try {
            //检查是否需要compact的时间间隔，一般情况是在比如memstore flush后或者其他事件触发compact的，但是有时也需要不同的compact策略，
            // 所以需要周期性的检查具体间隔=hbase.server.compactchecker.interval.multiplier * hbase.server.thread.wakefrequency，默认1000；
            long multiplier = s.getCompactionCheckMultiplier();
            assert multiplier > 0;
            // 未到整数倍，跳过，每当迭代因子iteration为合并检查倍增器multiplier的整数倍时，才会发起检查
            if (iteration % multiplier != 0) continue;
            if (s.needsCompaction()) {//// 需要合并的话，发起SystemCompaction请求，此处最终比较的是是否当前hfile数量减去正在compacting的文件数大于设置的compact min
　　　　　　　　　　　　　　　　　　　　　　//值。若满足则执行systemcompact
              // Queue a compaction. Will recognize if major is needed.
              this.instance.compactSplitThread.requestSystemCompaction(r, s, getName()
                  + " requests compaction");
            } else if (s.isMajorCompaction()) {
              if (majorCompactPriority == DEFAULT_PRIORITY
                  || majorCompactPriority > r.getCompactPriority()) {
                this.instance.compactSplitThread.requestCompaction(r, s, getName()
                    + " requests major compaction; use default priority", null);
              } else {
                this.instance.compactSplitThread.requestCompaction(r, s, getName()
                    + " requests major compaction; use configured priority",
                  this.majorCompactPriority, null);
              }
            }
          } catch (IOException e) {
            LOG.warn("Failed major compaction check on " + r, e);
          }
        }
      }
      iteration = (iteration == Long.MAX_VALUE) ? 0 : (iteration + 1);
    }
  }

　　其中判断是否需要compact比较简单，主要是isMajorCompaction的判断。最主要的逻辑如下:()

　　　　获取下一次majorcompact的时间mc

　　　　获取所有需要compact的file的modify time，已得到所有的file中最小的时间戳lowTimestamp，如果lowTimestamp<now - mc以为这需要进行major compact了。

　　　　　　如果此时只有一个file，则进行如下判断

　　　　　　　　如果未过期，且其block的本定性不要求满足，则进行majorcompact，否则不进行major compact

　　　　　　　　如果过期，则进行major compact

public boolean isMajorCompaction(final Collection<StoreFile> filesToCompact)
    throws IOException {
  boolean result = false;
  //获取下一次major compact的时间
  long mcTime = getNextMajorCompactTime(filesToCompact);
  if (filesToCompact == null || filesToCompact.isEmpty() || mcTime == 0) {
    return result;
  }
  // TODO: Use better method for determining stamp of last major (HBASE-2990)
  //获取待合并文件中modify的最小时间戳 以及当前时间
  long lowTimestamp = StoreUtils.getLowestTimestamp(filesToCompact);
  long now = System.currentTimeMillis();
  if (lowTimestamp > 0l && lowTimestamp < (now - mcTime)) {
    //lowTimestamp < (now - mcTime)即意味着当前时间位于进行major compact的时间范围之内，要进行compact
    // Major compaction time has elapsed.
    long cfTtl = this.storeConfigInfo.getStoreFileTtl();
    if (filesToCompact.size() == 1) {
      // Single file
      StoreFile sf = filesToCompact.iterator().next();
      Long minTimestamp = sf.getMinimumTimestamp();
      //文件存留时间oldest
      long oldest = (minTimestamp == null)
          ? Long.MIN_VALUE
          : now - minTimestamp.longValue();
      if (sf.isMajorCompaction() &&
          (cfTtl == HConstants.FOREVER || oldest < cfTtl)) {
        float blockLocalityIndex = sf.getHDFSBlockDistribution().getBlockLocalityIndex(
            RSRpcServices.getHostname(comConf.conf, false)
        );
        if (blockLocalityIndex < comConf.getMinLocalityToForceCompact()) {
          if (LOG.isDebugEnabled()) {
            LOG.debug("Major compaction triggered on only store " + this +
                "; to make hdfs blocks local, current blockLocalityIndex is " +
                blockLocalityIndex + " (min " + comConf.getMinLocalityToForceCompact() +
                ")");
          }
          result = true;
        } else {
          if (LOG.isDebugEnabled()) {
            LOG.debug("Skipping major compaction of " + this +
                " because one (major) compacted file only, oldestTime " +
                oldest + "ms is < ttl=" + cfTtl + " and blockLocalityIndex is " +
                blockLocalityIndex + " (min " + comConf.getMinLocalityToForceCompact() +
                ")");
          }
        }
      } else if (cfTtl != HConstants.FOREVER && oldest > cfTtl) {////只有一个hfile（最早的ts>ttl）整个文件过期 => 进行marjor compact
        LOG.debug("Major compaction triggered on store " + this +
          ", because keyvalues outdated; time since last major compaction " +
          (now - lowTimestamp) + "ms");
        result = true;
      }
    } else {
      if (LOG.isDebugEnabled()) {
        LOG.debug("Major compaction triggered on store " + this +
            "; time since last major compaction " + (now - lowTimestamp) + "ms");
      }
      result = true;
    }
  }
  return result;
}