Replication and Foreign Keys

Replication is based upon the binary log. However, cascading deletes or updates based on foreign key relations are an internal mechanism, and are not written to the binary log.

Because of this, an identical statement run on the master and the slave may result in different outcomes if the foreign key relations are not identical on both master and slave This could be the case if the storage engine on one supports cascading deletes (e.g. InnoDB) and the storage engine on the other does not (e.g. MyISAM), or the one has specified a foreign key relation, and the other hasn't.

Take the following example:

CREATE TABLE employees (
    x INT PRIMARY KEY,
    name VARCHAR(10)
) ENGINE = InnoDB;

CREATE TABLE children (
    y INT PRIMARY KEY,
    f INT,
    name VARCHAR(10),
    FOREIGN KEY fk (f) REFERENCES employees (x)
        ON DELETE CASCADE
) ENGINE = InnoDB;

The slave, however, has been set up without InnoDB support, and defaults to MyISAM, so the foreign key restrictions are not in place.

INSERT INTO employees VALUES (1, 'Yaser'), (2, 'Prune');

INSERT INTO children VALUES (1, 1, 'Haruna'), (2, 1, 'Hera'), (3, 2, 'Eva');

At this point, the slave and the master are in sync:

SELECT * FROM employees;
+---+-------+
| x | name  |
+---+-------+
| 1 | Yaser |
| 2 | Prune |
+---+-------+
2 rows in set (0.00 sec)

SELECT * FROM children;
+---+------+--------+
| y | f    | name   |
+---+------+--------+
| 1 |    1 | Haruna |
| 2 |    1 | Hera   |
| 3 |    2 | Eva    |
+---+------+--------+

However, after:

DELETE FROM employees WHERE x=1;

there are different outcomes on the slave and the master.

On the master, the cascading deletes have taken effect:

SELECT * FROM children;
+---+------+------+
| y | f    | name |
+---+------+------+
| 3 |    2 | Eva  |
+---+------+------+

On the slave, the cascading deletes did not take effect:

SELECT * FROM children;
+---+------+--------+
| y | f    | name   |
+---+------+--------+
| 1 |    1 | Haruna |
| 2 |    1 | Hera   |
| 3 |    2 | Eva    |
+---+------+--------+