社区投稿 | 基于 WRITESET 的并行复制方式

整个逻辑就在函数 Writeset_trx_dependency_tracker::get_dependency 中，下面是一些关键代码，代码稍多：

if (can_use_writesets) //如果能够使用writeset 方式
  {
    /*
     Check if adding this transaction exceeds the capacity of the writeset
     history. If that happens, m_writeset_history will be cleared only after  而 add_pke
     using its information for current transaction.
    */
    exceeds_capacity=
      m_writeset_history.size() + writeset->size() > m_opt_max_history_size;
//如果大于参数binlog_transaction_dependency_history_size设置清理标记
    /*
     Compute the greatest sequence_number among all conflicts and add the
     transaction's row hashes to the history.
    */
    int64 last_parent= m_writeset_history_start;
//临时变量，首先设置为最小的一个seq number
    for (std::set<uint64>::iterator it= writeset->begin(); it != writeset->end(); ++it)
//循环每一个Writeset中的每一个元素
    {
      Writeset_history::iterator hst= m_writeset_history.find(*it);
//是否在writeset history中 已经存在了。map中的元素是 key是writeset 值是sequence number
      if (hst != m_writeset_history.end()) //如果存在
      {
        if (hst->second > last_parent && hst->second < sequence_number)
          last_parent= hst->second;
//如果已经大于了不需要设置
        hst->second= sequence_number;
//更改这行记录的sequence_number
      }
      else
      {
        if (!exceeds_capacity)
          m_writeset_history.insert(std::pair<uint64, int64>(*it, sequence_number));
//没有冲突则插入。
      }
    }

......
    if (!write_set_ctx->get_has_missing_keys())
//如果没有主键和唯一键那么不更改last commit
    {
      /*
       The WRITESET commit_parent then becomes the minimum of largest parent
       found using the hashes of the row touched by the transaction and the
       commit parent calculated with COMMIT_ORDER.
      */；
      commit_parent= std::min(last_parent, commit_parent);
//这里对last commit做更改了。降低他的last commit
    }
  }
    }
  }

  if (exceeds_capacity || !can_use_writesets)
  {
    m_writeset_history_start= sequence_number;
//如果超过最大设置 清空writeset history。从当前sequence 重新记录 也就是最小的那个事务seqnuce number
    m_writeset_history.clear();//清空真个MAP
  }