MySQL Trigger & Stored Procedure Replication

Question

Ok,I'm running a setup with a single master and a number of slaves. All writes go through the master and are replicated down to the slaves which are used strictly for reads.

Now I have a stored procedure (not function) which is called by a trigger on an insert. According to the MySQL docs, for replication triggers log the call to the trigger while stored procedures actually log the result of the stored procedure.

So my question is, when my trigger gets fired, will it replicate both the trigger and the results of the procedure that the trigger calls (resulting in the procedure effectively being run twice)? Or will it simply replicate the trigger have the slaves re-run the stored procedure on their own?

Thanks

Answer 1

In MySQL 5.0 (and MySQL 5.1 with statement based binary logging), only the calling query is logged, so in your case, the INSERT would be logged.

On the slave, the INSERT will be executed and then the trigger will be re-run on the slave. So the trigger needs to exist on the slave, and assuming it does, then it will be executed in exactly the same way as the master.

In MySQL 5.1, there is row-based binary logging, which will log only the rows being changed, so the trigger would not be re-fired on the slave, but all rows that changed would still be propagated.

Answer 2

In addition to Harrison's excellent answer:

• Assuming the databases are in sync (schema, data, same version) to start with, it should just work
• If it doesn't, then it may be that you're using something non deterministic in your queries or trigger. Fix that.
• Regardless of how you use replication, you need to have monitoring to check that the slaves are always in sync. Without any monitoring, they will become out of sync (subtly) and you won't notice. MySQL has no automatic built-in feature for checking this or fixing it.