Lets start by considering a scenario where records are being inserted in a single auto-increment table via different nodes of a multi-master cluster. One issue that might arise is ‘collision’ of generated auto-increment values on different nodes, which is precisely the subject of this article.

As the cluster is multi-master, it allows writes on all master nodes. As a result of which a table might get same auto-incremented values on different nodes on INSERTs. This issue is discovered only after the writeset is replicated and that’s a problem!

Galera cluster suffers with the similar problem.

Lets try to emulate this on a 2-node Galera cluster :

As expected, the second commit could not succeed because of the collision.

So, how do we handle this issue? Enter @@auto_increment_increment and @@auto_increment_offset! Using these two system variables one can control the sequence of auto-generated values on a MySQL/MariaDB server. The trick is to set them in such a way that every node in the cluster generates a sequence of non-colliding numbers.

For instance, lets discuss this for a 3-node cluster (n=3):

As you can see, by setting each node’s auto_increment_increment to the total number of nodes (n) in the cluster and auto_increment_offset to a number between [1,n], we can assure that auto-increment values, thus generated, would be unique across the cluster, thus, would avoid any conflict or collision.

In Galera cluster this is already taken care of by default. As and when a node joins the cluster, the two auto-increment variables are adjusted automatically to avoid collision. However, this capability can be controlled by using wsrep_auto_increment_control variable.

With this setting the last COMMIT in the above example would succeed.

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