The document describes transactions in a MySQL database and demonstrates concurrent transactions. It shows two transactions (T1 and T2) operating on a bank database table concurrently. T1 and T2 both set autocommit to 0, indicating explicit transactions. They perform select, insert, update, and delete operations. This causes locking between the transactions and eventually a deadlock, demonstrating how the database handles concurrent transactions and locking.

1. Bases de Datos.
SET AUTOCOMMIT = {0 | 1}
Si el modo de autocommit está en apagado SET AUTOCOMMIT = 0, entonces se
asume que la misma transacción continua hasta que se realice un COMMIT o un
ROLLBACK. Por default el modo de autocommit está encendido, de manera que cada
operación es considerada una transacción y todo cambio se va reflejando
automáticamente en la base de datos.
Partiendo del esquema siguiente se presentan las acciones de 2 transacciones concurrentes.
mysql> desc bank;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id | int(11) | | PRI | 0 | |
| debit | float | YES | | NULL | |
+-------+---------+------+-----+---------+-------+
2 rows in set (0.08 sec)
T1 T2
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows affected (0.09 Query OK, 0
sec) rows affected (0.00 sec)
mysql> select * from bank; mysql> select * from bank;
+------+-------+ +------
| id | debit | +-------+
+------+-------+ | id | debit |
| 32 | 999 | +------
| 64 | 7865 | +-------+
+------+-------+ | 32 | 999 |
2 rows in set | 64 | 7865 |
(0.00 sec) +------
mysql> insert into bank +-------+
values(66,3453); 2 rows in set
Query OK, 1 row (0.00 sec)
affected (0.09 sec)
mysql> select * from bank;
+------
+-------+
| id | debit |
+------
+-------+
| 32 | 999 |
| 64 | 7865 |
+------
+-------+
2 rows in set
(0.15 sec)
mysql> select * from bank;
mysql> select * from bank; +------
+------+-------+ +-------+
| id | debit | | id | debit |
+------+-------+ +------
| 32 | 999 | +-------+
| 64 | 7865 | | 32 | 999 |
1

2. Bases de Datos.
| 66 | 3453 | | 64 | 7865 |
+------+-------+ +------
3 rows in set +-------+
(0.00 sec) 2 rows in set
mysql> delete from bank where (0.00 sec)
id=64;
Query OK, 1 row
affected (0.12 sec)
mysql> select * from bank;
+------+-------+
| id | debit |
+------+-------+
| 32 | 999 |
| 66 | 3453 |
+------+-------+
2 rows in set mysql> select * from bank;
(0.00 sec) +------
+-------+
| id | debit |
mysql> commit; +------
Query OK, 0 rows affected (0.07 +-------+
sec) | 32 | 999 |
| 64 | 7865 |
+------
+-------+
mysql> set autocommit=0; 2 rows in set
Query OK, 0 rows affected (0.00 (0.00 sec)
sec) mysql> commit;
mysql> select * from bank; Query OK, 0 rows affected
+------+-------+ (0.00 sec)
| id | debit |
+------+-------+ mysql> set autocommit=0;
| 32 | 999 | Query OK, 0 rows affected
| 66 | 3453 | (0.00 sec)
+------+-------+ mysql> select * from bank;
2 rows in set +------
(0.00 sec) +-------+
mysql> rollback; | id | debit |
Query OK, 0 rows +------
affected (0.00 sec) +-------+
| 32 | 999 |
| 66 | 3453 |
+------
+-------+
2 rows in set
(0.00 sec)
mysql> rollback;
Query OK, 0
rows affected (0.01 sec)
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows Query OK, 0
affected (0.01 sec) rows affected (0.00 sec)
mysql> select * from bank where mysql> update bank set
id=32 for update; debit=666 where id=32;
+------+-------+ .
| id | debit | .
+------+-------+ .
| 32 | 999 | .
2

3. Bases de Datos.
+------+-------+ .
1 row in set .
(0.00 sec) .Wait unlock
mysql> rollback; .
Query OK, 0 rows .
affected (0.00 sec) .
.
mysql> update bank set
debit=666 where id=32;
Query OK, 1
row affected (13.02 sec)
Rows matched:
1 Changed: 1 Warnings: 0
mysql> select * from bank;
+------+-------+ mysql> select * from bank;
| id | debit | +------
+------+-------+ +-------+
| 32 | 999 | | id | debit |
| 66 | 3453 | +------
+------+-------+ +-------+
2 rows in set | 32 | 666 |
(0.00 sec) | 66 | 3453 |
+------
+-------+
2 rows in set
(0.00 sec)
mysql> commit;
Query OK, 0
rows affected (0.07 sec)
mysql> select * from bank;
+------+-------+
| id | debit | mysql> select * from bank;
+------+-------+ +------
| 32 | 999 | +-------+
| 66 | 3453 | | id | debit |
+------+-------+ +------
2 rows in set +-------+
(0.00 sec) | 32 | 666 |
| 66 | 3453 |
mysql> commit; +------
Query OK, 0 rows +-------+
affected (0.07 sec) 2 rows in set
mysql> select * from bank; (0.01 sec)
+------+-------+
| id | debit |
+------+-------+
| 32 | 666 |
| 66 | 3453 | mysql> select * from bank;
+------+-------+ +------
2 rows in set +-------+
(0.01 sec) | id | debit |
+------
+-------+
| 32 | 666 |
| 66 | 3453 |
+------
+-------+
2 rows in set
(0.01 sec)
3

4. Bases de Datos.
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows Query OK, 0
affected (0.00 sec) rows affected (0.00 sec)
mysql> select * from bank where mysql> update bank set
id=32 lock in share mode; debit=777 where id=32;
+------+-------+ .
| id | debit | .
+------+-------+ .
| 32 | 666 | .>Wait unlock
+------+-------+ .
1 row in set .
(0.00 sec) .
mysql> update bank set .
debit=888 where id=32; .
ERROR 1213 (40001): Deadlock .
found when trying to get lock; .
Try restarting transaction .
.
.
.
>T1 tiene bloqueado a 32, T2 .
espera la liberacion .
>al hacer el update en T1 ahora .
T1 espera a T2 mysql> update bank set
>por lo tanto se produce un debit=777 where id=32;
deadlock Query OK, 1
>el dbms hace rollback en T1 row affected (23.10 sec)
(ultimo en espera) y procesa T2 Rows matched:
1 Changed: 1 Warnings: 0
mysql> select * from bank;
+------
+-------+
| id | debit |
mysql> select * from bank; +------
+------+-------+ +-------+
| id | debit | | 32 | 777 |
+------+-------+ | 66 | 3453 |
| 32 | 666 | +------
| 66 | 3453 | +-------+
+------+-------+ 2 rows in set
2 rows in set (0.00 sec)
(0.00 sec) mysql> commit;
Query OK, 0
rows affected (0.00 sec)
mysql> select * from bank;
+------
+-------+
mysql> select * from bank; | id | debit |
+------+-------+ +------
| id | debit | +-------+
+------+-------+ | 32 | 777 |
| 32 | 666 | | 66 | 3453 |
| 66 | 3453 | +------
+------+-------+ +-------+
2 rows in set 2 rows in set
(0.00 sec) (0.00 sec)
mysql> rollback;
Query OK, 0 rows
affected (0.00 sec)
mysql> select * from bank;
4

5. Bases de Datos.
+------+-------+
| id | debit |
+------+-------+
| 32 | 777 |
| 66 | 3453 |
+------+-------+
2 rows in set
(0.01 sec)
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows Query OK, 0
affected (0.00 sec) rows affected (0.00 sec)
mysql> select * from bank where mysql> select * from bank
id=32 for update; where id=32 for update;
+------+-------+ .
| id | debit | .
+------+-------+ .
| 32 | 777 | .
+------+-------+ .
1 row in set .
(0.00 sec) .> Wait unlock
mysql> update bank set .
debit=111 where id=32; .
Query OK, 1 row .
affected (0.00 sec) .
Rows matched: 1 .
Changed: 1 Warnings: 0
mysql> select * from bank
where id=32 for update;
mysql> commit; +------
Query OK, 0 rows +-------+
affected (0.01 sec) | id | debit |
+------
+-------+
| 32 | 111 |
+------
+-------+
1 row in set
mysql> select * from bank; (32.45 sec)
+------+-------+
| id | debit |
+------+-------+
| 32 | 111 |
| 66 | 3453 | mysql> rollback;
+------+-------+ Query OK, 0
2 rows in set rows affected (0.00 sec)
(0.00 sec)
5

6. Bases de Datos.
mysql> set autocommit=0;
Query OK, 0 rows
affected (0.00 sec)
mysql> update bank set
debit=000 where id=32; mysql> select * from bank;
Query OK, 1 row +------
affected (6.88 sec) +-------+
Rows matched: 1 | id | debit |
Changed: 1 Warnings: 0 +------
mysql> select * from bank; +-------+
+------+-------+ | 32 | 111 |
| id | debit | | 66 | 3453 |
+------+-------+ +------
| 32 | 0 | +-------+
| 66 | 3453 | 2 rows in set
+------+-------+ (0.00 sec)
2 rows in set
(0.00 sec)
mysql> commit;
Query OK, 0 rows
affected (0.00 sec)
mysql> select * from bank;
+------+-------+
| id | debit |
+------+-------+
| 32 | 0 |
| 66 | 3453 |
+------+-------+
2 rows in set
(0.00 sec)
mysql> select * from bank;
+------
+-------+
| id | debit |
+------
+-------+
| 32 | 0 |
| 66 | 3453 |
+------
+-------+
2 rows in set
(0.00 sec)
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows Query OK, 0
affected (0.00 sec) rows affected (0.00 sec)
mysql> select * from bank
6

7. Bases de Datos.
where id=32 for update;
+------
+-------+
| id | debit |
+------
mysql> update bank set +-------+
debit=333 where id=32; | 32 | 0 |
. +------
. +-------+
. 1 row in set
. (0.00 sec)
.
.> Wait unlock
.
.
.
.
. mysql> commit;
mysql> update bank set Query OK, 0
debit=333 where id=32; rows affected (0.00 sec)
Query OK, 1 row
affected (3.20 sec)
Rows matched: 1
Changed: 1 Warnings: 0
mysql> select * from bank;
+------+-------+
| id | debit |
+------+-------+
| 32 | 333 |
| 66 | 3453 | mysql> select * from bank;
+------+-------+ +------
2 rows in set +-------+
(0.00 sec) | id | debit |
mysql> commit; +------
Query OK, 0 rows +-------+
affected (0.00 sec) | 32 | 0 |
mysql> select * from bank; | 66 | 3453 |
+------+-------+ +------
| id | debit | +-------+
+------+-------+ 2 rows in set
| 32 | 333 | (0.00 sec)
| 66 | 3453 |
+------+-------+
2 rows in set
(0.00 sec)
mysql> select * from bank;
+------
+-------+
| id | debit |
+------
+-------+
| 32 | 0 |
| 66 | 3453 |
+------
+-------+
2 rows in set
(0.00 sec)
mysql> commit;
Query OK, 0
rows affected (0.00 sec)
7

8. Bases de Datos.
mysql> select * from bank;
+------
+-------+
| id | debit |
+------
+-------+
| 32 | 333 |
| 66 | 3453 |
+------
+-------+
2 rows in set
(0.00 sec)
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows Query OK, 0
affected (0.00 sec) rows affected (0.00 sec)
mysql> select * from bank
where id=32 lock in share
mode;
+------
+-------+
| id | debit |
+------
+-------+
| 32 | 333 |
+------
mysql> select * from bank where +-------+
id=32 lock in share mode; 1 row in set
+------+-------+ (0.00 sec)
| id | debit |
+------+-------+
| 32 | 333 |
+------+-------+
1 row in set
(0.00 sec)
mysql> update bank set
debit=444 where id=32;
.
.
.
.
.> Wait unlock
.
.
. mysql> update bank set
. debit=777 where id=32;
. ERROR 1213 (40001): Deadlock
. found when trying to get
. lock;
Try restarting transaction
mysql> update bank set
debit=444 where id=32;
Query OK, 1 row >T2 tiene bloqueado a 32, T1
affected (30.83 sec) espera la liberacion
Rows matched: 1 >al hacer el update en T2
Changed: 1 Warnings: 0 ahora T2 espera a T1
>por lo tanto se produce un
mysql> select * from bank; deadlock
+------+-------+ >el dbms hace rollback en T2
8

9. Bases de Datos.
| id | debit | (ultimo en espera) y procesa
+------+-------+ T1
| 32 | 444 |
| 66 | 3453 |
+------+-------+
2 rows in set
(0.00 sec)
mysql> commit;
Query OK, 0 rows mysql> select * from bank;
affected (0.00 sec) +------
+-------+
mysql> select * from bank; | id | debit |
+------+-------+ +------
| id | debit | +-------+
+------+-------+ | 32 | 333 |
| 32 | 444 | | 66 | 3453 |
| 66 | 3453 | +------
+------+-------+ +-------+
2 rows in set 2 rows in set
(0.00 sec) (0.00 sec)
mysql> commit;
Query OK, 0
rows affected (0.00 sec)
mysql> select * from bank;
+------
+-------+
| id | debit |
+------
+-------+
| 32 | 444 |
| 66 | 3453 |
+------
+-------+
2 rows in set
(0.00 sec)
mysql> set autocommit=0; mysql> set autocommit=0;
Query OK, 0 rows affected (0.00 Query OK, 0 rows affected
sec) (0.01 sec)
mysql> select * from bank where
id=32 lock in share mode;
+----+-------+
| id | debit |
+----+-------+
| 32 | 444 | mysql> update bank set
+----+-------+ debit=555 where id=32;
1 row in set .
(0.00 sec) .
> .
> Nadie puede modificar la .
tupla 32 .> Wait unlock
> .
.
mysql> commit; .
Query OK, 0 rows .
affected (0.00 sec) .
.
mysql> update bank set
debit=555 where id=32;
Query OK, 1
9

10. Bases de Datos.
row affected (13.21 sec)
mysql> select * from bank; Rows
+----+-------+ matched: 1 Changed: 1
| id | debit | Warnings: 0
+----+-------+
| 32 | 444 |
| 66 | 3453 |
+----+-------+
2 rows in set
(1.02 sec)
mysql> commit;
Query OK, 0 rows affected
mysql> select * from bank; (0.14 sec)
+----+-------+ mysql> select * from bank;
| id | debit | +----
+----+-------+ +-------+
| 32 | 555 | | id | debit
| 66 | 3453 | |
+----+-------+ +----
2 rows in set +-------+
(0.01 sec) | 32 | 555 |
| 66 | 3453
|
+----
+-------+
2 rows in
set (0.00 sec)
10