HBase源码分析之HRegion上MemStore的flsuh流程(二)

        继上篇《HBase源码分析之HRegion上MemStore的flsuh流程(一)》之后,我们继续分析下HRegion上MemStore flush的核心方法internalFlushcache(),它的主要流程如图所示:

HBase源码分析之HRegion上MemStore的flsuh流程(二)

        其中,internalFlushcache()方法的代码如下:

/**
   * Flush the memstore. Flushing the memstore is a little tricky. We have a lot of updates in the
   * memstore, all of which have also been written to the wal. We need to write those updates in the
   * memstore out to disk, while being able to process reads/writes as much as possible during the
   * flush operation.
   * <p>This method may block for some time.  Every time you call it, we up the regions
   * sequence id even if we don't flush; i.e. the returned region id will be at least one larger
   * than the last edit applied to this region. The returned id does not refer to an actual edit.
   * The returned id can be used for say installing a bulk loaded file just ahead of the last hfile
   * that was the result of this flush, etc.
   * @return object describing the flush's state
   *
   * @throws IOException general io exceptions
   * @throws DroppedSnapshotException Thrown when replay of wal is required
   * because a Snapshot was not properly persisted.
   */
  protected FlushResult internalFlushcache(MonitoredTask status)
      throws IOException {
    return internalFlushcache(this.wal, -1, status);
  }

  /**
   * @param wal Null if we're NOT to go via wal.
   * @param myseqid The seqid to use if <code>wal</code> is null writing out flush file.
   * @return object describing the flush's state
   * @throws IOException
   * @see #internalFlushcache(MonitoredTask)
   */
  protected FlushResult internalFlushcache(
      final WAL wal, final long myseqid, MonitoredTask status) throws IOException {
    
	//  如果RegionServerServices类型的rsServices不为空,且为夭折的,直接抛出异常
	if (this.rsServices != null && this.rsServices.isAborted()) {
      // Don't flush when server aborting, it's unsafe
      throw new IOException("Aborting flush because server is aborted...");
    }
	
	// 获取开始时间
    final long startTime = EnvironmentEdgeManager.currentTime();
    // If nothing to flush, return, but we need to safely update the region sequence id
    // 如果没有可以刷新的缓存,直接返回,但是我们需要安全的更新Region的sequence id
    if (this.memstoreSize.get() <= 0) {
      // Take an update lock because am about to change the sequence id and we want the sequence id
      // to be at the border of the empty memstore.
      // 获取一个更新锁,因为我们即将要更新一个序列ID,并且我们想让这个序列ID成为一个空的memstore的边界
      MultiVersionConsistencyControl.WriteEntry w = null;
      
      // 获取更新锁的写锁
      this.updatesLock.writeLock().lock();
      try {
        if (this.memstoreSize.get() <= 0) {
          // Presume that if there are still no edits in the memstore, then there are no edits for
          // this region out in the WAL subsystem so no need to do any trickery clearing out
          // edits in the WAL system. Up the sequence number so the resulting flush id is for
          // sure just beyond the last appended region edit (useful as a marker when bulk loading,
          // etc.)
          // wal can be null replaying edits.
          // 假设如果有memstore仍然没有数据,
          if (wal != null) {
            w = mvcc.beginMemstoreInsert();
            long flushSeqId = getNextSequenceId(wal);
            FlushResult flushResult = new FlushResult(
                FlushResult.Result.CANNOT_FLUSH_MEMSTORE_EMPTY, flushSeqId, "Nothing to flush");
            w.setWriteNumber(flushSeqId);
            mvcc.waitForPreviousTransactionsComplete(w);
            w = null;
            return flushResult;
          } else {
            return new FlushResult(FlushResult.Result.CANNOT_FLUSH_MEMSTORE_EMPTY,
                "Nothing to flush");
          }
        }
      } finally {
        this.updatesLock.writeLock().unlock();
        if (w != null) {
          mvcc.advanceMemstore(w);
        }
      }
    }

    LOG.info("Started memstore flush for " + this +
      ", current region memstore size " +
      StringUtils.byteDesc(this.memstoreSize.get()) +
      ((wal != null)? "": "; wal is null, using passed sequenceid=" + myseqid));

    // Stop updates while we snapshot the memstore of all of these regions' stores. We only have
    // to do this for a moment.  It is quick. We also set the memstore size to zero here before we
    // allow updates again so its value will represent the size of the updates received
    // during flush
    // 当我们更新所有这些region存储的memstore的快照时,停止更新操作。
    // 我们这样做一瞬间,它是非常迅速的。
    // 在我们允许再次更新时,我们也设置memstore的大小为0,所以它的大小也代表了在flush期间接收到的更新的大小
    // 创建多版本一致性控制器中的写条目
    MultiVersionConsistencyControl.WriteEntry w = null;

    // We have to take an update lock during snapshot, or else a write could end up in both snapshot
    // and memstore (makes it difficult to do atomic rows then)
    // 我们需要在快照期间的一个更新锁,否则一个写入最终在快照与内存之前完成(届时将很难做原子行的保证)
    // 获得锁以阻塞并发的更新
    // 设置状态跟踪器的状态:获取锁以阻塞并发的更新
    status.setStatus("Obtaining lock to block concurrent updates");
    
    // block waiting for the lock for internal flush
    // 阻塞,等待flush的锁
    // 获得updatesLock的写锁,阻塞所有对于该Region的更新操作。
    this.updatesLock.writeLock().lock();
    
    long totalFlushableSize = 0;
    
    // 设置状态跟踪器的状态:正在准备通过创建存储的快照刷新
    status.setStatus("Preparing to flush by snapshotting stores in " +
      getRegionInfo().getEncodedName());
    
    // 创建两个缓存容器:storeFlushCtxs列表和committedFiles映射集合,用来存储刷新过程中的刷新上下文和已完成文件路径
    List<StoreFlushContext> storeFlushCtxs = new ArrayList<StoreFlushContext>(stores.size());
    TreeMap<byte[], List<Path>> committedFiles = new TreeMap<byte[], List<Path>>(
        Bytes.BYTES_COMPARATOR);
    
    // 刷新的序列号ID
    long flushSeqId = -1L;

    long trxId = 0;
    try {
      try {
    	// mvcc推进一次写操作事务,此时w中的写序号为0
        w = mvcc.beginMemstoreInsert();
        
        // 获取刷新序列号ID,如果wal不为空,通过wal取下一个序列号,否则设置为-1
        if (wal != null) {// 如果wal不为空
        	
          // startCacheFlush实际上做了两件事:
          // 1、调用closeBarrier.beginOp()方法,确定开始一个flush操作;
          // 2、Region名对应的最近序列化Id从数据结构
          //   oldestUnflushedRegionSequenceIds移动到lowestFlushingRegionSequenceIds中
        	// 疑问:oldestUnflushedRegionSequenceIds中数据是何时放入的?用它来做什么呢?
        	// 在FSHLog的append()方法中,如果entry.isInMemstore(),putIfAbsent放入的
          if (!wal.startCacheFlush(this.getRegionInfo().getEncodedNameAsBytes())) {
            // This should never happen.
            String msg = "Flush will not be started for ["
                + this.getRegionInfo().getEncodedName() + "] - because the WAL is closing.";
            status.setStatus(msg);
            return new FlushResult(FlushResult.Result.CANNOT_FLUSH, msg);
          }
          // Get a sequence id that we can use to denote the flush. It will be one beyond the last
          // edit that made it into the hfile (the below does not add an edit, it just asks the
          // WAL system to return next sequence edit).
          // wal不为空的话,获取下一个序列号
          flushSeqId = getNextSequenceId(wal);
        } else {
          // use the provided sequence Id as WAL is not being used for this flush.
          // 这里myseqid传递进来的是-1
          flushSeqId = myseqid;
        }

        // 循环该Region所有的store,预处理storeFlushCtxs、committedFiles
        // 1、累加每个store可以flush的memstore大小至totalFlushableSize;
        // 2、初始化storeFlushCtxs,为每个store创建对应的flush上下文信息StoreFlusherImpl实例,这些上下文实例携带了同一个刷新序列号
        // 2、将每个store对应的StoreFlushContext添加到ArrayList列表storeFlushCtxs中,实际生成的是StoreFlusherImpl实例
        // 3、将每个store对应的FamilyName添加到TreeMap集合committedFiles中,以备
        // 3、初始化committedFiles:将每个store对应的列名放置到committedFiles的key中,value暂时为null
        for (Store s : stores.values()) {
          totalFlushableSize += s.getFlushableSize();
          
          // 这里只是构造一个StoreFlusherImpl对象,该对象只有cacheFlushSeqNum一个变量被初始化为flushSeqId
          // 然后,加入到storeFlushCtxs列表
          storeFlushCtxs.add(s.createFlushContext(flushSeqId));
          committedFiles.put(s.getFamily().getName(), null); // for writing stores to WAL
        }

        // write the snapshot start to WAL
        // 在WAL中写一个刷新的开始标记,并获取一个事务ID
        if (wal != null) {
        	
          // 其实就是往WAL中append一条记录:row为Region所在的startKey,
          // family为METAFAMILY,
          // qualifier为HBASE::FLUSH,
          // value为FlushDescriptor
          FlushDescriptor desc = ProtobufUtil.toFlushDescriptor(FlushAction.START_FLUSH,
            getRegionInfo(), flushSeqId, committedFiles);
          trxId = WALUtil.writeFlushMarker(wal, this.htableDescriptor, getRegionInfo(),
            desc, sequenceId, false); // no sync. Sync is below where we do not hold the updates lock
        }

        // Prepare flush (take a snapshot)
        // 循环storeFlushCtxs,为每个StoreFlushContext做准备工作,主要是生成memstore的快照
        for (StoreFlushContext flush : storeFlushCtxs) {
          /**
           * 刷新前的准备工作
           * 1、获取memstore的快照,并赋值到snapshot;
           * 2、获取flush的数目,即待刷新cell数目,并赋值到cacheFlushCount;
           * 3、获取flush的大小,并赋值到cacheFlushSize;
           * 4、创建空的已提交文件列表,大小为1。
           */
          flush.prepare();
        }
      } catch (IOException ex) {
        if (wal != null) {
          if (trxId > 0) { // check whether we have already written START_FLUSH to WAL
            try {
              FlushDescriptor desc = ProtobufUtil.toFlushDescriptor(FlushAction.ABORT_FLUSH,
                getRegionInfo(), flushSeqId, committedFiles);
              WALUtil.writeFlushMarker(wal, this.htableDescriptor, getRegionInfo(),
                desc, sequenceId, false);
            } catch (Throwable t) {
              LOG.warn("Received unexpected exception trying to write ABORT_FLUSH marker to WAL:" +
                  StringUtils.stringifyException(t));
              // ignore this since we will be aborting the RS with DSE.
            }
          }
          // we have called wal.startCacheFlush(), now we have to abort it
          // 我们已经调用了wal的startCacheFlush()方法,现在我们不得不放弃它。
          // 1、将Region名对应的SeqId从数据结构lowestFlushingRegionSequenceIds移回至oldestUnflushedRegionSequenceIds
          // 2、调用closeBarrier.endOp(),终止一个操作
          wal.abortCacheFlush(this.getRegionInfo().getEncodedNameAsBytes());
          throw ex; // let upper layers deal with it.
        }
      } finally {
    	// 快照创建好后,释放写锁updatesLock
        this.updatesLock.writeLock().unlock();
      }
      
      // 设置状态跟踪器的状态:完成了memstore的snapshot创建
      String s = "Finished memstore snapshotting " + this +
        ", syncing WAL and waiting on mvcc, flushsize=" + totalFlushableSize;
      status.setStatus(s);
      if (LOG.isTraceEnabled()) LOG.trace(s);
      // sync unflushed WAL changes
      // see HBASE-8208 for details
      if (wal != null) {
        try {
          wal.sync(); // ensure that flush marker is sync'ed
        } catch (IOException ioe) {
          LOG.warn("Unexpected exception while wal.sync(), ignoring. Exception: "
              + StringUtils.stringifyException(ioe));
        }
      }

      // wait for all in-progress transactions to commit to WAL before
      // we can start the flush. This prevents
      // uncommitted transactions from being written into HFiles.
      // We have to block before we start the flush, otherwise keys that
      // were removed via a rollbackMemstore could be written to Hfiles.
      
      // 在我们可以开始flush之前等待所有进行中的事务提交到WAL。这可以防止未提交的事务被写入HFiles。
      // 我们在开始刷新之前,不得不阻塞,否则通过一个rollbackMemstore被删除的keys可能被写入到Hfiles。
      
      // 真正flush之前,先设置一个多版本一致性控制器的写序号,值为本次flush的序列号
      w.setWriteNumber(flushSeqId);
      
      // 然后,调用多版本控制器的方法,等待其他的事务完成
      mvcc.waitForPreviousTransactionsComplete(w);
      // set w to null to prevent mvcc.advanceMemstore from being called again inside finally block
      
      // 设置w为null,防止mvcc.advanceMemstore在finally模块再次被调用
      w = null;
      
      // 设置状态跟踪器的状态:刷新stores进行中...
      s = "Flushing stores of " + this;
      status.setStatus(s);
      if (LOG.isTraceEnabled()) LOG.trace(s);
    } finally {
      if (w != null) {
        // in case of failure just mark current w as complete
    	// 失败的情况下,标记当前w为已完成
        mvcc.advanceMemstore(w);
      }
    }

    // Any failure from here on out will be catastrophic requiring server
    // restart so wal content can be replayed and put back into the memstore.
    // Otherwise, the snapshot content while backed up in the wal, it will not
    // be part of the current running servers state.
    boolean compactionRequested = false;
    try {
      // A.  Flush memstore to all the HStores.
      // Keep running vector of all store files that includes both old and the
      // just-made new flush store file. The new flushed file is still in the
      // tmp directory.

    	// 循环storeFlushCtxs,对每个StoreFlushContext执行刷新操作flushCache,将数据真正写入文件
      for (StoreFlushContext flush : storeFlushCtxs) {
    	 // 调用HStore对象的flushCache()方法,将数据真正写入文件
        flush.flushCache(status);
      }

      // Switch snapshot (in memstore) -> new hfile (thus causing
      // all the store scanners to reset/reseek).
      Iterator<Store> it = stores.values().iterator(); // stores.values() and storeFlushCtxs have
      // same order
      
      // 循环storeFlushCtxs,对每个StoreFlushContext执行commit操作
      for (StoreFlushContext flush : storeFlushCtxs) {
        boolean needsCompaction = flush.commit(status);
        if (needsCompaction) {
          compactionRequested = true;
        }
        committedFiles.put(it.next().getFamily().getName(), flush.getCommittedFiles());
      }
      storeFlushCtxs.clear();

      // Set down the memstore size by amount of flush.
      // 设置flush之后的memstore的大小
      this.addAndGetGlobalMemstoreSize(-totalFlushableSize);

      if (wal != null) {
        // write flush marker to WAL. If fail, we should throw DroppedSnapshotException
    	// 将flush标记写入WAL,同时执行sync
        FlushDescriptor desc = ProtobufUtil.toFlushDescriptor(FlushAction.COMMIT_FLUSH,
          getRegionInfo(), flushSeqId, committedFiles);
        WALUtil.writeFlushMarker(wal, this.htableDescriptor, getRegionInfo(),
          desc, sequenceId, true);
      }
    } catch (Throwable t) {
      // An exception here means that the snapshot was not persisted.
      // The wal needs to be replayed so its content is restored to memstore.
      // Currently, only a server restart will do this.
      // We used to only catch IOEs but its possible that we'd get other
      // exceptions -- e.g. HBASE-659 was about an NPE -- so now we catch
      // all and sundry.
      if (wal != null) {
        try {
          FlushDescriptor desc = ProtobufUtil.toFlushDescriptor(FlushAction.ABORT_FLUSH,
            getRegionInfo(), flushSeqId, committedFiles);
          WALUtil.writeFlushMarker(wal, this.htableDescriptor, getRegionInfo(),
            desc, sequenceId, false);
        } catch (Throwable ex) {
          LOG.warn("Received unexpected exception trying to write ABORT_FLUSH marker to WAL:" +
              StringUtils.stringifyException(ex));
          // ignore this since we will be aborting the RS with DSE.
        }
        wal.abortCacheFlush(this.getRegionInfo().getEncodedNameAsBytes());
      }
      DroppedSnapshotException dse = new DroppedSnapshotException("region: " +
          Bytes.toStringBinary(getRegionName()));
      dse.initCause(t);
      status.abort("Flush failed: " + StringUtils.stringifyException(t));

      // Callers for flushcache() should catch DroppedSnapshotException and abort the region server.
      // However, since we may have the region read lock, we cannot call close(true) here since
      // we cannot promote to a write lock. Instead we are setting closing so that all other region
      // operations except for close will be rejected.
      this.closing.set(true);

      if (rsServices != null) {
        // This is a safeguard against the case where the caller fails to explicitly handle aborting
        rsServices.abort("Replay of WAL required. Forcing server shutdown", dse);
      }

      throw dse;
    }

    // If we get to here, the HStores have been written.
    if (wal != null) {
      // 调用WAL的completeCacheFlush()方法完成MemStore的flush
      // 将Region对应的最近一次序列化ID从数据结构lowestFlushingRegionSequenceIds中删除
      // 调用closeBarrier.endOp()终止一个操作
      wal.completeCacheFlush(this.getRegionInfo().getEncodedNameAsBytes());
    }

    // Record latest flush time
    // 记录当前时间为上次flush时间
    this.lastFlushTime = EnvironmentEdgeManager.currentTime();

    // Update the last flushed sequence id for region. TODO: This is dup'd inside the WAL/FSHlog.
    // 将本次flush序列号ID赋值给lastFlushSeqId
    this.lastFlushSeqId = flushSeqId;

    // C. Finally notify anyone waiting on memstore to clear:
    // e.g. checkResources().
    // 最后唤醒等待memstore的线程
    synchronized (this) {
      notifyAll(); // FindBugs NN_NAKED_NOTIFY
    }

    long time = EnvironmentEdgeManager.currentTime() - startTime;
    long memstoresize = this.memstoreSize.get();
    String msg = "Finished memstore flush of ~" +
      StringUtils.byteDesc(totalFlushableSize) + "/" + totalFlushableSize +
      ", currentsize=" +
      StringUtils.byteDesc(memstoresize) + "/" + memstoresize +
      " for region " + this + " in " + time + "ms, sequenceid=" + flushSeqId +
      ", compaction requested=" + compactionRequested +
      ((wal == null)? "; wal=null": "");
    LOG.info(msg);
    // 设置状态追踪状态:完成
    status.setStatus(msg);

    // 返回flush结果
    return new FlushResult(compactionRequested ? FlushResult.Result.FLUSHED_COMPACTION_NEEDED :
        FlushResult.Result.FLUSHED_NO_COMPACTION_NEEDED, flushSeqId);
  }


        又是一个大方法。莫慌,我们慢慢来分析:
        1、首先,需要判断下HRegion上的RegionServer相关的服务是否正常;
        2、获取开始时间,方便记录耗时,以体现系统的性能;
        3、如果没有可以刷新的缓存,直接返回,但是我们需要安全的更新Region的sequence id;
        4、设置状态跟踪器的状态:获取锁以阻塞并发的更新,即Obtaining lock to block concurrent updates;
        5、获得updatesLock的写锁,阻塞所有对于该Region上数据的更新操作,注意,这里用的是updatesLock,而不是lock;
        6、设置状态跟踪器的状态:正在准备通过创建存储的快照刷新,即Preparing to flush by snapshotting stores in...;
        7、创建两个缓存容器:storeFlushCtxs列表和committedFiles映射集合,用来存储刷新过程中的刷新上下文和已完成文件路径;
        8、创建刷新的序列号ID,即flushSeqId,初始化为-1;
        9、mvcc推进一次写操作事务,此时w中的写序号为0,获得多版本一致性控制器中的写条目;
        10、获取刷新序列号ID,如果wal不为空,通过wal取下一个序列号,否则设置为-1:
                10.1、调用wal的startCacheFlush()方法,在HRegion上开启一个flush操作:
                           10.1.1、调用closeBarrier.beginOp()方法,确定开始一个flush操作;
                           10.1.2、Region名对应的最近序列化Id从数据结构oldestUnflushedRegionSequenceIds移动到lowestFlushingRegionSequenceIds中;
                10.2、 wal不为空的话,获取下一个序列号,赋值给flushSeqId;
        11、循环该Region所有的store,预处理storeFlushCtxs、committedFiles:
                11.1、累加每个store可以flush的memstore大小至totalFlushableSize;
                11.2、将每个store对应的StoreFlushContext添加到ArrayList列表storeFlushCtxs中,实际生成的是StoreFlusherImpl实例,该对象只有cacheFlushSeqNum一个变量被初始化为flushSeqId;
                11.3、初始化committedFiles:将每个store对应的列名放置到committedFiles的key中,value暂时为null;
        12、在WAL中写一个刷新的开始标记,并获取一个事务ID--trxId,其实就是往WAL中append一条记录:row为Region所在的startKey,family为METAFAMILY,qualifier为HBASE::FLUSH,value为FlushDescriptor;
        13、循环storeFlushCtxs,为每个StoreFlushContext做准备工作,主要是生成memstore的快照,刷新前的准备工作如下:
                13.1、获取memstore的快照,并赋值到snapshot;
                13.2、获取flush的数目,即待刷新cell数目,并赋值到cacheFlushCount;
                13.3、获取flush的大小,并赋值到cacheFlushSize;
                13.4、创建空的已提交文件列表,大小为1;
        14、快照创建好后,释放写锁updatesLock;
        15、设置状态跟踪器的状态:完成了memstore的snapshot创建;
        16、真正flush之前,先设置一个多版本一致性控制器的写序号,值为本次flush的序列号;
        17、然后,调用多版本控制器的方法,等待其他的事务完成;
        18、设置w为null,防止mvcc.advanceMemstore在finally模块再次被调用;
        19、设置状态跟踪器的状态:刷新stores进行中...;
        20、失败的情况下,标记当前w为已完成;
        21、循环storeFlushCtxs,对每个StoreFlushContext执行刷新操作flushCache,将数据真正写入文件:
                 21.1、调用HStore对象的flushCache()方法,将数据真正写入文件;
        22、循环storeFlushCtxs,对每个StoreFlushContext执行commit操作;
        23、设置flush之后的memstore的大小,减去totalFlushableSize;
        24、将flush标记写入WAL,同时执行sync;
        25、调用WAL的completeCacheFlush()方法完成MemStore的flush:将Region对应的最近一次序列化ID从数据结构lowestFlushingRegionSequenceIds中删除,并调用closeBarrier.endOp()终止一个操作;
        26、记录当前时间为上次flush时间;
        27、将本次flush序列号ID赋值给lastFlushSeqId;
        28、最后唤醒等待memstore的线程;
        29、设置状态追踪状态:完成;
        30、返回flush结果。


        我的天哪!在没有考虑异常的情况下,居然有整整30个步骤!这样一看,显得很啰嗦、麻烦,我们不如化繁为简,把握主体流程。实际上,整个flush的核心流程不外乎以下几大步骤:

        第一步,上锁,标记状态,而且是上了两把锁:外层是控制HRegion整体行为的锁lock,内层是控制HRegion读写的锁updatesLock;
        第二步,获取flush的序列化ID,并通过多版本一致性控制器mvcc推进一次写事务;
        第三步,通过closeBarrier.beginOp()在HRegion上开启一个操作,避免其他操作(比如compact、split等)同时执行;
        第四步,在WAL中写一个flush的开始标记,并获取一个事务ID;         

        第五步,生成memstore的快照;

        第六步,快照创建好后,释放第一把锁updatesLock,此时客户端又可以发起读写请求;

        第七步,利用多版本一致性控制器mvcc等待其他事务完成;
        第八步,将数据真正写入文件,并提交;
        第九步,在WAL中写一个flush的结束标记;
        第十步,通过调用closeBarrier.endOp()在HRegion上终止一个操作,允许其他操作继续执行。

        这样的话,我们看着就比较顺,比较简单了。不得不说,整个flush设计的还是比较严谨和巧妙地。为什么这么说呢?

        首先,严谨之处体现在,宏观上,它利用closeBarrier.beginOp()和closeBarrier.endOp()很好的控制了HRegion上的多种整体行为,比如flush、compact、split等操作,使其不相互冲突;微观上,针对HRegion上,增加了updatesLock锁,使得数据的更新在flush期间不能进行,保证了数据的准确性;同时,还利用序列号在WAL中标记开始与结束,使得在flush过程中,如果出现异常,系统也能知道开始flush之后数据发生的变化,因为WAL的序列号是递增的,最后,也利用了多版本一致性控制器,保障了写数据时读数据的一致性和完整性,关于多版本一致性控制器相关的内容,将会撰写专门的文章进行介绍,请读者莫急。

        其次,巧妙之处体现在,flush流程采用采用了两把锁,使得Region内部的行为和对外的服务互不影响,同时,利用快照技术,快速生成即将被flush的内存,生成之后立马释放控制写数据的写锁,极大地提高了HBase高并发低延迟的写性能。

        这里,先简单说下写锁和快照的引入,是如何体现HBase高并发写的性能的。

        整个flush的过程是比较繁琐,同时涉及到写真正的物理文件,也是比较耗时的。试想下,如果我们对整个flush过程全程加写锁,结果会怎么样?针对该HRegion的数据读写请求就必须等待整个flush过程的结束,那么对于客户端来说,将不得不经常陷入莫名其妙的等待。
        通过对MemStore生成快照snapshot,并在生成前加更新锁updatesLock的写锁,阻止客户端对MemStore数据的读取与更新,确保了数据的一致性,同时,在快照snapshot生成后,立即释放更新锁updatesLock的写锁,让客户端的后续读写请求与快照flush到物理磁盘文件同步进行,使得客户端的访问请求得到快速的响应,不得不说是HBase团队一个巧妙地设计,也值得我们在以后的系统开发过程中借鉴。

        身体是革命的本钱,不早了,要保证在12点前睡觉啊,还是先休息吧!剩下的细节,只能寄希望于(三)和其他博文了!

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