本文最初于2016年底发表在我的个人微信公众号里面，现略有修订后发布在这里。本文的技术信息针对的是mysql-5.7.x和mariadb-10.1.9。

MariaDB和MySQL两者对分布式事务的支持有所不同，总的来说MySQL的支持更好，是完备的和可靠的(尽管后来也陆续发现了不少bug)，而MariaDB的支持还有诸多问题，先举例说明。

本文测试用例使用MySQL-5.7.16和MariaDB-10.1.9 进行测试。

MySQL/MariaDB对分布式事务的支持是根据 X/Open CAE document Distributed Transaction Processing: The XA Specification (http://www.opengroup.org/public/pubs/catalog/c193.htm) ，主要包括下面这几个语句：

xa start 'gtid';

xa end 'gtid';

xa prepare 'gtid';

xa commit 'gtid';

xa rollback 'gtid';

xa recover;

外部的分布式事务管理器(transaction manager, TM) 可以使用这套XA命令来操作mysql 数据库，按照XA标准的两阶段提交算法(2PC) 完成分布式事务提交。各个语句的意义见官方文档。

其中mariadb的问题在于 xa prepare 'gtid' 之后，这个事务分支(transaction branch) 可能会丢失。详见下文。

事先创建1个简单的表并且插入4行数据：

create table t1(a int primary key);

insert into t1 values(1),(2),(3),(4);

一。两者的公共行为(相同点)

本节的查询是要说明，xa prepare之前(无论是xa end 之前还是之后)，xa事务的改动对外部不可见，也不持久化，退出连接就会丢失修改。xa事务信息本身也会在退出连接后消失，重新连接后xa recover不会显示它。

这些行为mysql与mariadb相同，都是正确的。

mysql> use test;

Reading table information for completion of table and column names

You can turn off this feature to get a quicker startup with -A

Database changed

mysql> xa start '124';

Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values(5);

Query OK, 1 row affected (0.00 sec)

mysql> insert into t1 values(6);

Query OK, 1 row affected (0.00 sec)

mysql> quit;

Bye

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ ./bin/mysql -S ../../mysql-instances/a/mysql.sock

Welcome to the MySQL monitor. Commands end with ; or \g.

Your MySQL connection id is 5

Server version: 5.7.16-debug-log Source distribution

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its

affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> xa recover;

Empty set (0.00 sec)

mysql> use test;

Reading table information for completion of table and column names

You can turn off this feature to get a quicker startup with -A

Database changed

mysql> xa start '124';

Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values(5);

Query OK, 1 row affected (0.00 sec)

mysql> insert into t1 values(6);

Query OK, 1 row affected (0.00 sec)

mysql> xa end '124';

Query OK, 0 rows affected (0.00 sec)

mysql> quit;

Bye

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ ./bin/mysql -S ../../mysql-instances/a/mysql.sock

Welcome to the MySQL monitor. Commands end with ; or \g.

Your MySQL connection id is 6

Server version: 5.7.16-debug-log Source distribution

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its

affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> xa recover;

Empty set (0.00 sec)

mysql> select*from t1;

ERROR 1046 (3D000): No database selected

mysql> use test;

Reading table information for completion of table and column names

You can turn off this feature to get a quicker startup with -A

Database changed

mysql> select*from t1;

+------+

| a |

+------+

| 1 |

| 2 |

| 3 |

| 4 |

+------+

4 rows in set (0.00 sec)

二。MySQL的XA PREPARE 的行为

1. 对MySQL来说，在一个事务中执行xa prepare之后退出连接，xa事务不丢失，它的更新也不会丢失。

mysql> create table t1(a int);

Query OK, 0 rows affected (0.05 sec)

mysql> xa start '123';

Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values(1),(2);

Query OK, 2 rows affected (0.00 sec)

Records: 2 Duplicates: 0 Warnings: 0

mysql> insert into t1 values(3),(4);

Query OK, 2 rows affected (0.00 sec)

Records: 2 Duplicates: 0 Warnings: 0

mysql> xa end '123';

Query OK, 0 rows affected (0.00 sec)

mysql> xa prepare '123';

Query OK, 0 rows affected (0.07 sec)

mysql> quit;

Bye

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ ./bin/mysql -S ../../mysql-instances/a/mysql.sock

Welcome to the MySQL monitor. Commands end with ; or \g.

Your MySQL connection id is 4

Server version: 5.7.16-debug-log Source distribution

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its

affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> xa recover;

+----------+--------------+--------------+------+

| formatID | gtrid_length | bqual_length | data |

+----------+--------------+--------------+------+

| 1 | 3 | 0 | 123 |

+----------+--------------+--------------+------+

1 row in set (0.00 sec)

mysql> xa commit '123';

Query OK, 0 rows affected (0.01 sec)

mysql> select*From test.t1;

+------+

| a |

+------+

| 1 |

| 2 |

| 3 |

| 4 |

+------+

4 rows in set (0.00 sec)

2. 对mysql来说，在一个事务中执行xa prepare后，kill掉 mysqld ，xa 事务的改动及其元数据不丢失，mysql binlog系统与innodb做了协调一致的事务恢复，非常完美。重启mysqld后，客户端连接上去，执行xa recover可以看到这个prepared事务，执行 xa commit 可以完成该事务的提交。

mysql> xa start '124';

Query OK, 0 rows affected (0.00 sec)

mysql> insert into t1 values(5);

Query OK, 1 row affected (0.00 sec)

mysql> insert into t1 values(6);

Query OK, 1 row affected (0.00 sec)

mysql> xa end '124';

Query OK, 0 rows affected (0.00 sec)

mysql> xa prepare '124';

Query OK, 0 rows affected (0.00 sec)

mysql> quit;

Bye

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ ./bin/mysql -S ../../mysql-instances/a/mysql.sock

Welcome to the MySQL monitor. Commands end with ; or \g.

Your MySQL connection id is 7

Server version: 5.7.16-debug-log Source distribution

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its

affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> xa recover;

+----------+--------------+--------------+------+

| formatID | gtrid_length | bqual_length | data |

+----------+--------------+--------------+------+

| 1 | 3 | 0 | 124 |

+----------+--------------+--------------+------+

1 row in set (0.00 sec)

mysql> quit

Bye

[1]+ Killed ./bin/mysqld_safe --defaults-file=../../mysql-instances/a/my.cnf

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ ./bin/mysqld_safe --defaults-file=../../mysql-instances/a/my.cnf &

[1] 22109

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ 2016-12-02T06:53:49.336023Z mysqld_safe Logging to '/home/david/mysql-instances/a/datadir/david-VirtualBox.err'.

2016-12-02T06:53:49.350561Z mysqld_safe Starting mysqld daemon with databases from /home/david/mysql-instances/a/datadir

david@david-VirtualBox:~/mysql_installs/mysql-5.7.16$ ./bin/mysql -S ../../mysql-instances/a/mysql.sock

Welcome to the MySQL monitor. Commands end with ; or \g.

Your MySQL connection id is 2

Server version: 5.7.16-debug-log Source distribution

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its

affiliates. Other names may be trademarks of their respective

owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> xa recover;

+----------+--------------+--------------+------+

| formatID | gtrid_length | bqual_length | data |

+----------+--------------+--------------+------+

| 1 | 3 | 0 | 124 |

+----------+--------------+--------------+------+

1 row in set (0.00 sec)

mysql> select*from t1;

ERROR 1046 (3D000): No database selected

mysql> use test;

Reading table information for completion of table and column names

You can turn off this feature to get a quicker startup with -A

Database changed

mysql> select*from t1;

+------+

| a |

+------+

| 1 |

| 2 |

| 3 |

| 4 |

+------+

4 rows in set (0.00 sec)

mysql> xa commit '124';

Query OK, 0 rows affected (0.00 sec)

mysql> select*from t1;

+------+

| a |

+------+

| 1 |

| 2 |

| 3 |

| 4 |

| 5 |

| 6 |

+------+

6 rows in set (0.00 sec)

mysql>

3. mysql之所以能做到上述行为，是因为mysql在xa prepare 做了binlog刷盘，并且允许其后的xa commit被其他事务的binlog隔开。

本例中，新启动一个xa事务 T1，在xa prepare 之后，另启动一个连接在其中执行2个普通本地事务，然后再做xa commit T1，可以看到这个xa commit 的binlog与xa prepare被那两个新事务的binlog隔开了。对MySQL来说这不是问题。

三。MariaDB的XA支持

mariadb没有把xa start到xa prepare这一块操作作为一个单独的binlog事务并且在xa prepare时刻做binlog刷盘，因此xa prepare之后一旦数据库连接断开或者mysqld重启，那么binlog子系统中prepared状态的事务的binlog就消失了，但是innodb当中这个prepared事务是存在的也就是说binlog与innodb数据不一致了。

1. xa prepare 后连接断开，那么这个prepared事务就消失了，包括事务修改数据和元数据。binlog上面没有它的binlog，xa recover也不能列出这个事务。这就错的更加离谱了，可能是连接断开后mariadb把innodb中prepared的事务也回滚了。

MariaDB [(none)]> xa start '124';

Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> insert into t1 values(5);

ERROR 1046 (3D000): No database selected

MariaDB [(none)]> use test;

Database changed

MariaDB [test]> insert into t1 values(5);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> insert into t1 values(6);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> xa end '124';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> xa prepare '124';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> xa recover;

+----------+--------------+--------------+------+

| formatID | gtrid_length | bqual_length | data |

+----------+--------------+--------------+------+

| 1 | 3 | 0 | 124 |

+----------+--------------+--------------+------+

1 row in set (0.00 sec)

MariaDB [test]> quit;

Bye

[tdengine@TENCENT64 ~/davie/mysql_installs/tdsql-mariadb-10.1.9-bin-release3/install]$./jmysql.sh 6678

cmd: e

/data/6678/prod/mysql.sock

/data/home/tdengine/davie/mysql_installs/tdsql-mariadb-10.1.9-bin-release3

Welcome to the MariaDB monitor. Commands end with ; or \g.

Your MariaDB connection id is 9

Server version: 10.1.9-MariaDBV1.0R030D002-20161123-1511 Source distribution

Copyright (c) 2000, 2015, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> xa recover;

Empty set (0.00 sec)

MariaDB [(none)]> use test;

Database changed

MariaDB [test]> select*from t1;

Empty set (0.00 sec)

MariaDB [test]>

2. xa prepare 后kill掉mysqld，那么这个prepared事务在binlog上面不存在，不过xa recover能列出这个事务，并且它的改动也存在。

在innodb当中，执行了XA PREPARE之后，这个事务已经prepare了，但是它的binlog没有写到binlog文件中。mysqld被kill掉重新拉起后，innodb做了事务恢复，所以可以看到它之前的改动；但是binlog上面却没有这个事务。不过做xa commit 却又可以提交这个事务。之后，这个事务的改动就可见了。但是，innodb中的数据与binlog已经不一致了。备机上面没有事务内容

MariaDB [test]> xa start '125';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> insert into t1 values(9);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> insert into t1 values(10);

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> xa end '125';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> xa prepare '125';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> quit

Bye

[tdengine@TENCENT64 ~/davie/mysql_installs/tdsql-mariadb-10.1.9-bin-release3/install]$./jmysql.sh 6678

cmd: e

/data/6678/prod/mysql.sock

/data/home/tdengine/davie/mysql_installs/tdsql-mariadb-10.1.9-bin-release3

Welcome to the MariaDB monitor. Commands end with ; or \g.

Your MariaDB connection id is 3

Server version: 10.1.9-MariaDBV1.0R030D002-20161123-1511 Source distribution

Copyright (c) 2000, 2015, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [(none)]> xa recover;

+----------+--------------+--------------+------+

| formatID | gtrid_length | bqual_length | data |

+----------+--------------+--------------+------+

| 1 | 3 | 0 | 125 |

+----------+--------------+--------------+------+

1 row in set (0.00 sec)

MariaDB [(none)]> select*from t1;

ERROR 1046 (3D000): No database selected

MariaDB [(none)]> use test;

Database changed

MariaDB [test]> select*from t1;

+---+

| a |

+---+

| 5 |

| 6 |

| 7 |

| 8 |

+---+

4 rows in set (0.01 sec)

MariaDB [test]> xa commit '125';

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> select*from t1;

+----+

| a |

+----+

| 5 |

| 6 |

| 7 |

| 8 |

| 9 |

| 10 |

+----+

6 rows in set (0.00 sec)

MariaDB [test]>

从最初的测例开始，一直没有任何插入数据行的binlog(write_row)写入。