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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?

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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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