hbase mvcc

  MVCC相关源码：

  有mvcc用hregion的mvcc的write point，在生成mvcc的readpoint

  KeyValueHeap类的next获取cell的时候使用keyvalue的mvcc的readpoint，判断是否取当前kv

/**
   * Gets the next row of keys from the top-most scanner.
   * <p>
   * This method takes care of updating the heap.
   * <p>
   * This can ONLY be called when you are using Scanners that implement
   * InternalScanner as well as KeyValueScanner (a {@link StoreScanner}).
   * @param result
   * @param limit
   * @return true if there are more keys, false if all scanners are done
   */
  public boolean next(List<Cell> result, int limit) throws IOException {
    if (this.current == null) {
      return false;
    }
    InternalScanner currentAsInternal = (InternalScanner)this.current;
    boolean mayContainMoreRows = currentAsInternal.next(result, limit);
    KeyValue pee = this.current.peek();
    /*
     * By definition, any InternalScanner must return false only when it has no
     * further rows to be fetched. So, we can close a scanner if it returns
     * false. All existing implementations seem to be fine with this. It is much
     * more efficient to close scanners which are not needed than keep them in
     * the heap. This is also required for certain optimizations.
     */
    if (pee == null || !mayContainMoreRows) {
      this.current.close();
    } else {
      this.heap.add(this.current);
    }
    this.current = pollRealKV();
    return (this.current != null);
  }