Question

我正在尝试阻止数据库迁移期间的并发修改，我的方法是使用t锁定现有表SELECT ... FOR UPDATE（prereq）并在锁定期间实现迁移。

因此，我创建了一个简单的示例，如下所示：

Connection conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/testdb", "root", "");

conn.setAutoCommit(false);
statement = conn.createStatement();
ResultSet rs = statement.executeQuery("SELECT id FROM t FOR UPDATE");
rs.next();
System.out.println(rs.getInt(1));

System.out.println("Sleeping...");
Thread.sleep(10000);

statement.executeUpdate("CREATE TABLE t2 (id int PRIMARY KEY)");
// Release lock
statement.executeUpdate("UPDATE t SET id = 2 WHERE id = 1");

conn.commit();

statement.close();
conn.close();

其中t是一个只有id int PRIMARY KEY的虚拟表。

我设置Thread.sleep(10000);来模拟漫长的过程。在Thread.sleep(10000);我正在运行第二个（类似的代码片段）来模拟并发

Connection conn = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/testdb", "root", "");

Statement statement = conn.createStatement();
conn.setAutoCommit(false);
statement = conn.createStatement();
ResultSet rs = statement.executeQuery("SELECT id FROM t FOR UPDATE");
rs.next();
System.out.println(rs.getInt(1));

conn.commit();

statement.close();
conn.close();

但我希望代码打印2 ..它打印1。

当我查看MySQL常规日志时，我看到了那个东西

438 Query   SET autocommit=0
438 Query   SELECT id FROM t FOR UPDATE
439 Query   /* mysql-connector-java-5.1.35 ( Revision: 5fb9c5849535c13917c2cf9baaece6ef9693ef27 ) */SHOW VARIABLES WHERE Variable_name ='language' OR Variable_name = 'net_write_timeout' OR Variable_name = 'interactive_timeout' OR Variable_name = 'wait_timeout' OR Variable_name = 'character_set_client' OR Variable_name = 'character_set_connection' OR Variable_name = 'character_set' OR Variable_name = 'character_set_server' OR Variable_name = 'tx_isolation' OR Variable_name = 'transaction_isolation' OR Variable_name = 'character_set_results' OR Variable_name = 'timezone' OR Variable_name = 'time_zone' OR Variable_name = 'system_time_zone' OR Variable_name = 'lower_case_table_names' OR Variable_name = 'max_allowed_packet' OR Variable_name = 'net_buffer_length' OR Variable_name = 'sql_mode' OR Variable_name = 'query_cache_type' OR Variable_name = 'query_cache_size' OR Variable_name = 'license' OR Variable_name = 'init_connect'
439 Query   /* mysql-connector-java-5.1.35 ( Revision: 5fb9c5849535c13917c2cf9baaece6ef9693ef27 ) */SELECT @@session.auto_increment_increment
439 Query   SET character_set_results = NULL
439 Query   SET autocommit=1
439 Query   SET sql_mode='NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES'
439 Query   SET autocommit=0
439 Query   SELECT id FROM t FOR UPDATE
438 Query   CREATE TABLE t2 (id int PRIMARY KEY)
439 Query   commit
439 Query   rollback
439 Quit    
438 Query   UPDATE t SET id = 2 WHERE id = 1
438 Query   commit
438 Query   rollback

提交CREATE TABLE t2...事务（会话2）后释放锁定，但我在autoCommit(false)为什么MySQL强制commit在这种情况下。

我可能也错过了解SELECT ... FOR UPDATE的工作原理......

更新：使用mysql客户端在没有Java和JDBC的情况下进行测试

SESSION1> CREATE TABLE t (id int PRIMARY KEY) ENGINE=INNODB;
SESSION1> INSERT INTO t VALUES (1);
SESSION1> BEGIN;
SESSION1> SELECT id FROM t FOR UPDATE;

SESSION2> BEGIN;
SESSION2> SELECT id FROM t FOR UPDATE;
--> HERE SESSION2 is stuck (expected behavior)

SESSION1> CREATE TABLE t2 (id int PRIMARY KEY) ENGINE=INNODB;
--> HERE SESSION2 is unlock returning 
+----+
| id |
+----+
|  1 |
+----+
1 row in set (4.15 sec)

似乎SELECT ... FOR UPDATE的DLL语句不能正常工作

Answer 1

如果您的存储引擎是MyISAM，您将会发现此行为。 MyISAM不支持事务，因此，您的第一个代码不会放置任何锁定，因为所有语句都会自动提交，因此您的第二个代码将很乐意浏览自己的SELECT FOR UPDATE并直接进入SELECT ID。

长话短说：将数据库切换到InnoDB存储以使其正常工作。

参考文献：

https://dev.mysql.com/doc/refman/5.0/en/myisam-storage-engine.html

https://dev.mysql.com/doc/refman/5.5/en/innodb-storage-engine.html

Answer 2

最后，在独占锁定（CREATE TABLE）期间无法创建SELECT ... FOR UPDATE之类的DLL语句。

但是，为了满足我的需求，我将使用与flyway相同的策略。确实，flyway使用两个会话：

可锁定的Metatable会话（Metatable是flyway的核心表）
执行迁移的对象会话

SESSION-METATABLE> CREATE TABLE t (id int PRIMARY KEY) ENGINE=INNODB;
SESSION-METATABLE> INSERT INTO t VALUES (1);
SESSION-METATABLE> BEGIN;
SESSION-METATABLE> SELECT id FROM t FOR UPDATE;

SESSION-CONCURRENT> BEGIN;
SESSION-CONCURRENT> SELECT id FROM t FOR UPDATE;
--> HERE SESSION-CONCURRENT is stuck (expected behavior)

SESSION-OBJECT> BEGIN;
SESSION-OBJECT> CREATE TABLE t2 (id int PRIMARY KEY);
--> HERE SESSION-CONCURRENT is not unlock
SESSION-OBJECT> COMMIT;
--> SESSION-CONCURRENT still stuck even after COMMIT;

SESSION-METATABLE>UPDATE t SET id=2 WHERE id=1;
SESSION-METATABLE>COMMIT;
--> HERE SESSION-CONCURRENT is unlock and print
+----+
| id |
+----+
|  2 |
+----+
1 row in set (23.33 sec)

SELECT FOR UPDATE在迁移期间锁定数据库

2 个答案: