More things to know about events in MySQL

Events in MySQL are very useful for having some action kick off on a regular basis. I've used them for adding/removing partitions, killing long running queries and archiving data. However, when working with replication, they can some times be tricky. The data charmer wrote a blog post a few years ago which is useful:

http://datacharmer.blogspot.com/2009/03/something-to-know-about-event-scheduler.html

This is also a good reference:
http://anothermysqldba.blogspot.com/2013/05/using-mysql-event-scheduler.html

In MySQL only the master has running events. The event itself is replicated to the slave but is "SLAVESIDE_DISABLED". The tricky part is when you want to promote a slave to be a master. The event does not become ENABLED until you run an alter command like this:

ALTER EVENT <schema_name>.<event_name> ENABLE;

Also turning on the event scheduler does not enable individual events.

Your failover process could include a script that checks the slave for all events that are SLAVESIDE_DISABLED and then changes them to ENABLE when promoting a slave to a master. One of the tricky parts I have also experienced is when adding a new slave-replica to a cluster. I would copy over a previous backup from XtraBackup and then my script would add the slave into the cluster. If the backup contained events all the events on the SLAVE would be ENABLED. If the event was doing some thing like adding or removing partitions then I would notice that replication would break because the slave was doing what the master was also doing and when the statements were replicated to the slave, they would fail. I needed to add a check to my script which adds new slaves to a cluster and make all the events SLAVESIDE_DISABLED by running this on each event on the new replica:

ALTER EVENT zabbix.RotatePartitionedTables disable on slave;

Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'Could not find first log file name in binary log index file'

Today I was trying to restore a backup onto a server and then add that new server into a replication cluster. I have a script that automatically does this by capturing the binary log position and log file that the slave should use when attaching to the master. Then the slave can use the binary log and relay file to catch up.

Each time I attempted this I would get this error:

Last_IO_Error: Got fatal error 1236 from master when reading data from binary log: 'Could not find first log file name in binary log index file'

I was wondering, who the heck keeps purging the binary logs before my new slave has time to replicate the data?

I checked the my.cnf file for the master and the logs are set to expire after 14 days. I was using a backup taken from this morning.

I checked the root's crontab and there was a little bash script running every hour doing this:

#!/bin/bash
time_string=$(date +"%Y-%m-%d %H:%M:%S" -d  "1 hour ago"); mysql -uUSERNAME --password=PASSWORD -e "purge binary logs before '${time_string}'"

That little bash script is purging all the binary logs from an hour before it starts so my script wasn't able to attach to the master and replicate because the log file which my backup was referencing files that had already been purged.

This is an extremely busy server that generates a lot of logs so I can see why this was here.

I temporary commented out the entry, started a new backup and finished adding my new slave into the server.

I also added an FYI to /etc/motd so that when someone logs in to the serverr via SSH they get a message indicating that this cron job is running.

pt-deadlock-logger

There are a number of options to run the deadlock logging tool but there is an actual example:

pt-deadlock-logger h=my_logging_server.com --user=USERNAME --password=PASSWORD --dest h=my_server.com,D=percona_schema,t=deadlocks,u=USERNAME,p=PASSWORD --create-dest-table --daemonize

This is the most useful free way I've found to capture deadlocks in MySQL.