Do you need triggers on a mysql database slave? - mysql

Question

Say you are running two mysql servers: one a master, the other the slave. The master has triggers set that update columns with the COUNT of the number of rows in other tables. For instance, you have a news table and a comments table. News contains an INT column called "total_comments" which is incremented via trigger every time a new row is put into "comments." Does the slave need this trigger as well (to keep "news.total_comments" up to date) or will it get be told to update the appropriate "news.total_comments" directly?

Answer 1

From the docs http://dev.mysql.com/doc/refman/5.0/en/faqs-triggers.html:
22.5.4: How are actions carried out through triggers on a master
replicated to a slave? First, the
triggers that exist on a master must
be re-created on the slave server.
Once this is done, the replication
flow works as any other standard DML
statement that participates in
replication. For example, consider a
table EMP that has an AFTER insert
trigger, which exists on a master
MySQL server. The same EMP table and
AFTER insert trigger exist on the
slave server as well. The replication
flow would be: An INSERT statement is
made to EMP. The AFTER trigger on EMP
activates. The INSERT statement is
written to the binary log. The
replication slave picks up the INSERT
statement to EMP and executes it. The
AFTER trigger on EMP that exists on
the slave activates.
And
22.5.4 Actions carried out through triggers on a master are not
replicated to a slave server.
Thus, you DO need the triggers on the slave.

Answer 2

It depends on the replication you're using. If you use statement based replication, then you must use matching triggers in the master and the slave. If you use row-based replication, then you must not include the triggers on the slave.

Answer 3

You can have the action of requests made by triggers in the binary log with federated tables (MySQL5) by adding the same table with a local connection.
---------------------------------------------------------------------------------------
-- EXEMPLE :
-- We want install a replication of the table test_table that will be managed by Trg_Update triggers.
---------------------------------------------------------------------------------------
Create database TEST;
USE TEST;
CREATE TABLE test_trigger (
id INT(20) NOT NULL AUTO_INCREMENT,
name VARCHAR(32) NOT NULL DEFAULT '',
PRIMARY KEY (id),
INDEX name (name),
) ;
DELIMITER |
CREATE TRIGGER Trg_Update AFTER INSERT ON test_trigger
FOR EACH ROW BEGIN
INSERT INTO federated_table (name, other) values (NEW.name, ‘test trigger on federated table -> OK’)
END|
DELIMITER ;
CREATE TABLE test_table (
id INT(20) NOT NULL AUTO_INCREMENT,
name VARCHAR(32) NOT NULL DEFAULT '',
other VARCHAR(32) NOT NULL DEFAULT '',
PRIMARY KEY (id),
INDEX name (name),
INDEX other_key (other)
) ;
CREATE TABLE federated_table (
id INT(20) NOT NULL AUTO_INCREMENT,
name VARCHAR(32) NOT NULL DEFAULT '',
other VARCHAR(32) NOT NULL DEFAULT '',
PRIMARY KEY (id),
INDEX name (name),
INDEX other_key (other)
)
ENGINE=FEDERATED
CONNECTION='mysql://root#localhost/TEST/test_table';
---------------------------------------------------------------------------------------