一、级联时间计算方式

比如A->B->C那么C的延迟是计算B还是A的。因为Event的中timestamp没变，但是C计算时间差值却是以B为准。如果时钟同步的情况下，其实基本可以考虑为A的。计算方法可以参考我的《深入理解MySQL主从原理》专栏

逻辑如下：

级联中的Event依然是主库的时间，因此其延迟还是相对主库而言。
虽然apply_event_and_update_pos函数中由设置为当前时间thd->set_time()
但是最终设置还是在Query_log_event::do_apply_event和Query_log_event::do_apply_event中的
的
THD::set_time (this=0x7ffe74007da0, t=0x7ffe74007828)

因为只有做了数据库修改才会触发记录Event的工作

设个设置就是设置为event header的timestamp，因此还是级联中记录的Event的时间还是
主库的时间，计算延迟就是相对主库的时间。



栈帧

#0  THD::set_time (this=0x7ffe74007da0, t=0x7ffe7493c4d0) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/sql_class.h:3526
#1  0x00000000018459ab in Query_log_event::do_apply_event (this=0x7ffe7493c3b0, rli=0x676be60, query_arg=0x7ffe740061dc "BEGIN", q_len_arg=5)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/log_event.cc:4714
#2  0x0000000001845287 in Query_log_event::do_apply_event (this=0x7ffe7493c3b0, rli=0x676be60)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/log_event.cc:4567
#3  0x00000000018420d4 in Log_event::apply_event (this=0x7ffe7493c3b0, rli=0x676be60) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/log_event.cc:3570
#4  0x00000000018bc078 in apply_event_and_update_pos (ptr_ev=0x7fffec094830, thd=0x7ffe74007da0, rli=0x676be60)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/rpl_slave.cc:4766
#5  0x00000000018bd773 in exec_relay_log_event (thd=0x7ffe74007da0, rli=0x676be60) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/rpl_slave.cc:5300
#6  0x00000000018c46e4 in handle_slave_sql (arg=0x6675d30) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/rpl_slave.cc:7543
#7  0x0000000001945620 in pfs_spawn_thread (arg=0x7ffe7c02c6c0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/perfschema/pfs.cc:2190
#8  0x00007ffff7bc6aa1 in start_thread () from /lib64/libpthread.so.0
#9  0x00007ffff6719bcd in clone () from /lib64/libc.so.6


#0  THD::set_time (this=0x7ffe74007da0, t=0x7ffe7493c4d0) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/sql_class.h:3526
#1  0x00000000018459ab in Query_log_event::do_apply_event (this=0x7ffe7493c3b0, rli=0x676be60, query_arg=0x7ffe740061dc "BEGIN", q_len_arg=5)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/log_event.cc:4714
#2  0x0000000001845287 in Query_log_event::do_apply_event (this=0x7ffe7493c3b0, rli=0x676be60)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/log_event.cc:4567
#3  0x00000000018420d4 in Log_event::apply_event (this=0x7ffe7493c3b0, rli=0x676be60) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/log_event.cc:3570
#4  0x00000000018bc078 in apply_event_and_update_pos (ptr_ev=0x7fffec094830, thd=0x7ffe74007da0, rli=0x676be60)
    at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/rpl_slave.cc:4766
#5  0x00000000018bd773 in exec_relay_log_event (thd=0x7ffe74007da0, rli=0x676be60) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/rpl_slave.cc:5300
#6  0x00000000018c46e4 in handle_slave_sql (arg=0x6675d30) at /root/mysqlall/percona-server-locks-detail-5.7.22/sql/rpl_slave.cc:7543
#7  0x0000000001945620 in pfs_spawn_thread (arg=0x7ffe7c02c6c0) at /root/mysqlall/percona-server-locks-detail-5.7.22/storage/perfschema/pfs.cc:2190
#8  0x00007ffff7bc6aa1 in start_thread () from /lib64/libpthread.so.0
#9  0x00007ffff6719bcd in clone () from /lib64/libc.so.6


query map event在修改的第一条数据 记录时间 这个时间肯定是Log_event::apply_event的时间，
GTID自然是在最后commit的时候 ,XID EVENT也是正确的主库时间。

实际上第一个THD::set_time(header->timestamp)后user_time 就正确，下面的逻辑不会设置为当前时间。

inline void set_time()
  {
    start_utime= utime_after_lock= my_micro_time();
    if (user_time.tv_sec || user_time.tv_usec)
    {
      start_time= user_time;
    }
    else
      my_micro_time_to_timeval(start_utime, &start_time);

#ifdef HAVE_PSI_THREAD_INTERFACE
    PSI_THREAD_CALL(set_thread_start_time)(start_time.tv_sec);
#endif
  }


但是主库dispatch_command的时候不会设置user_time user_time为0，因此设置为当前时间。

#0  THD::set_time (this=0x7ffedc0009c0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_class.h:3514
#1  0x00000000015c5fe8 in dispatch_command (thd=0x7ffedc0009c0, com_data=0x7fffec5bdd70, command=COM_QUERY)
    at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1247
#2  0x00000000015c58ff in do_command (thd=0x7ffedc0009c0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/sql_parse.cc:1021
#3  0x000000000170e578 in handle_connection (arg=0x67d01a0) at /mysqldata/percona-server-locks-detail-5.7.22/sql/conn_handler/connection_handler_per_thread.cc:312
#4  0x0000000001945538 in pfs_spawn_thread (arg=0x67c9dc0) at /mysqldata/percona-server-locks-detail-5.7.22/storage/perfschema/pfs.cc:2190
#5  0x00007ffff7bcfaa1 in start_thread () from /lib64/libpthread.so.0
#6  0x00007ffff6b37c4d in clone () from /lib64/libc.so.6

二、header中的timestamp和query_event的exe time计算方式

common_header：中的时间来自 命令发起的时间。下面是其初始化  
  
Log_event::Log_event(THD* thd_arg, uint16 flags_arg,
                     enum_event_cache_type cache_type_arg,
                     enum_event_logging_type logging_type_arg,
                     Log_event_header *header, Log_event_footer *footer)
  : is_valid_param(false), temp_buf(0), exec_time(0),
    event_cache_type(cache_type_arg), event_logging_type(logging_type_arg),
    crc(0), common_header(header), common_footer(footer), thd(thd_arg)
{
  server_id= thd->server_id;
  common_header->unmasked_server_id= server_id;
  common_header->when= thd->start_time;
  common_header->log_pos= 0;
  common_header->flags= flags_arg;
}
 
dispatch_command调用时间


Query_log_event::Query_log_event(THD* thd_arg, const char* query_arg,
                 size_t query_length, bool using_trans,
                 bool immediate, bool suppress_use,
                                 int errcode, bool ignore_cmd_internals)

: binary_log::Query_event(query_arg,
                          thd_arg->catalog().str,
                          thd_arg->db().str,
                          query_length,
                          thd_arg->thread_id(),
                          thd_arg->variables.sql_mode,
                          thd_arg->variables.auto_increment_increment,
                          thd_arg->variables.auto_increment_offset,
                          thd_arg->variables.lc_time_names->number,
                          (ulonglong)thd_arg->table_map_for_update,
                          errcode,
                          thd_arg->db().str ? strlen(thd_arg->db().str) : 0,
                          thd_arg->catalog().str ? strlen(thd_arg->catalog().str) : 0),
  Log_event(thd_arg,
            (thd_arg->thread_specific_used ? LOG_EVENT_THREAD_SPECIFIC_F :
             0) |
            (suppress_use ? LOG_EVENT_SUPPRESS_USE_F : 0),
            using_trans ? Log_event::EVENT_TRANSACTIONAL_CACHE :
                          Log_event::EVENT_STMT_CACHE,
            Log_event::EVENT_NORMAL_LOGGING,
            header(), footer()),
  data_buf(0)
{
  DBUG_EXECUTE_IF("debug_lock_before_query_log_event",
                  DBUG_SYNC_POINT("debug_lock.before_query_log_event", 10););

  /* save the original thread id; we already know the server id */
  slave_proxy_id= thd_arg->variables.pseudo_thread_id;
  if (query != 0)
    is_valid_param= true;
  /*
  exec_time calculation has changed to use the same method that is used
  to fill out "thd_arg->start_time"
  */

  struct timeval end_time;
  ulonglong micro_end_time= my_micro_time();//这里获取时间 query event
  my_micro_time_to_timeval(micro_end_time, &end_time);

  exec_time= end_time.tv_sec - thd_arg->start_time.tv_sec;//这里计算时间