详解MySQL的Seconds_Behind_Master
对于mysql主备实例，seconds_behind_master是衡量master与slave之间延时的一个重要参数。通过在slave上执行'show slave status;'可以获取seconds_behind_master的值。
原始实现Definition：The number of seconds that the slave SQL thread is behind processing the master binary log.
Type：time_t（long）
计算方式如下：
rpl_slave.cc::show_slave_status_send_data()if ((mi->get_master_log_pos() == mi->rli->get_group_master_log_pos()) && (!strcmp(mi->get_master_log_name(),mi->rli->get_group_master_log_name()))) { if (mi->slave_running == MYSQL_SLAVE_RUN_CONNECT) protocol->store(0LL); else protocol->store_null(); } else { long time_diff = ((long)(time(0) - mi->rli->last_master_timestamp) - mi->clock_diff_with_master); protocol->store( (longlong)(mi->rli->last_master_timestamp ? max(0L, time_diff) : 0)); }
主要分为以下两种情况：
SQL线程等待IO线程获取主机binlog，此时seconds_behind_master为0，表示备机与主机之间无延时； SQL线程处理relay log，此时seconds_behind_master通过(long)(time(0) ? mi->rli->last_master_timestamp) ? mi->clock_diff_with_master计算得到；last_master_timestamp定义：
主库binlog中事件的时间。
type: time_t (long)
计算方式：
last_master_timestamp根据备机是否并行复制有不同的计算方式。
非并行复制：
rpl_slave.cc:exec_relay_log_event()if ((!rli->is_parallel_exec() || rli->last_master_timestamp == 0) && !(ev->is_artificial_event() || ev->is_relay_log_event() || (ev->common_header->when.tv_sec == 0) || ev->get_type_code() == binary_log::FORMAT_DESCRIPTION_EVENT || ev->server_id == 0)){ rli->last_master_timestamp= ev->common_header->when.tv_sec + (time_t) ev->exec_time; DBUG_ASSERT(rli->last_master_timestamp >= 0);}
在该模式下，last_master_timestamp表示为每一个event的结束时间，其中when.tv_sec表示event的开始时间，exec_time表示事务的执行时间。该值的计算在apply_event之前，所以event还未执行时，last_master_timestamp已经被更新。由于exec_time仅在Query_log_event中存在，所以last_master_timestamp在应用一个事务的不同event阶段变化。以一个包含两条insert语句的事务为例，在该代码段的调用时，打印出event的类型、时间戳和执行时间
create table t1(a int PRIMARY KEY AUTO_INCREMENT ,b longblob) engine=innodb;begin;insert into t1(b) select repeat(’a’,104857600);insert into t1(b) select repeat(’a’,104857600);commit;
10T06:41:32.628554Z 11 [Note] [MY-000000] [Repl] event_type: 33 GTID_LOG_EVENT
2020-02-10T06:41:32.628601Z 11 [Note] [MY-000000] [Repl] event_time: 1581316890
2020-02-10T06:41:32.628614Z 11 [Note] [MY-000000] [Repl] event_exec_time: 0
2020-02-10T06:41:32.628692Z 11 [Note] [MY-000000] [Repl] event_type: 2 QUERY_EVENT
2020-02-10T06:41:32.628704Z 11 [Note] [MY-000000] [Repl] event_time: 1581316823
2020-02-10T06:41:32.628713Z 11 [Note] [MY-000000] [Repl] event_exec_time: 35
2020-02-10T06:41:32.629037Z 11 [Note] [MY-000000] [Repl] event_type: 19 TABLE_MAP_EVENT
2020-02-10T06:41:32.629057Z 11 [Note] [MY-000000] [Repl] event_time: 1581316823
2020-02-10T06:41:32.629063Z 11 [Note] [MY-000000] [Repl] event_exec_time: 0
2020-02-10T06:41:33.644111Z 11 [Note] [MY-000000] [Repl] event_type: 30 WRITE_ROWS_EVENT
2020-02-10T06:41:33.644149Z 11 [Note] [MY-000000] [Repl] event_time: 1581316823
2020-02-10T06:41:33.644156Z 11 [Note] [MY-000000] [Repl] event_exec_time: 0
2020-02-10T06:41:43.520272Z 0 [Note] [MY-011953] [InnoDB] Page cleaner took 9185ms to flush 3 and evict 0 pages
2020-02-10T06:42:05.982458Z 11 [Note] [MY-000000] [Repl] event_type: 19 TABLE_MAP_EVENT
2020-02-10T06:42:05.982488Z 11 [Note] [MY-000000] [Repl] event_time: 1581316858
2020-02-10T06:42:05.982495Z 11 [Note] [MY-000000] [Repl] event_exec_time: 0
2020-02-10T06:42:06.569345Z 11 [Note] [MY-000000] [Repl] event_type: 30 WRITE_ROWS_EVENT
2020-02-10T06:42:06.569376Z 11 [Note] [MY-000000] [Repl] event_time: 1581316858
2020-02-10T06:42:06.569384Z 11 [Note] [MY-000000] [Repl] event_exec_time: 0
2020-02-10T06:42:16.506176Z 0 [Note] [MY-011953] [InnoDB] Page cleaner took 9352ms to flush 8 and evict 0 pages
2020-02-10T06:42:37.202507Z 11 [Note] [MY-000000] [Repl] event_type: 16 XID_EVENT
2020-02-10T06:42:37.202539Z 11 [Note] [MY-000000] [Repl] event_time: 1581316890
2020-02-10T06:42:37.202546Z 11 [Note] [MY-000000] [Repl] event_exec_time: 0
并行复制：
rpl_slave.cc mts_checkpoint_routinets = rli->gaq->empty() ? 0 : reinterpret_cast<Slave_job_group *>(rli->gaq->head_queue())->ts; rli->reset_notified_checkpoint(cnt, ts, true); /* end-of 'Coordinator::'commit_positions' */
在该模式下备机上存在一个分发队列gaq，如果gaq为空，则设置last_commit_timestamp为0；如果gaq不为空，则此时维护一个checkpoint点lwm，lwm之前的事务全部在备机上执行完成，此时last_commit_timestamp被更新为lwm所在事务执行完成后的时间。该时间类型为time_t类型。
ptr_group->ts = common_header->when.tv_sec + (time_t)exec_time; // Seconds_behind_master relatedrli->rli_checkpoint_seqno++;
if (update_timestamp) { mysql_mutex_lock(&data_lock); last_master_timestamp = new_ts; mysql_mutex_unlock(&data_lock);}
在并行复制下，event执行完成之后才会更新last_master_timestamp，所以非并行复制和并行复制下的seconds_behind_master会存在差异。
clock_diff_with_master定义：
The difference in seconds between the clock of the master and the clock of the slave (second - first). It must be signed as it may be <0 or >0. clock_diff_with_master is computed when the I/O thread starts; for this the I/O thread does a SELECT UNIX_TIMESTAMP() on the master. type: longrpl_slave.cc::get_master_version_and_clock()if (!mysql_real_query(mysql, STRING_WITH_LEN('SELECT UNIX_TIMESTAMP()')) && (master_res= mysql_store_result(mysql)) && (master_row= mysql_fetch_row(master_res))) { mysql_mutex_lock(&mi->data_lock); mi->clock_diff_with_master= (long) (time((time_t*) 0) - strtoul(master_row[0], 0, 10)); DBUG_EXECUTE_IF('dbug.mts.force_clock_diff_eq_0', mi->clock_diff_with_master= 0;); mysql_mutex_unlock(&mi->data_lock); }
该差值仅被计算一次，在master与slave建立联系时处理。
其他exec_time定义：
the difference from the statement’s original start timestamp and the time at which it completed executing. type: unsigned longstruct timeval end_time;ulonglong micro_end_time = my_micro_time();my_micro_time_to_timeval(micro_end_time, &end_time);exec_time = end_time.tv_sec - thd_arg->query_start_in_secs();时间函数
（1）time_t time(time_t timer) time_t为long类型，返回的数值仅精确到秒；
（2）int gettimeofday (struct timeval *tv, struct timezone *tz) 可以获得微秒级的当前时间；
（3）timeval结构
#include <time.h>stuct timeval { time_t tv_sec; /*seconds*/ suseconds_t tv_usec; /*microseconds*/}总结
使用seconds_behind_master衡量主备延时只能精确到秒级别，且在某些场景下，seconds_behind_master并不能准确反映主备之间的延时。主备异常时，可以结合seconds_behind_master源码进行具体分析。
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