MySQL 事務功能
==============================================================================
事務：
1.介紹
★事務：
一組原子性的SQL查詢、或者是一個或多個SQL語句組成的獨立工作單元；
★ACID測試：
☉A：AUTOMICITY，原子性；
整個事務中的所有操作要麼全部成功執行，要麼全部失敗後回滾；
☉C：CONSISTENCY，一致性；
數據庫總是應該從一個一致性狀態轉爲另一個一致性狀態；
☉I：ISOLATION，隔離性；
一個事務所做出的操作在提交之前，是否能爲其它事務可見；出於保證併發操作之目的，隔離有多種級別；
☉D：DURABILITY，持久性；
事務一旦提交，其所做出的修改會永久保存；
2.事務日誌：
mysql會最大程度的使用緩存機制來提高數據庫的訪問效率，但是萬一數據庫發生斷電，因爲緩存的數據沒有寫入磁盤，導致緩存在內存中的數據丟失而導致數據不一致怎麼辦？
★Innodb主要是通過事務日誌實現ACID特性；
★事務日誌包括：
☉重做日誌redo
Redo記錄的是已經全部完成的事務，就是執行了commit（提交）的事務，記錄文件是ib_logfile0 ib_logfile1；
☉回滾日誌undo
Undo記錄的是已部分完成並且寫入硬盤的未完成的事務，默認情況下回滾日誌是記錄在表空間中的（共享表空間或者獨享表空間）；
★執行過程：
一般情況下，mysql在崩潰之後，重啓服務，innodb通過回滾日誌undo將所有已完成並寫入磁盤的未完成事務進行rollback，然後redo中的事務全部重新執行一遍（後臺啓動相關線程自動執行）即可恢復數據，但是隨着redo的量增加，每次從redo的第一條開始恢復就會浪費長的時間，所以引入了checkpoint（檢查點）機制；
★事務日誌相關配置參數：
innodb_log_files_in_group //一組中有幾個文件，默認爲2個
innodb_log_group_home_dir //日誌目錄的存放位置，默認在數據目錄中
innodb_log_file_size //日誌文件大小
innodb_mirrored_log_groups //鏡像日誌組
演示：
1.查看事物日誌相關的參數
MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE '%innodb%log%'; +-------------------------------------------+---------+ | Variable_name | Value | +-------------------------------------------+---------+ | innodb_flush_log_at_trx_commit | 1 | 要不要在提交時從內存中同步數據到事物日誌所在磁盤上，1表示ON開啓 | innodb_locks_unsafe_for_binlog | OFF | | innodb_log_block_size | 512 | | innodb_log_buffer_size | 8388608 | | innodb_log_file_size | 5242880 | 日誌文件大小，默認爲5M | innodb_log_files_in_group | 2 | 一組中有幾個文件，默認爲2個 | innodb_log_group_home_dir | ./ | 日誌目錄的存放位置，默認在數據目錄中 | innodb_mirrored_log_groups | 1 | 鏡像日誌組（自己） | innodb_recovery_update_relay_log | OFF | | innodb_use_global_flush_log_at_trx_commit | ON | +-------------------------------------------+---------+ 10 rows in set (0.00 sec)
3.自動提交：單語句事務
★自動提交：單語句事務
默認情況下mysql開啓的是單語句事物功能（即每一次執行完成後都會自動提交至事物日誌中，然後再從日誌中同步到數據文件中，非常影響性能），我們要想手動控制事物，就要把自動提交事物的功能關閉。
★查看自動提交命令：
mysql> SELECT @@autocommit;
mysql> SHOW GLOBAL|SESSION VARIABLES LIKE 'autocommit'
★關閉當前會話單語句功能：
mysql> SET @@session.autocommit=0；
演示：
1.爲了演示效果，支持事物功能，我這裏修改了之前hellodb.sql的存儲引擎爲InnoDB，並重新導入mysql；現在，我們要想手動控制事物，就需要關閉單語句事物功能，如下：
MariaDB [hellodb]> show table status like 'students'\G *************************** 1. row *************************** Name: students Engine: InnoDB # 存儲引擎爲InnoDB Version: 10 Row_format: Compact Rows: 25 Avg_row_length: 655 Data_length: 16384 Max_data_length: 0 Index_length: 0 Data_free: 0 Auto_increment: 26 Create_time: 2016-11-28 11:42:19 Update_time: NULL Check_time: NULL Collation: utf8_general_ci Checksum: NULL Create_options: Comment: 1 row in set (0.02 sec) MariaDB [hellodb]> SHOW VARIABLES LIKE 'autocommit'; # 查看當前會話的單語句事物功能爲ON +---------------+-------+ | Variable_name | Value | +---------------+-------+ | autocommit | ON | +---------------+-------+ 1 row in set (0.00 sec) MariaDB [hellodb]> SET @@session.autocommit=0; Query OK, 0 rows affected (0.01 sec) MariaDB [hellodb]> SHOW VARIABLES LIKE 'autocommit'; # 再次查看變爲OFF +---------------+-------+ | Variable_name | Value | +---------------+-------+ | autocommit | OFF | +---------------+-------+ 1 row in set (0.01 sec)
--------------------------------------------------------------------------------
4.手動控制事務：
★啓動：START TRANSACTION
★提交：COMMIT
★回滾：ROLLBACK
★事務支持 savepoints：
SAVEPOINT identifier
ROLLBACK [WORK] TO [SAVEPOINT] identifier
RELEASE SAVEPOINT identifier
演示：
1.在關閉單語句事務的基礎上，手動啓動事物功能,並演示回滾操作
MariaDB [hellodb]> START TRANSACTION; # 啓動事物功能
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> DESC courses;
+----------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+----------------+
| CourseID | smallint(5) unsigned | NO | PRI | NULL | auto_increment |
| Course | varchar(100) | NO | | NULL | |
+----------+----------------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)
MariaDB [hellodb]> SELECT * FROM courses;
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
+----------+----------------+
7 rows in set (0.01 sec)
MariaDB [hellodb]> INSERT INTO courses (Course) VALUES ('Zabbix'),('Puppet'); # 插入一行數據
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
MariaDB [hellodb]> SELECT * FROM courses;
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 8 | Zabbix |
| 9 | Puppet |
+----------+----------------+
9 rows in set (0.00 sec)
MariaDB [hellodb]> DELETE FROM courses WHERE CourseID=3; # 刪除一行數據
Query OK, 1 row affected (0.00 sec)
MariaDB [hellodb]> SELECT * FROM courses;
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 8 | Zabbix |
| 9 | Puppet |
+----------+----------------+
8 rows in set (0.00 sec)
MariaDB [hellodb]> ROLLBACK; # 如果這時我們後悔了，可以執行回滾操作
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> SELECT * FROM courses; # 查看可以發現又會到原來開始的狀態了
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
+----------+----------------+
7 rows in set (0.00 sec)2.在操作上添加savepoints（存儲點），可以指定回滾到存儲點的位置，如下：
MariaDB [hellodb]> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> INSERT INTO courses (Course) VALUES ('Zabbix'),('Puppet');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
MariaDB [hellodb]> SAVEPOINT first; # 添加第一個存儲點
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> DELETE FROM courses WHERE CourseID=3;
Query OK, 1 row affected (0.00 sec)
MariaDB [hellodb]> SAVEPOINT second; # 添加第二個存儲點
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> UPDATE courses SET Course='nginx' WHERE CourseID=4;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
MariaDB [hellodb]> SELECT * FROM courses; # 查看修改後的表數據
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 4 | nginx |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 10 | Zabbix |
| 11 | Puppet |
+----------+----------------+
8 rows in set (0.00 sec)
MariaDB [hellodb]> ROLLBACK TO second; # 回滾到第二個存儲點
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> SELECT * FROM courses;
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 10 | Zabbix |
| 11 | Puppet |
+----------+----------------+
8 rows in set (0.00 sec)
MariaDB [hellodb]> ROLLBACK TO first;
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> SELECT * FROM courses; # 回滾到第一個存儲點
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 10 | Zabbix |
| 11 | Puppet |
+----------+----------------+
9 rows in set (0.01 sec)
MariaDB [hellodb]> ROLLBACK; # 回滾到原始狀態
Query OK, 0 rows affected (0.01 sec)
MariaDB [hellodb]> SELECT * FROM courses;
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
+----------+----------------+
7 rows in set (0.00 sec)3.啓動事物，執行操作後提交（COMMIT），發現再回滾就無效了，因爲這時已經同步到數據中了；
MariaDB [hellodb]> START TRANSACTION; # 啓動事物
Query OK, 0 rows affected (0.01 sec)
MariaDB [hellodb]> INSERT INTO courses (Course) VALUES ('Zabbix'),('Puppet');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
MariaDB [hellodb]> COMMIT; # 提交
Query OK, 0 rows affected (0.01 sec)
MariaDB [hellodb]> SELECT * FROM courses; # 提交後查看數據
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 12 | Zabbix |
| 13 | Puppet |
+----------+----------------+
9 rows in set (0.00 sec)
MariaDB [hellodb]> ROLLBACK;
Query OK, 0 rows affected (0.00 sec)
MariaDB [hellodb]> SELECT * FROM courses; # 執行回滾命令後發現無效
+----------+----------------+
| CourseID | Course |
+----------+----------------+
| 1 | Hamo Gong |
| 2 | Kuihua Baodian |
| 3 | Jinshe Jianfa |
| 4 | Taiji Quan |
| 5 | Daiyu Zanghua |
| 6 | Weituo Zhang |
| 7 | Dagou Bangfa |
| 12 | Zabbix |
| 13 | Puppet |
+----------+----------------+
9 rows in set (0.00 sec)5.事務隔離級別：
★事務隔離級別：
READ-UNCOMMITTED：讀未提交 --> 髒讀；
READ-COMMITTED：讀提交--> 不可重複讀；
REPEATABLE-READ：可重複讀 --> 幻讀；（默認級別）
SERIALIZABLE：串行化；
★查看及設定事物隔離級別：
mysql> SELECT @@session.tx_isolation;
mysql> SET @@session.tx_isolation='...'
演示：
1.READ-UNCOMMITTED：讀未提交 --> 髒讀；即當node1和node2兩個會話同時訪問修改同一張表時，node1用戶的事物還沒有提交，node2用戶就可以看到，導致的結果就是髒讀（看到了別人沒能提交的數據），如下：
1）在node1會話中，修改隔離級別爲讀未提交，然後啓動事物，修改一行數據，並回滾
MariaDB [hellodb]> SELECT @@tx_isolation; # 查看隔離級別 +-----------------+ | @@tx_isolation | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> SET @@session.tx_isolation='READ-UNCOMMITTED'; # 修改隔離級別爲讀未提交 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT @@tx_isolation; +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> START TRANSACTION; # 啓動事物 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> UPDATE courses SET Course='nginx' WHERE CourseID=4; # 修改數據 Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [hellodb]> select * from courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | nginx | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> rollback; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> select * from courses; # 回滾後查看數據 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec)
2）在node2會話中可以發現，它沒有做任何操作，前後兩次查看時數據發生了變化，所以讀未提交可以看到別人沒能提交的數據
MariaDB [hellodb]> set @@session.tx_isolation='READ-UNCOMMITTED'; # 同樣設置運行級別爲讀未提交 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT @@tx_isolation； +------------------+ | @@tx_isolation | +------------------+ | READ-UNCOMMITTED | +------------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> start transaction; # 同樣啓動事物 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> select * from courses; # node1會話修改數據後查看，可以看到修改的數據nginx +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | nginx | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.01 sec) MariaDB [hellodb]> select * from courses; # node1會話執行回滾後查看，可以看到又爲原來的狀態 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec)
2.READ-COMMITTED：讀提交--> 不可重複讀；即當node1和node2兩個會話同時訪問修改同一張表時，node1用戶的事物只有提交了，node2用戶纔可以看到，但node2用戶並沒有修改數據。所以，node2會話如果重複讀的話，每次的結果可能是不一樣的，如下：
1）在node1會話中，修改隔離級別爲讀提交，然後啓動事物，修改一行數據，並提交；
MariaDB [hellodb]> set @@session.tx_isolation='READ-COMMITTED'; #修改隔離級別爲讀提交 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT @@tx_isolation； +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) MariaDB [hellodb]> start transaction; # 啓動事物 Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> UPDATE courses SET Course='Nagios' WHERE CourseID=12; # 修改數據 Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 MariaDB [hellodb]> select * from courses; +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Nagios | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> COMMIT; # 提交數據 Query OK, 0 rows affected (0.01 sec)
2）在node2會話中，可以發現在node1會話未提交數據之前，node2查看到的數據一直沒有發生改變，直到node1會話提交數據之後，node2才發現數據發生了改變，但node2沒有做任何操作
MariaDB [hellodb]> set @@session.tx_isolation='READ-COMMITTED'; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> start transaction; Query OK, 0 rows affected (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # 在node1會話沒有提交數據前，數據爲改變 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Zabbix | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> SELECT * FROM courses; # node1會話提交數據後，可以看到數據發生改變 +----------+----------------+ | CourseID | Course | +----------+----------------+ | 1 | Hamo Gong | | 2 | Kuihua Baodian | | 3 | Jinshe Jianfa | | 4 | Taiji Quan | | 5 | Daiyu Zanghua | | 6 | Weituo Zhang | | 7 | Dagou Bangfa | | 12 | Nagios | | 13 | Puppet | +----------+----------------+ 9 rows in set (0.00 sec) MariaDB [hellodb]> COMMIT; Query OK, 0 rows affected (0.00 sec)
3.REPEATABLE-READ：可重複讀 --> 幻讀，也是mysql和mariadb的默認運行級別，即兩個會話爲各自獨立的事務。node1和node2會話只要建立事務，第一次查看數據的結果將貫穿整個事物過程中，期間底層數據的變化是看不到的，只有在結束事物後纔可以看到數據的變化，這樣直接導致的結果就是幻讀，即各自會話看到的數據並非真實的數據，底層數據可能已經發生了改變；