Mysql 5.7 Gtid內部學習(三) Gtid和Last_commt/sequnce_number的生成時機

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一、Gtid生成型別

這裡首先使用原始碼的解釋給出三種型別：

• AUTOMATIC_GROUP
• GTID_GROUP
• ANONYMOUS_GROUP

其中AUTOMATIC_GROUP通常用於主庫開啟Gtid的情況，GTID_GROUP通常用於備庫和使用了GTID_NEXT的情況下。

原始碼中有詳細解釋如下：

 /**
    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的生成時機

Gtid其實是在commit的時候呼叫MYSQL_BIN_LOG::ordered_commit執行到flush 階段產生Gtid event的時候才生成，生成後會將這個Gtid 加入到Gtid_state的Owned_gtids中，實際上這個過程不僅要生成Gtid還會生成sequence_number和last_commit並且會構造Gtid_event寫入到binlog cache最後將binlog cache寫入到binlog file，下面是binlog_cache_data::flush函式的片段：

if (!error)
      if ((error= mysql_bin_log.write_gtid(thd, this, &writer))) //生成Gtid和Last_commt/sequnce_number構造好Gtid event並且寫入到到binlog cache中
        thd->commit_error= THD::CE_FLUSH_ERROR;
if (!error)
      error= mysql_bin_log.write_cache(thd, this, &writer);  //將binlog cache寫入到檔案 

下面是mysql_bin_log.write_gtid中生成Gtid和Last_commt/sequnce_number的程式碼片段：

 if (thd->variables.gtid_next.type == AUTOMATIC_GROUP)//如果過是非指定的Gtid則需要自動生成呼叫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(); 

其呼叫棧幀如下：

#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) 

接下來我們就需要具體研究下一個Gtid是依靠什麼邏輯生成的。我們需要檢視函式Gtid_state::generate_automatic_gtid和Gtid_state::get_automatic_gno邏輯，他們用於生成一個Gtid。

三、Gtid_state::generate_automatic_gtid邏輯

 // 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則生成GTID
  {
    Gtid automatic_gtid= { specified_sidno, specified_gno };

    if (automatic_gtid.sidno == 0)//如果是備庫則sidno>0，如果是主庫sidno==0，因為主庫的Gtid這個時候才生成，但是備庫則是使用GTID_GROUP指定生成
      automatic_gtid.sidno= get_server_sidno();//此處返回本server的sidno

    lock_sidno(automatic_gtid.sidno);//此處對併發生成GNO的多個執行緒進行控制

    if (automatic_gtid.gno == 0)//如果是備庫則gno>0，如果是主庫gno == 0，因為主庫的Gtid這個時候才生成，但是備庫則是使用GTID_GROUP指定生成
      automatic_gtid.gno= get_automatic_gno(automatic_gtid.sidno);//此處返回最後指定sidno的end gno

    if (automatic_gtid.gno != -1)
      acquire_ownership(thd, automatic_gtid);//此處將這個gtid 及上面的SIDNO:gno加入到owned_gtids中 並且賦予給執行緒 經過本步驟 可以顯示
    else
      ret= RETURN_STATUS_REPORTED_ERROR;

    unlock_sidno(automatic_gtid.sidno);//分配完成其他執行緒可以分配
  }
  else //如果是OFF_PERMISSIVE或者OFF狀態如何處理 這裡不做討論了
  {
    // 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();//釋放讀寫鎖 

接下來看看gno的生成邏輯Gtid_state::get_automatic_gno。

四、Gtid_state::generate_automatic_gtid邏輯

while (true)
  {
    const Gtid_set::Interval *iv= ivit.get(); //定義Interval指標指向 這個連結串列指標開頭,如果在進行下次迴圈會獲得NULL
    rpl_gno next_interval_start= iv != NULL ? iv->start : MAX_GNO; //正常情況下不會為NULL因此 next_interval_start 等於第一個interval的start，當然如果初始化會為NULL，
                                                                   //如果Interval->next =NULL 則標示沒有區間了。
    while (next_candidate.gno < next_interval_start &&
           DBUG_EVALUATE_IF("simulate_gno_exhausted", false, true)) //這裡next_candidate.gno正常不會小於next_interval_start ，如果Interval->next =NULL或者初始化
                                                                    //next_interval_start會被製為MAX_GNO那麼條件成立  
                                                                    //DBUG_RETURN(next_candidate.gno);返回了這個gno 則GTID生成
    {
      if (owned_gtids.get_owner(next_candidate) == 0) //如果本GTID已經被其他執行緒佔用則next_candidate.gno++;返回這個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 則指向了本區間最大的值+1
    ivit.next();
  } 

五、本節小結

學習完本節至少能夠學習到：

• 1、Gtid在主庫什麼時候時候生成。
• 2、Last_commit/seqence_number什麼時候生成。
• 3、Gtid的生成邏輯是怎麼樣的。

如果有原始碼閱讀能力的同學可以按照這個框架繼續深入學習。

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