# Replication and Foreign Keys

{% hint style="info" %}
The terms *master* and *slave* have historically been used in replication, and MariaDB has begun the process of adding *primary* and *replica* synonyms. The old terms will continue to be used to maintain backward compatibility - see [MDEV-18777](https://jira.mariadb.org/browse/MDEV-18777) to follow progress on this effort.
{% endhint %}

Replication is based upon the [binary log](https://mariadb.com/docs/server/server-management/server-monitoring-logs/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:

```sql
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.

```sql
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:

```sql
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:

```sql
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:

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

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

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

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