Mysql – Is it possible for unique key to break master-master replication in thesql

MySQLreplication

I have a master-master mysql setup with 2 servers running the exact same application making writes to such a table:

 CREATE TABLE `metric` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `host` varchar(50) NOT NULL,
  `userid` int(10) unsigned DEFAULT NULL,
  `name` varchar(255) NOT NULL,
  `sampleid` tinyint(3) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `unique-metric` (`userid`,`host`,`name`,`sampleid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

auto_increment_increment is 2 and offsets are 0 and 1, so PK ids don't clash, but is it possible that with bad timing, 2 applications will create a row with an equal unique-metric index breaking replication on both mysql servers, since replication thread won't be able to insert replicated row into table due to another row already having the exact same index?

Best Answer

Though I've not tried this myself, I've done a quick documentation hunt and found that this part of the MySQL documentation is helpful.

The relevant part is:

If a statement produces different errors on the master and the slave, the slave SQL thread terminates, and the slave writes a message to its error log and waits for the database administrator to decide what to do about the error. This includes the case that a statement produces an error on the master or the slave, but not both.

The next question would then be: "How do I know there has been an error?", to which I've found a useful script here, which could be run frequently as a cron task.

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