MariaDB与MySQL对比 --- 对分布式事务的支持

Posted 2021-04-25 数据库开发者

MariaDB与mysql对比 --- 对分布式事务的支持

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（也发现了一个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 在 xa prepare时候不会刷盘binlog，因此xa prepare 之后，这个事务的binlog仍然可能丢失。

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。

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binlog replication 可以找到这个事务，所以 xa recover 可以列出来，并且做xa commit 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。

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。