MySQL Slave同一server_id的冲突原因分析

2024-07-24 12:39:20

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今天分析一个诡异问题,一个模拟Slave线程的程序,不断的被Master Server给kill掉,最终发现是因为有两个Slave使用同样一个server id去连接Master Server,为什么两个Slave用同一个server id会被Master Server给Kill呢？分析了源码,这源于MySQL Replication的重连机制.

我们首先看看一个Slave注册到Master会发生什么,首先Slave需要向Master发送一个COM_REGISTER_SLAVE类型的请求(sql_parse.cc)命令请求,这里Master会使用register_slave函数注册一个Slave到slave_list,代码如下:

case COM_REGISTER_SLAVE:

  {

    if (!register_slave(thd, (uchar*)packet, packet_length))

      my_ok(thd);

    break;

  }

在注册Slave线程的时候会发生什么呢？我们略去无用的代码直接看重点,repl_failsafe.cc代码如下:

int register_slave(THD* thd, uchar* packet, uint packet_length)

{

  int res;

  SLAVE_INFO *si;

  uchar *p= packet, *p_end= packet + packet_length;

.... //省略

  if (!(si->master_id= uint4korr(p)))

    si->master_id= server_id;

  si->thd= thd;

  pthread_mutex_lock(&LOCK_slave_list);

  unregister_slave(thd,0,0); //关键在这里，先取消注册server_id相同的Slave线程

  res= my_hash_insert(&slave_list, (uchar*) si); //把新的Slave线程注册到slave_list  //Vevb.com

  pthread_mutex_unlock(&LOCK_slave_list);

  return res;

.....

}

这是什么意思呢？这就是重连机制,slave_list是一个Hash表,server_id是Key,每一个线程注册上来,需要删掉同样server_id的Slave线程,再把新的Slave线程加到slave_list表中.

线程注册上来后,请求Binlog,发送COM_BINLOG_DUMP请求,Master会发送binlog给Slave,代码如下:

case COM_BINLOG_DUMP:

    {

      ulong pos;

      ushort flags;

      uint32 slave_server_id;

      status_var_increment(thd->status_var.com_other);

      thd->enable_slow_log= opt_log_slow_admin_statements;

      if (check_global_access(thd, REPL_SLAVE_ACL))

        break;

      /* TODO: The following has to be changed to an 8 byte integer */

      pos = uint4korr(packet);

      flags = uint2korr(packet + 4);

      thd->server_id=0; /* avoid suicide */

      if ((slave_server_id= uint4korr(packet+6))) // mysqlbinlog.server_id==0

        kill_zombie_dump_threads(slave_server_id);

      thd->server_id = slave_server_id;

      general_log_print(thd, command, "Log: '%s'  Pos: %ld", packet+10,

                      (long) pos);

      mysql_binlog_send(thd, thd->strdup(packet + 10), (my_off_t) pos, flags); //不断的发送日志给slave端

      unregister_slave(thd,1,1); //发送完成后清理Slave线程，因为执行到这一步肯定是binlog dump线程被kill了

      /*  fake COM_QUIT -- if we get here, the thread needs to terminate */

      error = TRUE;

      break;

    }

mysql_binlog_send函数在sql_repl.cc,里面是轮询Master binlog,发送给Slave,再来简单看看unregister_slave做了什么,repl_failsafe.cc,代码如下:

void unregister_slave(THD* thd, bool only_mine, bool need_mutex)

{

  if (thd->server_id)

  {

    if (need_mutex)

      pthread_mutex_lock(&LOCK_slave_list);

    SLAVE_INFO* old_si;

    if ((old_si = (SLAVE_INFO*)hash_search(&slave_list,

                                           (uchar*)&thd->server_id, 4)) &&

        (!only_mine || old_si->thd == thd)) //拿到slave值

    hash_delete(&slave_list, (uchar*)old_si); //从slave_list中拿掉

    if (need_mutex)

      pthread_mutex_unlock(&LOCK_slave_list);

  }

}