03全局序列號篇詳細介紹了分片表, 業務在使用分片表時, 很自然的可能會遇到一個事務中操作的數據分佈在多個分片節點上, 即分佈式事務. 先來直觀感受下Mycat處理事務的過程.
登陸tb3表的dnTest2節點主機, 操作如下.
mysql> set global innodb_lock_wait_timeout = 5;
Query OK, 0 rows affected (0.00 sec)
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> select user_name from tb3 where user_id = 59 for update;
+-----------+
| user_name |
+-----------+
| mnop_f |
+-----------+
1 row in set (0.00 sec)
登陸Mycat, 開啓一個事務, 結合日誌看下該過程.
mysql> start transaction;
Query OK, 0 rows affected (0.01 sec)
mysql> update tb3 set user_name = 'igkl_2f' where user_id = 4;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update tb3 set user_name = 'mnop_2f' where user_id = 59;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> commit;
ERROR 1003 (HY000): Transaction error, need to rollback.
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
關鍵日誌如下.
03/27 12:07:09.189 DEBUG [$_NIOREACTOR-1-RW] (ServerQueryHandler.java:56) -ServerConnection [id=2, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=true, schema=testdb]begin
03/27 12:16:33.019 DEBUG [$_NIOREACTOR-2-RW] (ServerQueryHandler.java:56) -ServerConnection [id=6, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=false, schema=testdb]update tb3 set user_name = 'igkl_2f' where user_id = 4
03/27 12:16:33.021 DEBUG [$_NIOREACTOR-2-RW] (NonBlockingSession.java:113) -ServerConnection [id=6, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=false, schema=testdb]update tb3 set user_name = 'igkl_2f' where user_id = 4, route={
1 -> dnTest1{update tb3 set user_name = 'igkl_2f' where user_id = 4}
03/27 12:16:33.021 DEBUG [$_NIOREACTOR-2-RW] (MySQLConnection.java:459) -con need syn ,total syn cmd 3 commands SET names utf8;SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;SET autocommit=0;schema change:false con:MySQLConnection [id=8, lastTime=1522124193021, user=appacc, schema=test1, old shema=test1, borrowed=true, fromSlaveDB=false, threadId=28, charset=utf8, txIsolation=0, autocommit=true, attachment=dnTest1{update tb3 set user_name = 'igkl_2f' where user_id = 4}, respHandler=SingleNodeHandler [node=dnTest1{update tb3 set user_name = 'igkl_2f' where user_id = 4}, packetId=0], host=192.168.4.235, port=3306, statusSync=null, writeQueue=0, modifiedSQLExecuted=true]
03/27 12:16:52.795 DEBUG [$_NIOREACTOR-2-RW] (ServerQueryHandler.java:56) -ServerConnection [id=6, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=false, schema=testdb]update tb3 set user_name = 'mnop_2f' where user_id = 59
03/27 12:16:52.796 DEBUG [$_NIOREACTOR-2-RW] (NonBlockingSession.java:113) -ServerConnection [id=6, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=false, schema=testdb]update tb3 set user_name = 'mnop_2f' where user_id = 59, route={
1 -> dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}
03/27 12:16:52.797 DEBUG [$_NIOREACTOR-2-RW] (MySQLConnection.java:459) -con need syn ,total syn cmd 3 commands SET names utf8;SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;SET autocommit=0;schema change:false con:MySQLConnection [id=20, lastTime=1522124212797, user=appacc, schema=test2, old shema=test2, borrowed=true, fromSlaveDB=false, threadId=8, charset=utf8, txIsolation=0, autocommit=true, attachment=dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}, respHandler=SingleNodeHandler [node=dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}, packetId=0], host=192.168.4.151, port=3306, statusSync=null, writeQueue=0, modifiedSQLExecuted=true]
03/27 12:16:58.800 WARN [$_NIOREACTOR-1-RW] (SingleNodeHandler.java:232) -execute sql err : errno:1205 Lock wait timeout exceeded; try restarting transaction con:MySQLConnection [id=20, lastTime=1522124212784, user=appacc, schema=test2, old shema=test2, borrowed=true, fromSlaveDB=false, threadId=8, charset=utf8, txIsolation=3, autocommit=false, attachment=dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}, respHandler=SingleNodeHandler [node=dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}, packetId=1], host=192.168.4.151, port=3306, statusSync=org.opencloudb.mysql.nio.MySQLConnection$StatusSync@52530466, writeQueue=0, modifiedSQLExecuted=true] frontend host:192.168.4.184/59858/test_user
03/27 12:17:05.660 DEBUG [$_NIOREACTOR-2-RW] (ServerQueryHandler.java:56) -ServerConnection [id=6, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=false, schema=testdb]commit
03/27 12:17:08.868 DEBUG [$_NIOREACTOR-2-RW] (ServerQueryHandler.java:56) -ServerConnection [id=6, schema=testdb, host=192.168.4.184, user=test_user,txIsolation=3, autocommit=false, schema=testdb]rollback
03/27 12:17:08.870 DEBUG [$_NIOREACTOR-2-RW] (RollbackNodeHandler.java:79) -rollback job run for MySQLConnection [id=20, lastTime=1522124212784, user=appacc, schema=test2, old shema=test2, borrowed=true, fromSlaveDB=false, threadId=8, charset=utf8, txIsolation=3, autocommit=false, attachment=dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}, respHandler=SingleNodeHandler [node=dnTest2{update tb3 set user_name = 'mnop_2f' where user_id = 59}, packetId=1], host=192.168.4.151, port=3306, statusSync=org.opencloudb.mysql.nio.MySQLConnection$StatusSync@52530466, writeQueue=0, modifiedSQLExecuted=true]
03/27 12:17:08.870 DEBUG [$_NIOREACTOR-2-RW] (RollbackNodeHandler.java:79) -rollback job run for MySQLConnection [id=8, lastTime=1522124193010, user=appacc, schema=test1, old shema=test1, borrowed=true, fromSlaveDB=false, threadId=28, charset=utf8, txIsolation=3, autocommit=false, attachment=dnTest1{update tb3 set user_name = 'igkl_2f' where user_id = 4}, respHandler=SingleNodeHandler [node=dnTest1{update tb3 set user_name = 'igkl_2f' where user_id = 4}, packetId=1], host=192.168.4.235, port=3306, statusSync=null, writeQueue=0, modifiedSQLExecuted=true]
Mycat目前對於未分片的節點, 是可以保證事務的完整性的; 若是多個分片節點, 在執行事務時, 遇到任何分片出錯, 也是能保證所有分片回滾的, 即上邊展示的情況. 可是應用一旦進入commit過程, 若此時出現問題, 其就無能爲力了, 這也是Mycat稱之爲弱XA的原因.
上述commit過程是指: InnoDB prepare, write/sync Binlog, InnoDB commit(其歷經5.5至5.7版本的多次迭代優化, 這塊內容也是精彩紛呈). 雖該階段一般不會出現問題, 這也正暗示了Mycat在特殊情況下還不能保證分佈式事務安全. 那應用架構中又如何實現可靠的分佈式事務呢, 這又是另一個宏大的話題了...
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