一、官方释义
1.1、gtid_executed、gtid_purged
https://dev.mysql.com/doc/refman/5.7/en/replication-options-gtids.html#sysvar_gtid_executed
• gtid_executed
When used with global scope, this variable contains a representation of the set of all transactions executed on the server and GTIDs that have been set by a SET gtid_purged statement. This is the same as the value of the Executed_Gtid_Set column in the output of SHOW MASTER STATUS and SHOW SLAVE STATUS.
• gtid_purged
The set of all transactions that have been purged from the binary log. This is a subset of the set of transactions in gtid_executed.
gtid_executed(global)：MySQL数据库已经执行过的Gtid事务，处于内存中。show master status/show slave status中的Executed_Gtid_Set也取自这里
gtid_purged(global)：由于binlog文件的删除(如purge binary logs或者超过expire_logs_days设置)已经丢失的Gtid事务，它是gtid_executed的子集
1.2、binlog_gtid_simple_recovery
https://dev.mysql.com/doc/refman/5.7/en/replication-options-gtids.html#sysvar_binlog_gtid_simple_recovery
binlog_gtid_simple_recovery=FALSE
• To initialize gtid_executed, binary log files are iterated from the newest file, stopping at the first binary log that has any Previous_gtids_log_event. All GTIDs from Previous_gtids_log_event and Gtid_log_events are read from this binary log file. This GTID set is stored internally and called gtids_in_binlog. The value of gtid_executed is computed as the union of this set and the GTIDs stored in the mysql.gtid_executed table.
This process could take a long time if you had a large number of binary log files without GTID events, for example created when gtid_mode=OFF.
• To initialize gtid_purged, binary log files are iterated from the oldest to the newest, stopping at the first binary log that contains either a Previous_gtids_log_event that is non-empty (that has at least one GTID) or that has at least one Gtid_log_event. From this binary log it reads Previous_gtids_log_event. This GTID set is subtracted from gtids_in_binlog and the result stored in the internal variable gtids_in_binlog_not_purged. The value of gtid_purged is initialized to the value of gtid_executed, minus gtids_in_binlog_not_purged.
binlog_gtid_simple_recovery=TRUE
which is the default in MySQL 5.7.7 and later, the server iterates only the oldest and the newest binary log files and the values of gtid_purged and gtid_executed are computed based only on Previous_gtids_log_event or Gtid_log_event found in these files. This ensures only two binary log files are iterated during server restart or when binary logs are being purged.
数据库服务启动时，gtid_executed和gtid_purged按下面方式初始化
binlog_gtid_simple_recovery=FALSE
• gtid_executed：从mysql-bin.index的末行往首行所对应的binlog查找，直到首个被找到包含Previous_gtids_log_event的binlog。然后读取这个binlog的Previous_gtids_log_event和Gtid_log_events中的所有Gtid集合保存到内部变量gtids_in_binlog。然后使用gtids_in_binlog和mysql.gtid_executed表的并集初始化gtid_executed变量
如果你有大量非GTID的binlog(比如gtid_mode=off的情况下创建)，初始化gtid_executed的过程会消耗较长的时间
• gtid_purged：从mysql-bin.index的首行往末行所对应的binlog查找，直到首个被找到包含非空Previous_gtids_log_event或者Gtid_log_event的binlog。然后读取这个binlog的Previous_gtids_log_event，将gtids_in_binlog - Previous_gtids_log_event得到的集合保存到内部变量gtids_in_binlog_not_purged。最后使用gtid_executed - gtids_in_binlog_not_purged初始化gtid_purged变量
binlog_gtid_simple_recovery=TRUE(MySQL5.7.7及以上默认)
只迭代mysql-bin.index的首行和末行所对应的binlog，gtid_executed和gtid_purged的值就是取这两个binlog中的Previous_gtids_log_event/Gtid_log_event计算，当然gtid_executed变量的值还要结合mysql.gtid_executed
1.3、mysql.gtid_executed
https://dev.mysql.com/doc/refman/5.7/en/replication-gtids-concepts.html#replication-gtids-gtid-executed-table
GTIDs are stored in the mysql.gtid_executed table only when gtid_mode is ON or ON_PERMISSIVE. The point at which GTIDs are stored depends on whether binary logging is enabled or disabled:
• If binary logging is disabled (log_bin is OFF), or if log_slave_updates is disabled, the server stores the GTID belonging to each transaction together with the transaction in the table. In addition, the table is compressed periodically at a user-configurable rate; see mysql.gtid_executed Table Compression, for more information. This situation can only apply on a replication slave where binary logging or slave update logging is disabled. It does not apply on a replication master, because on a master, binary logging must be enabled for replication to take place.
• If binary logging is enabled (log_bin is ON), whenever the binary log is rotated or the server is shutdown, the server writes GTIDs for all transactions that were written into the previous binary log into the mysql.gtid_executed table. This situation applies on a replication master, or a replication slave where binary logging is enabled.
In the event of the server stopping unexpectedly, the set of GTIDs from the current binary log is not saved in the mysql.gtid_executed table. In this case, these GTIDs are added to the table and to the set of GTIDs in the gtid_executed system variable during recovery.
When binary logging is enabled, the mysql.gtid_executed table does not provide a complete record of the GTIDs for all executed transactions. That information is provided by the global value of the gtid_executed system variable.
如果没有开启binlog，每个事务提交前，会执行一个insert mysql.gtid_executed操作
如果开启了binlog，在binlog发生rotate(flush binary logs/达到max_binlog_size)或者关闭服务时，会把所有写入到binlog中的Gtid信息写入到mysql.gtid_executed表
如果服务异常停止，当前binlog中的Gtids信息没能写入到mysql.gtid_executed表，在恢复过程会把这些Gtids添加到mysql.gtid_executed表和gtid_executed系统变量(读取binlog中的Previous_gtids_log_event/Gtid_log_event信息？)
二、验证实验
2.1、oldest and newest file
占坑，过两天再补充
1、oldest、newest是如何介定
2、gtid_executed是来自gtids_in_binlog和mysql.gtid_executed的并集
3、如何通过binlog和mysql.gtid_executed得到gtid_executed、gtid_purged
