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Mysql 5.7 Gtid内部学习(三) Gtid和Last_commt/sequnce_number的生成时机

简书地址:

http://www.jianshu.com/p/6ee969dc2c9b

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(); 

其调用栈帧如下:

#0  Gtid_state::get_automatic_gno (this=0x2ff8bb0, sidno=1) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/rpl_gtid_state.cc:564  #1  0x0000000001803248 in Gtid_state::generate_automatic_gtid (this=0x2ff8bb0, thd=0x7fff2c000b70, specified_sidno=0, specified_gno=0)      at /root/mysql5.7.14/percona-server-5.7.14-7/sql/rpl_gtid_state.cc:628  #2  0x0000000001845703 in MYSQL_BIN_LOG::write_gtid (this=0x2dffc80, thd=0x7fff2c000b70, cache_data=0x7fff2c021178, writer=0x7ffff0358810)      at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:1167  #3  0x0000000001846307 in binlog_cache_data::flush (this=0x7fff2c021178, thd=0x7fff2c000b70, bytes_written=0x7ffff03588b8, wrote_xid=0x7ffff0358917)      at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:1454  #4  0x0000000001860e57 in binlog_cache_mngr::flush (this=0x7fff2c020ff0, thd=0x7fff2c000b70, bytes_written=0x7ffff0358918, wrote_xid=0x7ffff0358917)      at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:768  #5  0x0000000001856d46 in MYSQL_BIN_LOG::flush_thread_caches (this=0x2dffc80, thd=0x7fff2c000b70) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8470  #6  0x0000000001856f77 in MYSQL_BIN_LOG::process_flush_stage_queue (this=0x2dffc80, total_bytes_var=0x7ffff0358a88, rotate_var=0x7ffff0358a87,       out_queue_var=0x7ffff0358a78) at /root/mysql5.7.14/percona-server-5.7.14-7/sql/binlog.cc:8532  #7  0x0000000001858593 in MYSQL_BIN_LOG::ordered_commit (this=0x2dffc80, thd=0x7fff2c000b70, all=false, skip_commit=false) 

接下来我们就需要具体研究下一个Gtid是依靠什么逻辑生成的。我们需要查看函数Gtid_state::generate_automatic_gtid和Gtid_state::get_automatic_gno逻辑,他们用于生成一个Gtid。

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。

四、Gtid_state::generate_automatic_gtid逻辑

while (true)    {      const Gtid_set::Interval *iv= ivit.get(); //定义Interval指针指向 这个链表指针开头,如果在进行下次循环会获得NULL      rpl_gno next_interval_start= iv != NULL ? iv->start : MAX_GNO; //正常情况下不会为NULL因此 next_interval_start 等于第一个interval的start,当然如果初始化会为NULL,                                                                     //如果Interval->next =NULL 则标示没有区间了。      while (next_candidate.gno < next_interval_start &&             DBUG_EVALUATE_IF("simulate_gno_exhausted", false, true)) //这里next_candidate.gno正常不会小于next_interval_start ,如果Interval->next =NULL或者初始化                                                                      //next_interval_start会被制为MAX_GNO那么条件成立                                                                        //DBUG_RETURN(next_candidate.gno);返回了这个gno 则GTID生成      {        if (owned_gtids.get_owner(next_candidate) == 0) //如果本GTID已经被其他线程占用则next_candidate.gno++;返回这个gno。          DBUG_RETURN(next_candidate.gno);        next_candidate.gno++;      }      if (iv == NULL ||          DBUG_EVALUATE_IF("simulate_gno_exhausted", true, false))      {        my_error(ER_GNO_EXHAUSTED, MYF(0));        DBUG_RETURN(-1);      }      next_candidate.gno= iv->end; //iv->end 则指向了本区间最大的值+1      ivit.next();    } 

五、本节小结

学习完本节至少能够学习到:

  • 1、Gtid在主库什么时候时候生成。
  • 2、Last_commit/seqence_number什么时候生成。
  • 3、Gtid的生成逻辑是怎么样的。

如果有源码阅读能力的同学可以按照这个框架继续深入学习。

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