本篇内容主要讲解“Mysql 5.7中Gtid和Last_commt/sequnce_number的生成时机是什么”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“Mysql 5.7中Gtid和Last_commt/sequnce_number的生成时机是什么”吧!
 
一、Gtid生成类型
这里首先使用源码的解释给出三种类型：
 
AUTOMATIC_GROUP
GTID_GROUP
ANONYMOUS_GROUP
其中AUTOMATIC_GROUP通常用于主库开启Gtid的情况，GTID_GROUP通常用于备库和使用了GTID_NEXT的情况下。
 
源码中有详细解释如下：
 
 /**
    Specifies that the GTID has not been generated yet; it will be
    generated on commit.  It will depend on the GTID_MODE: if
    GTID_MODE<=OFF_PERMISSIVE, then the transaction will be anonymous;
    if GTID_MODE>=ON_PERMISSIVE, then the transaction will be assigned
    a new GTID.
 
    This is the default value: thd->variables.gtid_next has this state
    when GTID_NEXT="AUTOMATIC".
 
    It is important that AUTOMATIC_GROUP==0 so that the default value
    for thd->variables->gtid_next.type is AUTOMATIC_GROUP.
  */
  AUTOMATIC_GROUP= 0,
  /**
    Specifies that the transaction has been assigned a GTID (UUID:NUMBER).
 
    thd->variables.gtid_next has this state when GTID_NEXT="UUID:NUMBER".
 
    This is the state of GTID-transactions replicated to the slave.
  */
  GTID_GROUP,
  /**
    Specifies that the transaction is anonymous, i.e., it does not
    have a GTID and will never be assigned one.
 
    thd->variables.gtid_next has this state when GTID_NEXT="ANONYMOUS".
 
    This is the state of any transaction generated on a pre-GTID
    server, or on a server with GTID_MODE==OFF.
  */
ANONYMOUS_GROUP
二、Gtid和Last_commt/sequnce_number的生成时机
Gtid其实是在commit的时候调用MYSQL_BIN_LOG::ordered_commit执行到flush 阶段产生Gtid event的时候才生成，生成后会将这个Gtid 加入到Gtid_state的Owned_gtids中，实际上这个过程不仅要生成Gtid还会生成sequence_number和last_commit并且会构造Gtid_event写入到binlog cache最后将binlog cache写入到binlog file，下面是binlog_cache_data::flush函数的片段：
 
if (!error)
      if ((error= mysql_bin_log.write_gtid(thd, this, &writer))) //生成Gtid和Last_commt/sequnce_number构造好Gtid event并且写入到到binlog cache中
        thd->commit_error= THD::CE_FLUSH_ERROR;
if (!error)
      error= mysql_bin_log.write_cache(thd, this, &writer);  //将binlog cache写入到文件
下面是mysql_bin_log.write_gtid中生成Gtid和Last_commt/sequnce_number的代码片段：
 
 if (thd->variables.gtid_next.type == AUTOMATIC_GROUP)//如果过是非指定的Gtid则需要自动生成调用generate_automatic_gtid生成
  {
    if (gtid_state->generate_automatic_gtid(thd,
            thd->get_transaction()->get_rpl_transaction_ctx()->get_sidno(),
            thd->get_transaction()->get_rpl_transaction_ctx()->get_gno())
            != RETURN_STATUS_OK)
      DBUG_RETURN(true);
  }
…..
//下面生成sequence_number和last_committed
  int64 relative_sequence_number= trn_ctx->sequence_number – clock.get_offset();
  int64 relative_last_committed=
    trn_ctx->last_committed <= clock.get_offset() ?
    SEQ_UNINIT : trn_ctx->last_committed – clock.get_offset();
 
三、Gtid_state::generate_automatic_gtid逻辑
 // If GTID_MODE = ON_PERMISSIVE or ON, generate a new GTID
  if (get_gtid_mode(GTID_MODE_LOCK_SID) >= GTID_MODE_ON_PERMISSIVE)//如果GTID_MODE是ON_PERMISSIVE和ON则生成GTID
  {
    Gtid automatic_gtid= { specified_sidno, specified_gno };
 
    if (automatic_gtid.sidno == 0)//如果是备库则sidno>0，如果是主库sidno==0，因为主库的Gtid这个时候才生成，但是备库则是使用GTID_GROUP指定生成
      automatic_gtid.sidno= get_server_sidno();//此处返回本server的sidno
 
    lock_sidno(automatic_gtid.sidno);//此处对并发生成GNO的多个线程进行控制
 
    if (automatic_gtid.gno == 0)//如果是备库则gno>0，如果是主库gno == 0，因为主库的Gtid这个时候才生成，但是备库则是使用GTID_GROUP指定生成
      automatic_gtid.gno= get_automatic_gno(automatic_gtid.sidno);//此处返回最后指定sidno的end gno
 
    if (automatic_gtid.gno != -1)
      acquire_ownership(thd, automatic_gtid);//此处将这个gtid 及上面的SIDNO:gno加入到owned_gtids中 并且赋予给线程 经过本步骤 可以显示
    else
      ret= RETURN_STATUS_REPORTED_ERROR;
 
    unlock_sidno(automatic_gtid.sidno);//分配完成其他线程可以分配
  }
  else //如果是OFF_PERMISSIVE或者OFF状态如何处理 这里不做讨论了
  {
    // If GTID_MODE = OFF or OFF_PERMISSIVE, just mark this thread as
    // using an anonymous transaction.
    thd->owned_gtid.sidno= THD::OWNED_SIDNO_ANONYMOUS;
    thd->owned_gtid.gno= 0;
    acquire_anonymous_ownership();
    thd->owned_gtid.dbug_print(NULL,
                               "set owned_gtid (anonymous) in generate_automatic_gtid");
  }
 
  sid_lock->unlock();//释放读写锁
接下来看看gno的生成逻辑Gtid_state::get_automatic_gno。