(mysql.info) replication-auto-increment
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6.13 Auto-Increment in Multiple-Master Replication
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When multiple servers are configured as replication masters, special
steps must be taken to prevent key collisions when using
`AUTO_INCREMENT' columns, otherwise multiple masters may attempt to use
the same `AUTO_INCREMENT' value when inserting rows.
The `auto_increment_increment' and `auto_increment_offset' system
variables help to accommodate multiple-master replication with
`AUTO_INCREMENT' columns. Each of these variables has a default and
minimum value of 1, and a maximum value of 65,535. They were introduced
in MySQL 5.0.2.
These two variables effect `AUTO_INCREMENT' column behavior as follows:
* `auto_increment_increment' controls the increment between
successive `AUTO_INCREMENT' values.
* `auto_increment_offset' determines the starting point for
`AUTO_INCREMENT' column values.
By choosing non-conflicting values for these variables on different
masters, servers in a multiple-master configuration will not use
conflicting `AUTO_INCREMENT' values when inserting new rows into the
same table. To set up N master servers, set the variables like this:
* Set `auto_increment_increment' to N on each master.
* Set each of the N masters to have a different
`auto_increment_offset', using the values 1, 2, ..., N.
For additional information about `auto_increment_increment' and
`auto_increment_offset', see server-system-variables.
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