HBase中MVCC的实现机制及应用情况

MVCC(Multi-Version Concurrent Control),即多版本并发控制协议,广泛使用于数据库系统。本文将介绍HBase中对于MVCC的实现及应用情况。

MVCC基本原理

在介绍MVCC概念之前,我们先来想一下数据库系统里的一个问题:假设有多个用户同时读写数据库里的一行记录,那么怎么保证数据的一致性呢?一个基本的解决方法是对这一行记录加上一把锁,将不同用户对同一行记录的读写操作完全串行化执行,由于同一时刻只有一个用户在操作,因此一致性不存在问题。但是,它存在明显的性能问题:读会阻塞写,写也会阻塞读,整个数据库系统的并发性能将大打折扣。

MVCC(Multi-Version Concurrent Control),即多版本并发控制协议,它的目标是在保证数据一致性的前提下,提供一种高并发的访问性能。在MVCC协议中,每个用户在连接数据库时看到的是一个具有一致性状态的镜像,每个事务在提交到数据库之前对其他用户均是不可见的。当事务需要更新数据时,不会直接覆盖以前的数据,而是生成一个新的版本的数据,因此一条数据会有多个版本存储,但是同一时刻只有最新的版本号是有效的。因此,读的时候就可以保证总是以当前时刻的版本的数据可以被读到,不论这条数据后来是否被修改或删除。

更多关于MVCC基本思想的介绍,参考Wikipedia

一个MVCC实现类

见org.apache.hadoop.hbase.regionserver.MultiVersionConsistencyControl,用于控制Memstore中读写的一致性,其中维护两个long型的变量:

1)memstoreRead:用于记录当前全局可读的readPoint,同时为了每个客户端读请求能够记录自己发起请求时刻的readPoint,还有一个ThreadLocal的perThreadReadPoint变量,以及相关的set和get方法;

2)memstoreWrite:用于记录当前全局最大的writePoint,根据它为下个事务生成新的writePoint。

MultiVersionConsistencyControl中关键的实现方法如下:

1)WriteEntry beginMemstoreInsert():开始一个更新操作,将memstoreWrite加1,创建writeQueue并插入到writeQueue,并返回WriteEntry对象;

2)void completeMemstoreInsert(WriteEntry e):完成当前更新操作,将WriteEntry对象标记为可读,具体分两步:

  • boolean advanceMemstore(WriteEntry e):从头开始遍历writeQueue,移除所有已完成的WriteEntry对象,最后将memstoreRead更新为最新已完成的memstoreWrite;
  • void waitForRead(WriteEntry e):阻塞当前线程,直到memstoreRead等于当前WriteEntry的memstoreWrite,至此表明当前WriteEntry之前的所有更新事务都已经完成。

MVCC使用场景

见org.apache.hadoop.hbase.regionserver.HRegion.java,每个Region包含一个Memstore,维护一个MultiVersionConsistencyControl对象。

写操作

见HRegion.java中的以下写操作的方法:

1)put

2)checkAndPut

3)delete

4)checkAndDelete

5)internalFlushcache

6)mutateRow

7)mutateRowsWithLocks

8)batchMutate

最终会调用到applyFamilyMapToMemstore方法使用MVCC进行写操作:

  /**
   * Atomically apply the given map of family->edits to the memstore.
   * This handles the consistency control on its own, but the caller
   * should already have locked updatesLock.readLock(). This also does
   * <b>not</b> check the families for validity.
   *
   * @param familyMap Map of kvs per family
   * @param localizedWriteEntry The WriteEntry of the MVCC for this transaction.
   *        If null, then this method internally creates a mvcc transaction.
   * @return the additional memory usage of the memstore caused by the
   * new entries.
   */
  private long applyFamilyMapToMemstore(Map<byte[], List<KeyValue>> familyMap,
    MultiVersionConsistencyControl.WriteEntry localizedWriteEntry) {
    long size = 0;
    boolean freemvcc = false;

    try {
      if (localizedWriteEntry == null) {
        localizedWriteEntry = mvcc.beginMemstoreInsert();
        freemvcc = true;
      }

      for (Map.Entry<byte[], List<KeyValue>> e : familyMap.entrySet()) {
        byte[] family = e.getKey();
        List<KeyValue> edits = e.getValue();

        Store store = getStore(family);
        for (KeyValue kv: edits) {
          kv.setMemstoreTS(localizedWriteEntry.getWriteNumber());
          size += store.add(kv);
        }
      }
    } finally {
      if (freemvcc) {
        mvcc.completeMemstoreInsert(localizedWriteEntry);
      }
    }

     return size;
   }
View Code

读操作

HRegion.java中通过private ConcurrentHashMap<RegionScanner, Long> scannerReadPoints;维护各个查询请求的readPoint。

以get或scan请求为例,最终会通过getScanner方法需要构造RegionScannerImpl对象:

org.apache.hadoop.hbase.regionserver.HRegion.RegionScannerImpl

1)根据Scan对象构造时设置好readPoint,scan.getIsolationLevel()分为READ_UNCOMMITTED和READ_COMMITTED,只有当READ_COMMITTED时根据MultiVersionConsistencyControl.resetThreadReadPoint(mvcc);设置当前scanner线程的readPoint,并插入到scannerReadPoints维护起来。

2)根据scan需要读取的column family,创建StoreScanner(根据bloom filter、time range、ttl筛选需要的MemStoreScanner和StoreFileScanner),添加到scanners中,并最终根据scanners构造出一个KeyValueHeap

下面看下RegionScannerImpl中的next方法是每次查询时需要调用的函数:

boolean org.apache.hadoop.hbase.regionserver.HRegion.RegionScannerImpl.next(List<KeyValue> outResults, int limit) throws IOException

而上述方法会通过KeyValueHeap的next方法读取下一条数据:先定位到当前KeyValueScanner(即之前构造KeyValueHeap时传入的MemStoreScanner或StoreScanner),然后调用next方法。

StoreFileScanner和MemStoreScanner均为KeyValueScanner,通过其中的next()接口方法,分别调用到StoreFileScanner.java的skipKVsNewerThanReadpoint方法、Memstore.java中MemStoreScanner对象的getNext方法。

1)StoreFileScanner.java的skipKVsNewerThanReadpoint方法:

  protected boolean skipKVsNewerThanReadpoint() throws IOException {
    long readPoint = MultiVersionConsistencyControl.getThreadReadPoint();

    // We want to ignore all key-values that are newer than our current
    // readPoint
    while(enforceMVCC
        && cur != null
        && (cur.getMemstoreTS() > readPoint)) {
      hfs.next();
      cur = hfs.getKeyValue();
    }

    if (cur == null) {
      close();
      return false;
    }

    // For the optimisation in HBASE-4346, we set the KV's memstoreTS to
    // 0, if it is older than all the scanners' read points. It is possible
    // that a newer KV's memstoreTS was reset to 0. But, there is an
    // older KV which was not reset to 0 (because it was
    // not old enough during flush). Make sure that we set it correctly now,
    // so that the comparision order does not change.
    if (cur.getMemstoreTS() <= readPoint) {
      cur.setMemstoreTS(0);
    }
    return true;
  }
View Code

2)  Memstore.java中MemStoreScanner对象的getNext方法:

    protected KeyValue getNext(Iterator<KeyValue> it) {
      long readPoint = MultiVersionConsistencyControl.getThreadReadPoint();
    
      while (it.hasNext()) {
        KeyValue v = it.next();
        if (v.getMemstoreTS() <= readPoint) {
          return v;
        }
      }

      return null;
    }
View Code

 

posted on 2014-08-12 12:44 大圆那些事 阅读(...) 评论(...) 编辑 收藏

导航

公告