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15.18 InnoDB and MySQL Replication

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MySQL replication works for InnoDB tables as it does for MyISAM tables. It is also possible to use replication in a way where the storage engine on the slave is not the same as the original storage engine on the master. For example, you can replicate modifications to an InnoDB table on the master to a MyISAM table on the slave. For more information see, Section 17.3.4, “Using Replication with Different Master and Slave Storage Engines”.

For information about setting up a new slave for a master, see Section 17.1.2.6, “Setting Up Replication Slaves”, and Section 17.1.2.5, “Choosing a Method for Data Snapshots”. To make a new slave without taking down the master or an existing slave, use the MySQL Enterprise Backup product.

Transactions that fail on the master do not affect replication at all. MySQL replication is based on the binary log where MySQL writes SQL statements that modify data. A transaction that fails (for example, because of a foreign key violation, or because it is rolled back) is not written to the binary log, so it is not sent to slaves. See Section 13.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

Replication and CASCADE.  Cascading actions for InnoDB tables on the master are replicated on the slave only if the tables sharing the foreign key relation use InnoDB on both the master and slave. This is true whether you are using statement-based or row-based replication. Suppose that you have started replication, and then create two tables on the master using the following CREATE TABLE statements:

  1. CREATE TABLE fc1 (
  2.     i INT PRIMARY KEY,
  3.     j INT
  4. ) ENGINE = InnoDB;
  5.  
  6. CREATE TABLE fc2 (
  7.     m INT PRIMARY KEY,
  8.     n INT,
  9.     FOREIGN KEY ni (n) REFERENCES fc1 (i)
  10.         ON DELETE CASCADE
  11. ) ENGINE = InnoDB;

Suppose that the slave does not have InnoDB support enabled. If this is the case, then the tables on the slave are created, but they use the MyISAM storage engine, and the FOREIGN KEY option is ignored. Now we insert some rows into the tables on the master:

  1. master> INSERT INTO fc1 VALUES (1, 1), (2, 2);
  2. Query OK, 2 rows affected (0.09 sec)
  3. Records: 2  Duplicates: 0  Warnings: 0
  4.  
  5. master> INSERT INTO fc2 VALUES (1, 1), (2, 2), (3, 1);
  6. Query OK, 3 rows affected (0.19 sec)
  7. Records: 3  Duplicates: 0  Warnings: 0

At this point, on both the master and the slave, table fc1 contains 2 rows, and table fc2 contains 3 rows, as shown here:

  1. master> SELECT * FROM fc1;
  2. +---+------+
  3. | i | j    |
  4. +---+------+
  5. | 1 |    1 |
  6. | 2 |    2 |
  7. +---+------+
  8. 2 rows in set (0.00 sec)
  9.  
  10. master> SELECT * FROM fc2;
  11. +---+------+
  12. | m | n    |
  13. +---+------+
  14. | 1 |    1 |
  15. | 2 |    2 |
  16. | 3 |    1 |
  17. +---+------+
  18. 3 rows in set (0.00 sec)
  19.  
  20. slave> SELECT * FROM fc1;
  21. +---+------+
  22. | i | j    |
  23. +---+------+
  24. | 1 |    1 |
  25. | 2 |    2 |
  26. +---+------+
  27. 2 rows in set (0.00 sec)
  28.  
  29. slave> SELECT * FROM fc2;
  30. +---+------+
  31. | m | n    |
  32. +---+------+
  33. | 1 |    1 |
  34. | 2 |    2 |
  35. | 3 |    1 |
  36. +---+------+
  37. 3 rows in set (0.00 sec)

Now suppose that you perform the following DELETE statement on the master:

  1. master> DELETE FROM fc1 WHERE i=1;
  2. Query OK, 1 row affected (0.09 sec)

Due to the cascade, table fc2 on the master now contains only 1 row:

  1. master> SELECT * FROM fc2;
  2. +---+---+
  3. | m | n |
  4. +---+---+
  5. | 2 | 2 |
  6. +---+---+
  7. 1 row in set (0.00 sec)

However, the cascade does not propagate on the slave because on the slave the DELETE for fc1 deletes no rows from fc2. The slave's copy of fc2 still contains all of the rows that were originally inserted:

  1. slave> SELECT * FROM fc2;
  2. +---+---+
  3. | m | n |
  4. +---+---+
  5. | 1 | 1 |
  6. | 3 | 1 |
  7. | 2 | 2 |
  8. +---+---+
  9. 3 rows in set (0.00 sec)

This difference is due to the fact that the cascading deletes are handled internally by the InnoDB storage engine, which means that none of the changes are logged.


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